Signal transformations from cerebral cortex to superior colliculus for the generation of saccades

The ability of primates to make rapid and accurate saccadic eye movements for exploring the natural world is based on a neuronal system in the brain that has been studied extensively and is known to include multiple brain regions extending throughout the neuraxis. We examined the characteristics of signal flow in this system by recording from identified output neurons of two cortical regions, the lateral intraparietal area (LIP) and the frontal eye field (FEF), and from neurons in a brainstem structure targeted by these output neurons, the superior colliculus (SC). We compared the activity of neurons in these three populations while monkeys performed a delayed saccade task that allowed us to quantify visual responses, motor activity, and intervening delay activity. We examined whether delay activity was related to visual stimulation by comparing the activity during interleaved trials when a target was either present or absent during the delay period. We examined whether delay activity was related to movement by using a Go/Nogo task and comparing the activity during interleaved trials in which a saccade was either made (Go) or not (Nogo). We found that LIP output neurons, FEF output neurons, and SC neurons can all have visual responses, delay activity, and presaccadic bursts; hence in this way they are all quite similar. However, the delay activity tended to be more related to visual stimulation in the cortical output neurons than in the SC neurons. Complementing this, the delay activity tended to be more related to movement in the SC neurons than in the cortical output neurons. We conclude, first, that the signal flow leaving the cortex represents activity at nearly every stage of visuomotor transformation, and second, that there is a gradual evolution of signal processing as one proceeds from cortex to colliculus.

Progression in neuronal processing for saccadic eye movements from parietal cortex area lip to superior colliculus

Neurons in both the lateral intraparietal area (LIP) of the monkey parietal cortex and the intermediate layers of the superior colliculus (SC) are activated well in advance of the initiation of saccadic eye movements. To determine whether there is a progression in the covert processing for saccades from area LIP to SC, we systematically compared the discharge properties of LIP output neurons identified by antidromic activation with those of SC neurons collected from the same monkeys. First, we compared activity patterns during a delayed saccade task and found that LIP and SC neurons showed an extensive overlap in their responses to visual stimuli and in their sustained activity during the delay period. The saccade activity of LIP neurons was, however, remarkably weaker than that of SC neurons and never occurred without any preceding delay activity. Second, we assessed the dependence of LIP and SC activity on the presence of a visual stimulus by contrasting their activity in delayed saccade trials in which the presentation of the visual stimulus was either sustained (visual trials) or brief (memory trials). Both the delay and the presaccadic activity levels of the LIP neuronal sample significantly depended on the sustained presence of the visual stimulus, whereas those of the SC neuronal sample did not. Third, we examined how the LIP and SC delay activity relates to the future production of a saccade using a delayed GO/NOGO saccade task, in which a change in color of the fixation stimulus instructed the monkey either to make a saccade to a peripheral visual stimulus or to withhold its response and maintain fixation. The average delay activity of both LIP and SC neuronal samples significantly increased by the advance instruction to make a saccade, but LIP neurons were significantly less dependent on the response instruction than SC neurons, and only a minority of LIP neurons was significantly modulated. Thus despite some overlap in their discharge properties, the neurons in the SC intermediate layers showed a greater independence from sustained visual stimulation and a tighter relationship to the production of an impending saccade than the LIP neurons supplying inputs to the SC. Rather than representing the transmission of one processing stage in parietal cortex area LIP to a subsequent processing stage in SC, the differences in neuronal activity that we observed suggest instead a progressive evolution in the neuronal processing for saccades.

Frontal eye field sends delay activity related to movement, memory, and vision to the superior colliculus

Many neurons within prefrontal cortex exhibit a tonic discharge between visual stimulation and motor response. This delay activity may contribute to movement, memory, and vision. We studied delay activity sent from the frontal eye field (FEF) in prefrontal cortex to the superior colliculus (SC). We evaluated whether this efferent delay activity was related to movement, memory, or vision, to establish its possible functions. Using antidromic stimulation, we identified 66 FEF neurons projecting to the SC and we recorded from them while monkeys performed a Go/Nogo task. Early in every trial, a monkey was instructed as to whether it would have to make a saccade (Go) or not (Nogo) to a target location, which permitted identification of delay activity related to movement. In half of the trials (memory trials), the target disappeared, which permitted identification of delay activity related to memory. In the remaining trials (visual trials), the target remained visible, which permitted identification of delay activity related to vision. We found that 77% (51/66) of the FEF output neurons had delay activity. In 53% (27/51) of these neurons, delay activity was modulated by Go/Nogo instructions. The modulation preceded saccades made into only part of the visual field, indicating that the modulation was movement-related. In some neurons, delay activity was modulated by Go/Nogo instructions in both memory and visual trials and seemed to represent where to move in general. In other neurons, delay activity was modulated by Go/Nogo instructions only in memory trials, which suggested that it was a correlate of working memory, or only in visual trials, which suggested that it was a correlate of visual attention. In 47% (24/51) of FEF output neurons, delay activity was unaffected by Go/Nogo instructions, which indicated that the activity was related to the visual stimulus. In some of these neurons, delay activity occurred in both memory and visual trials and seemed to represent a coordinate in visual space. In others, delay activity occurred only in memory trials and seemed to represent transient visual memory. In the remainder, delay activity occurred only in visual trials and seemed to be a tonic visual response. In conclusion, the FEF sends diverse delay activity signals related to movement, memory, and vision to the SC, where the signals may be used for saccade generation. Downstream transmission of various delay activity signals may be an important, general way in which the prefrontal cortex contributes to the control of movement.

Interaction of the frontal eye field and superior colliculus for saccade generation

Both the frontal eye field (FEF) in the prefrontal cortex and the superior colliculus (SC) on the roof of the midbrain participate in the generation of rapid or saccadic eye movements and both have projections to the premotor circuits of the brain stem where saccades are ultimately generated. In the present experiments, we tested the contributions of the pathway from the FEF to the premotor circuitry in the brain stem that bypasses the SC. We assayed the contribution of the FEF to saccade generation by evoking saccades with direct electrical stimulation of the FEF. To test the role of the SC in conveying information to the brain stem, we inactivated the SC, thereby removing the circuit through the SC to the brain stem, and leaving only the direct FEF-brain stem pathway. If the contributions of the direct pathway were substantial, removal of the SC should have minimal effect on the FEF stimulation, whereas if the FEF stimulation were dependent on the SC, removal of the SC should alter the effect of FEF stimulation. By acutely inactivating the SC, instead of ablating it, we were able to test the efficiency of the direct FEF-brain stem pathway before substantial compensatory mechanisms could mask the effect of removing the SC. We found two striking effects of SC inactivation. In the first, we stimulated the FEF at a site that evoked saccades with vectors that were very close to those evoked at the site of the SC inactivation, and with such optimal alignment, we found that SC inactivation eliminated the saccades evoked by FEF stimulation. The second effect was evident when the FEF evoked saccades were disparate from those evoked in the SC, and in this case we observed a shift in the direction of the evoked saccade that was consistent with the SC inactivation removing a component of a vector average. Together these observations lead to the conclusion that in the nonablated monkey the direct FEF-brain stem pathway is not functionally sufficient to generate accurate saccades in the absence of the indirect pathway that courses from the FEF through the SC to the brain stem circuitry. We suggest that the recovery of function following SC ablation that has been seen in previous studies must result not from the use of an already functioning parallel pathway but from neural plasticity within the saccadic system.

Activation and inactivation of rostral superior colliculus neurons during smooth-pursuit eye movements in monkeys

Neurons in the intermediate and deep layers of the rostral superior colliculus (SC) of monkeys are active during attentive fixation, small saccades, and smooth-pursuit eye movements. Alterations of SC activity have been shown to alter saccades and fixation, but similar manipulations have not been shown to influence smooth-pursuit eye movements. Therefore we both activated (electrical stimulation) and inactivated (reversible chemical injection) rostral SC neurons to establish a causal role for the activity of these neurons in smooth pursuit. First, we stimulated the rostral SC during pursuit initiation as well as pursuit maintenance. For pursuit initiation, stimulation of the rostral SC suppressed pursuit to ipsiversive moving targets primarily and had modest effects on contraversive pursuit. The effect of stimulation on pursuit varied with the location of the stimulation with the most rostral sites producing the most effective inhibition of ipsiversive pursuit. Stimulation was more effective on higher pursuit speeds than on lower and did not evoke smooth-pursuit eye movements during fixation. As with the effects on pursuit initiation, ipsiversive maintained pursuit was suppressed, whereas contraversive pursuit was less affected. The stimulation effect on smooth pursuit did not result from a generalized inhibition because the suppression of smooth pursuit was greater than the suppression of smooth eye movements evoked by head rotations (vestibular-ocular reflex). Nor was the stimulation effect due to the activation of superficial layer visual neurons rather than the intermediate layers of the SC because stimulation of the superficial layers produced effects opposite to those found with intermediate layer stimulation. Second, we inactivated the rostral SC with muscimol and found that contraversive pursuit initiation was reduced and ipsiversive pursuit was increased slightly, changes that were opposite to those resulting from stimulation. The results of both the stimulation and the muscimol injection experiments on pursuit are consistent with the effects of these activation and inactivation experiments on saccades, and the effects on pursuit are consistent with the hypothesis that the SC provides a position signal that is used by the smooth-pursuit eye-movement system.

Discharge properties of neurons in the rostral superior colliculus of the monkey during smooth-pursuit eye movements

The intermediate and deep layers of the monkey superior colliculus (SC) comprise a retinotopically organized map for eye movements. The rostral end of this map, corresponding to the representation of the fovea, contains neurons that have been referred to as "fixation cells" because they discharge tonically during active fixation and pause during the generation of most saccades. These neurons also possess movement fields and are most active for targets close to the fixation point. Because the parafoveal locations encoded by these neurons are also important for guiding pursuit eye movements, we studied these neurons in two monkeys as they generated smooth pursuit. We found that fixation cells exhibit the same directional preferences during pursuit as during small saccades-they increase their discharge during movements toward the contralateral side and decrease their discharge during movements toward the ipsilateral side. This pursuit-related activity could be observed during saccade-free pursuit and was not predictive of small saccades that often accompanied pursuit. When we plotted the discharge rate from individual neurons during pursuit as a function of the position error associated with the moving target, we found tuning curves with peaks within a few degrees contralateral of the fovea. We compared these pursuit-related tuning curves from each neuron to the tuning curves for a saccade task from which we separately measured the visual, delay, and peri-saccadic activity. We found the highest and most consistent correlation with the delay activity recorded while the monkey viewed parafoveal stimuli during fixation. The directional preferences exhibited during pursuit can therefore be attributed to the tuning of these neurons for contralateral locations near the fovea. These results support the idea that fixation cells are the rostral extension of the buildup neurons found in the more caudal colliculus and that their activity conveys information about the size of the mismatch between a parafoveal stimulus and the currently foveated location. Because the generation of pursuit requires a break from fixation, the pursuit-related activity indicates that these neurons are not strictly involved with maintaining fixation. Conversely, because activity during the delay period was found for many neurons even when no eye movement was made, these neurons are also not obligatorily related to the generation of a movement. Thus the tonic activity of these rostral neurons provides a potential position-error signal rather than a motor command-a principle that may be applicable to buildup neurons elsewhere in the SC.

Multielectrode evidence for spreading activity across the superior colliculus movement map

The monkey superior colliculus (SC) has maps for both visual input and movement output in the superficial and intermediate layers, respectively, and activity on these maps is generally related to visual stimuli only in one part of the visual field and/or to a restricted range of saccadic eye movements to those stimuli. For some neurons within these maps, however, activity has been reported to spread from the caudal SC to the rostral SC during the course of a saccade. This spread of activity was inferred from averages of recordings at different sites on the SC movement map during saccades of different amplitudes and even in different monkeys. In the present experiments, SC activity was recorded simultaneously in pairs of neurons to observe the spread of activity during individual saccades. Two electrodes were positioned along the rostral-caudal axis of the SC with one being more caudal than the other, and 60 neuron pairs whose movement fields were large enough to see a spread of activity were studied. During individual saccades, the relative time of discharge of the two neurons was compared using 1) the time difference between peak discharge of the two neurons, 2) the difference between the "median activation time" of the two neurons, and 3) the shift required to align the two discharge patterns using cross-correlation. All three analysis methods gave comparable results. Many pairs of neurons were activated in sequence during saccade generation, and the order of activation was most frequently caudal to rostral. Such a sequence of activation was not observed in every neuron pair, but over the sample of neuron pairs studied, the spread was statistically significant. When we compared the time of neuronal activity to the time of saccade onset, we found that the caudal neuronal activity was more likely to be before the saccade, whereas the rostral neuronal activity was more likely to be during the saccade. These results demonstrate that when individual pairs of neurons are examined during single saccades there is evidence of a caudal to rostral spread of activity within the monkey SC, and they confirm the previous inferences of a spread of activity drawn from observations on averaged neuronal activity during multiple saccades. The functional contribution of this spread of activity remains to be determined.

Extent of compensation for variations in monkey saccadic eye movements

We investigated and quantified the ability of the primate saccadic system to generate accurate eye movements in spite of naturally occurring variations in saccadic speed and trajectory. We show that the amplitude of a series of saccades directed to the same target is positively correlated to their peak speed, i.e., the faster the saccade, the bigger its amplitude. We demonstrate that this result cannot be simply accounted for by the main sequence, and that on average the saccadic system is able to compensate for only 61% of the variability in speed. Deviations from the average trajectory are also only partially compensated: the underlying mechanism, which tends to bring the eyes back toward the desired trajectory, underperforms for small movements and overperforms for large movements. We also demonstrate that the performance of this compensatory mechanism, and the metrics of saccades in general, do not depend on the presence of visual information during the movement. By showing that deviations from the desired behavior are corrected during the saccade, our results further support the hypothesis that the innervation signal that generates saccadic eye movements is not pre-programmed but rather is dynamically adjusted during the movement. However, the compensation for deviations from the desired behavior is only partial, and the underlying mechanisms have yet to be completely understood. Although none of the current models of the saccadic system can account for our results, some of them, if appropriately modified, probably could.

Composition and topographic organization of signals sent from the frontal eye field to the superior colliculus

The frontal eye field (FEF) and superior colliculus (SC) contribute to saccadic eye movement generation, and much of the FEF's oculomotor influence may be mediated through the SC. The present study examined the composition and topographic organization of signals flowing from FEF to SC by recording from FEF neurons that were antidromically activated from rostral or caudal SC. The first and most general result was that, in a sample of 88 corticotectal neurons, the types of signals relayed from FEF to SC were highly diverse, reflecting the general population of signals within FEF rather than any specific subset of signals. Second, many neurons projecting from FEF to SC carried signals thought to reflect cognitive operations, namely tonic discharges during the delay period of a delayed-saccade task (delay signals), elevated discharges during the gap period of a gap task (gap increase signals), or both. Third, FEF neurons discharging during fixation were found to project to the SC, although they did not project preferentially to rostral SC, where similar fixation neurons are found. Neurons that did project preferentially to the rostral SC were those with foveal visual responses and those pausing during the gap period of the gap task. Many of the latter neurons also had foveal visual responses, presaccadic pauses in activity, and postsaccadic increases in activity. These two types of rostral-projecting neurons therefore may contribute to the activity of rostral SC fixation neurons. Fourth, conduction velocity was used as an indicator of cell size to correct for sampling bias. The outcome of this correction procedure suggested that among the most prevalent neurons in the FEF corticotectal population are those carrying putative cognitive-related signals, i.e., delay and gap increase signals, and among the least prevalent are those carrying presaccadic burst discharges but lacking peripheral visual responses. Fifth, corticotectal neurons carrying various signals were biased topographically across the FEF. Neurons with peripheral visual responses but lacking presaccadic burst discharges were biased laterally, neurons with presaccadic burst discharges but lacking peripheral visual responses were biased medially, and neurons carrying delay or gap increase signals were biased dorsally. Finally, corticotectal neurons were distributed within the FEF as a function of their visual or movement field eccentricity and projected to the SC such that eccentricity maps in both structures were closely aligned. We conclude that the FEF most likely influences the activity of SC neurons continuously from the start of fixation, through visual analysis and cognitive manipulations, until a saccade is generated and fixation begins anew. Furthermore, the projection from FEF to SC is highly topographically organized in terms of function at both its source and its termination.

Effects of attention on MT and MST neuronal activity during pursuit initiation

The responses of neurons in monkey extrastriate areas MT (middle temporal) and MST (medial superior temporal), and the initial metrics of saccadic and pursuit eye movements, have previously been shown to be better predicted by vector averaging or winner-take-all models depending on the stimulus conditions. To investigate the potential influences of attention on the neuronal activity, we measured the responses of single MT and MST neurons under identical stimulus conditions when one of two moving stimuli was the target for a pursuit eye movement. We found the greatest attentional modulation across neurons when two stimuli moved through the receptive field (RF) of the neuron and the stimulus motion was initiated at least 450 ms before reaching the center of the RF. These conditions were the same as those in which a winner-take-all model better predicted both the eye movements and the underlying neuronal activity. The modulation was almost always an increase of activity, and it was about equally frequent in MT and MST. A modulation of >50% was observed in approximately 41% of MT neurons and 27% of MST neurons. Responses to all directions of motion were modulated so that the direction tuning curves in the attended and unattended conditions were similar. Changes in the background activity with target selection were small and unlikely to account for the observed attentional modulation. In contrast, there was little change in the neuronal response with attention when the stimulus reached the RF center 150 ms after motion onset, which was also the condition in which the vector average model better predicted the initial eye movements and the activity of the neurons. These results are consistent with a competition model of attention in which top-down attention acts on the activity of one of two competing populations of neurons activated by the bottom-up input from peripheral stimuli. They suggest that there is a minimal separation of the populations necessary before attention can act on one population, similar to that required to produce a winner-take-all mode of behavior in pursuit initiation. The present experiments also suggest that it takes several hundred milliseconds to develop this top-down attention effect.

Disparity sensitivity of frontal eye field neurons

Information about depth is necessary to generate saccades to visual stimuli located in three-dimensional space. To determine whether monkey frontal eye field (FEF) neurons play a role in the visuo-motor processes underlying this behavior, we studied their visual responses to stimuli at different disparities. Disparity sensitivity was tested from 3 degrees of crossed disparity (near) to 3 degrees degrees of uncrossed disparity (far). The responses of about two thirds of FEF visual and visuo-movement neurons were sensitive to disparity and showed a broad tuning in depth for near or far disparities. Early phasic and late tonic visual responses often displayed different disparity sensitivity. These findings provide evidence of depth-related signals in FEF and suggest a role for FEF in the control of disconjugate as well as conjugate eye movements.

Response to motion in extrastriate area MSTl: disparity sensitivity

Many neurons in the lateral-ventral region of the medial superior temporal area (MSTl) have a clear center surround separation in their receptive fields. Either moving or stationary stimuli in the surround modulates the response to moving stimuli in the center, and this modulation could facilitate the perceptual segmentation of a moving object from its background. Another mechanism that could facilitate such segmentation would be sensitivity to binocular disparity in the center and surround regions of the receptive fields of these neurons. We therefore investigated the sensitivity of these MSTl neurons to disparity ranging from three degrees crossed disparity (near) to three degrees uncrossed disparity (far) applied to both the center and the surround regions. Many neurons showed clear disparity sensitivity to stimulus motion in the center of the receptive field. About (1)/(3) of 104 neurons had a clear peak in their response, whereas another (1)/(3) had broader tuning. Monocular stimulation abolished the tuning. The prevalence of cells broadly tuned to near and far disparity and the reversal of preferred directions at different disparities observed in MSTd were not found in MSTl. A stationary surround at zero disparity simply modulated up or down the response to moving stimuli at different disparities in the receptive field (RF) center but did not alter the disparity tuning curve. When the RF center motion was held at zero disparity and the disparity of the stationary surround was varied, some surround disparities produced greater modulation of MSTl neuron response than did others. Some neurons with different disparity preferences in center and surround responded best to the relative disparity differences between center and surround, whereas others were related to the absolute difference between center and surround. The combination of modulatory surrounds and the sensitivity to relative difference between center and surround disparity make these MSTl neurons particularly well suited for the segmentation of a moving object from the background.

Shift in smooth pursuit initiation and MT and MST neuronal activity under different stimulus conditions

The activity of neurons in extrastriate middle temporal (MT) and medial superior temporal (MST) areas were studied during the initiation of pursuit eye movements in macaque monkeys. The intersecting motion of two stimuli was used to test hypotheses about how these direction- and speed-sensitive neurons contribute to the generation of pursuit. The amplitude and direction of the initial saccade to the target and the initial speed and direction of pursuit were best predicted by a vector-average model of the underlying neuronal activity with relatively short time and spatial separation before a visual pursuit target and a distracter stimulus crossed in the visual field. The resulting eye movements were best described by a winner-take-all model when the time and spatial separation between the two stimuli was increased before the stimuli crossed. Neurons in MT and MST also shifted their activity from that best described by a vector average to a winner-take-all model under the same stimulus conditions. The changes in activity of neurons in both areas were generally similar to each other during these changes in pursuit initiation. Thus a slight alteration in the target motion produced a concurrent shift in both the neuronal processing and the movement execution. We propose that the differences in the oculomotor behavior can be accounted for by shifts in the overlap of active neuronal populations within MT and MST.

Frontal eye field neurons orthodromically activated from the superior colliculus

Frontal eye field neurons orthodromically activated from the superior colliculus. J. Neurophysiol. 80: 3331-3333, 1998. Anatomical studies have shown that the frontal eye field (FEF) and superior colliculus (SC) of monkeys are reciprocally connected, and a physiological study described the signals sent from the FEF to the SC. Nothing is known, however, about the signals sent from the SC to the FEF. We physiologically identified and characterized FEF neurons that are likely to receive input from the SC. Fifty-two FEF neurons were found that were orthodromically activated by electrical stimulation of the intermediate or deeper layers of the SC. All the neurons that we tested (n = 34) discharged in response to visual stimulation. One-half also discharged when saccadic eye movements were made. This provides the first direct evidence that the ascending pathway from SC to FEF might carry visual- and saccade-related signals. Our findings support a hypothesis that the SC and the FEF interact bidirectionally during the events leading up to saccade generation.

Modulation of neuronal activity in superior colliculus by changes in target probability

Complex visual scenes require that a target for an impending saccadic eye movement be selected from a larger number of possible targets. We investigated whether changing the probability that a visual stimulus would be selected as the target for a saccade altered activity of monkey superior colliculus (SC) neurons in two experiments. First, we changed the number of possible targets on each trial. Second, we kept the visual display constant and presented a single saccade target repeatedly so that target probability was established over time. Buildup neurons in the SC, those with delay period activity, showed a consistent reduction in activity as the probability of the saccade decreased, independent of the visual stimulus configuration. Other SC neurons, fixation and burst, were largely unaffected by the changes in saccade target probability. Because we had monkeys making saccades to many locations within the visual field, we could examine activity associated with saccades outside of the movement field of neurons. We found the activity of buildup neurons to be similar across the SC, before the target was identified, and reduced when the number of possible targets increased. The results of our experiments are consistent with a role for this activity in establishing a motor set. We found, consistent with this interpretation, that the activity of these neurons was predictive of the latency of a saccadic eye movement and not other saccade parameters such as end point or peak velocity.

Optic flow: A brain region devoted to optic flow analysis?

The visual motion - or optic flow - that results from an observer's own movement can indicate the direction of heading through the environment. Recent experiments have strengthened the argument that neurons in a specialized region of the cerebral cortex are critical for the analysis of this important class of visual stimuli.

Response to motion in extrastriate area MSTl: center-surround interactions

The medial superior temporal area of the macaque monkey extrastriate visual cortex can be divided into a dorsal medial (MSTd) and a lateral ventral (MSTl) region. The functions of the two regions may not be identical: MSTd may process optic flow information that results from the movement of the observer, whereas MSTl may be related more closely to processing visual motion related specifically to the motion of objects. If MSTl were related to such object motion, one would expect to see mechanisms for the segregation of objects from their surround. We investigated one of these mechanisms in MSTl neurons: the effect of stimuli falling in the region surrounding the receptive field center on the response to stimuli falling in the field center. We found the effects of the surround stimulation to be modulatory with little response to the surround stimulus itself but a clear effect on the response to the stimulus falling on the receptive field center. The response to motion in the center in the direction preferred for the neuron usually increased when the surround motion was in the opposite direction to that in the center and decreased when surround motion was in the same direction as that in the center. Fifty-seven percent of the neurons showed a ratio of response for center motion with a surround moving in the opposite direction to that in the center for center motion alone that was >1. The response to motion in the center also increased when the surround stimulus was stationary, and this increase was sometimes larger than that with a moving surround. Nearly 70% of the neurons showed a ratio of response to center motion with a stationary surround to center motion alone that was >1. This is in contrast to the minimal effect of stationary surrounds in middle temporal area neurons. When the stimulus presentation was reversed so that the stimulus in the center was stationary and the surround moved, some MSTl neurons responded when the direction of motion in the surround was in the direction opposite to the preferred direction of motion in the center of the receptive field. Stimulation of the surround thus had a profound effect on the response of MSTl neurons, and this pronounced effect of the surround is consistent with a role in the segmentation of objects using motion.

A screw microdrive for adjustable chronic unit recording in monkeys

A screw microdrive is described that attaches to the grid system used for recording single neurons from brains of awake behaving monkeys. Multiple screwdrives can be mounted on a grid over a single cranial opening. This method allows many electrodes to be implanted chronically in the brain and adjusted as needed to maintain isolation. rights reserved.

Reversible inactivation of monkey superior colliculus. II. Maps of saccadic deficits

Neurons in the superior colliculus (SC) are organized as maps of visual and motor space. The companion paper showed that muscimol injections into intermediate layers of the SC alter the trajectory of the movement and confirmed previously reported effects on latency, amplitude, and speed of saccades. In this paper we analyze the pattern of these deficits across the visual field by systematically comparing the magnitude of each deficit throughout a grid of targets covering a large fraction of the visual field. We also translate these deficits onto the SC map of the visual/movement fields to obtain a qualitative estimate of the extent of the deficit in the SC. We found a consistent pattern of substantially increased saccadic latency to targets in the contralateral visual hemifield, accompanied by slight and inconsistent increases and decreases for saccades to the ipsilateral hemifield. The initial and peak speed of saccades was reduced after the injection. The postinjection amplitude of the saccades were either hypometric or normometric, but rarely hypermetric. Although errors in the initial direction of the postinjection saccades were small, they consistently formed a simple pattern: an initial direction with minimal errors (a null direction) separating regions with clockwise and counterclockwise rotations of the initial direction. However, the null direction did not go through the center of the inactivated zone, as would be expected if the SC alone were determining saccade direction, e.g., with a population code. One hypothesis that can explain the misalignment of the null direction with the lesion site is that another system, acting in parallel with the SC, contributes to the determination of saccadic trajectory.

Reversible inactivation of monkey superior colliculus. I. Curvature of saccadic trajectory

The neurons in the intermediate layers of the monkey superior colliculus (SC) that discharge before saccadic eye movements can be divided into at least two types, burst and buildup neurons, and the differences in their characteristics are compatible with different functional contributions of the two cell types. It has been suggested that a spread of activity across the population of the buildup neurons during saccade generation may contribute to the control of saccadic eye movements. The influence of any such spread should be on both the horizontal and vertical components of the saccade because the map of the movement fields on the SC is a two-dimensional one; it should affect the trajectory of saccade. The present experiments used muscimol injections to inactivate areas within the SC to determine the functional contribution of such a spread of activity on the trajectory of the saccades. The analysis concentrated on saccades made to areas of the visual field that should be affected primarily by alteration of buildup neuron activity. Muscimol injections produced saccades with altered trajectories; they became consistently curved after the injection, and successive saccades to the same targets had similar curvatures. The curved saccades showed changes in their direction and speed at the very beginning of the saccade, and for those saccades that reached the target, the direction of the saccade was altered near the end to compensate for the initially incorrect direction. Postinjection saccades had lower peak speeds, longer durations, and longer latencies for initiation. The changes in saccadic trajectories resulting from muscimol injections, along with the previous observations on changes in speed of saccades with such injections, indicate that the SC is involved in influencing the eye position during the saccade as well as at the end of the saccade. The changes in trajectory when injections were made more rostral in the SC than the most active burst neurons also are consistent with a contribution of the buildup neurons to the control of the eye trajectory. The results do not, however, support the hypothesis that the buildup neurons in the SC act as a spatial integrator.

Monkey posterior parietal cortex neurons antidromically activated from superior colliculus

The connection between the posterior parietal cortex (PPC) and the superior colliculus (SC) was investigated by antidromically activating neurons within the lateral intraparietal (LIP) area with single-pulse stimulation delivered to the intermediate layers of the SC. To dissociate visual and saccade-related responses, the discharge properties of the identified efferent neurons were studied in the delayed visually guided saccade task and the memory guided saccade task. We found that the great majority (74%) of the identified LIP efferent neurons have a peripheral visual receptive field, typically with a broad spatial tuning. About two-thirds (64%) exhibited sustained activity during the delay period of the behavioral tasks, during which the monkeys had to withhold eye movements, and 80% of these increased their activity just before the onset of saccades. Both delay and presaccadic discharges in the delayed visually guided saccade task were higher than in the memory guided saccade task. These results establish that the neuronal signal sent by LIP to the SC carries both visual and saccade-related information.

Responses of MT and MST neurons to one and two moving objects in the receptive field

To test the effects of complex visual motion stimuli on the responses of single neurons in the middle temporal visual area (MT) and the medial superior temporal area (MST) of the macaque monkey, we compared the response elicited by one object in motion through the receptive field with the response of two simultaneously presented objects moving in different directions through the receptive field. There was an increased response to a stimulus moving in a direction other than the best direction when it was paired with a stimulus moving in the best direction. This increase was significant for all directions of motion of the non-best stimulus and the magnitude of the difference increased as the difference in the directions of the two stimuli increased. Similarly, there was a decreased response to a stimulus moving in a non-null direction when it was paired with a stimulus moving in the null direction. This decreased response in MT did not reach significance unless the second stimulus added to the null direction moved in the best direction, whereas in MST the decrease was significant when the second stimulus direction differed from the null by 90 degrees or more. Further analysis showed that the two-object responses were better predicted by taking the averaged response of the neuron to the two single-object stimuli than by summation, multiplication, or vector addition of the responses to each of the two single-object stimuli. Neurons in MST showed larger modulations than did neurons in MT with stimuli moving in both the best direction and in the null direction and the average better predicted the two-object response in area MST than in area MT. This indicates that areas MT and MST probably use a similar integrative mechanisms to create their responses to complex moving visual stimuli, but that this mechanism is further refined in MST. These experiments show that neurons in both MT and MST integrate the motion of all directions in their responses to complex moving stimuli. These results with the motion of objects were in sound agreement with those previously reported with the use of random dot patterns for the study of transparent motion in MT and suggest that these neurons use similar computational mechanisms in the processing of object and global motion stimuli.

Modulation of neuronal activity by target uncertainty

Visual scenes are composed of many elements and although we can appreciate a scene as a whole, we can only move our eyes to one element of the scene at a time. As visual scenes become more complex, the number of potential targets in the scene increases, and the uncertainty that any particular one will be selected for an eye movement also increases. How motor systems accommodate this target uncertainty remains unknown. The activities of neurons in both the cerebral cortex and superior colliculus are modulated by this selection process. We reasoned that activity associated with target uncertainty should be evident in the saccadic motor system at the final stages of neural processing, in the superior colliculus. By systematically changing the number of stimuli from which a selection must be made and recording from superior colliculus neurons, we found that as the target uncertainty increased, the neural activity preceding target selection decreased. These results indicate that neurons within the final common pathway for movement generation are active well in advance of the selection of a particular movement. This early activity varies with the probability that a particular movement will be selected.

Shared motor error for multiple eye movements

Most natural actions are accomplished with a seamless combination of individual movements. Such coordination poses a problem: How does the motor system orchestrate multiple movements to produce a single goal-directed action? The results from current experiments suggest one possible solution. Oculomotor neurons in the superior colliculus of a primate responded to mismatches between eye and target positions, even when the animal made two different types of eye movements. This neuronal activity therefore does not appear to convey a command for a specific type of eye movement but instead encodes an error signal that could be used by multiple movements. The use of shared inputs is one possible strategy for ensuring that different movements share a common goal.

Multiple temporal components of optic flow responses in MST neurons

Neurons in monkey medial superior temporal cortex (MST) respond to optic flow stimuli with early phasic, tonic, and after-phasic response components. In these experiments we characterized each response component to compare its potential contributions to visual motion processing. The early responses begin 60-100 ms after stimulus onset and last between 100 and 250 ms, the tonic responses begin 100-300 ms after stimulus onset and last for as long as the evoking stimulus persists, and the after-responses begin about 60 ms after the stimulus goes off and last for 100-350 ms. A neuron's tonic responses were evoked by specific optic flow stimuli: over two-thirds of the 264 neurons showed tonic responses evoked by two to five stimuli, whereas only 15% responded to either all or none of the stimuli. The tonic responses continued with stimulus presentations as long as 15 s, with their directional preferences being maintained throughout stimulation. However, the tonic response to a given stimulus was seen to change in amplitude when it was presented in random sequence with different sets of other stimuli. Thus, the tonic responses might convey substantial information about optic flow patterns, which continue with prolonged stimulation, but can be modified by the visual context created by other visual motion stimuli. Only about one-third of the 264 neurons had early responses that were selective for specific stimuli. In neurons yielding at least one early response, that neuron was most often activated by all the visual motion stimuli. After-responses occurred in only half the neurons, but they were more often specifically related to particular optic flow stimuli, regardless of whether those stimuli had evoked tonic excitatory or tonic inhibitory responses. The presence of early and after-responses complicates the interpretation of activity evoked when one stimulus immediately follows another. However, under those conditions, early responses and after-responses might contribute to signaling changes in the ongoing pattern of optic flow. We conclude that several components of MST responses should be recognized and that they potentially play different roles in the cortical analysis of optic flow. Tonic responses show the greatest specificity for particular optic flow stimuli, and possess characteristics which make them suitable neuronal participants in self-movement perception.

Medial superior temporal area neurons respond to speed patterns in optic flow

The speed of visual motion in optic flow fields can provide important cues about self-movement. We have studied the speed sensitivities of 131 neurons in the dorsal region of the medial superior temporal area (MSTd) that responded to either radial or circular optic flow stimuli. The responses of more than two-thirds of these neurons were strongly modulated by changes in the mean speed of motion in optic flow stimuli, with response profiles resembling simple filter characteristics. When we removed the normal gradient of speeds in optic flow (slower speeds in the center, faster speeds in the periphery), approximately two-thirds of the neurons showed changes in their responses. When the speed gradient was altered rather than eliminated, almost nine in 10 neurons preferred either a normal speed gradient or an inverted one (slower speeds near the periphery) over stimuli with no speed gradient. These speed gradient preferences do not come simply from different speed preferences in the central and peripheral segments of the stimulus area. Rather, these speed gradient preferences seemed to reflect interactions between simultaneously presented speeds within an optic flow stimulus. The sensitivity of MSTd neurons to patterns of speed, as well as patterns of direction, strengthens the view that these neurons are well suited to the analysis of optic flow. Sensitivity to speed gradients in optic flow might contribute to neuronal mechanisms for spatial orientation during self-movement and for representing the three-dimensional structure of the visual environment.

Planar directional contributions to optic flow responses in MST neurons

Many neurons in the dorsal region of the medial superior temporal area (MSTd) of monkey cerebral cortex respond to optic flow stimuli in which the center of motion is shifted off the center of the visual field. Each shifted-center-of-motion stimulus presents both different directions of planar motion throughout the visual field and a unique pattern of global motion across the visual field. We investigated the contribution of planar motion to the responses of these neurons in two experiments. In the first, we compared the responses of 243 neurons to planar motion and to shifted-center-of-motion stimuli created by vector summation of planar motion and radial or circular motion. We found that many neurons preferred the same directions of motion in the combined stimuli as in the planar stimuli, but other neurons did not. When we divided our sample into one group with stronger directionality to both planar and vector combination stimuli and one group with weaker directionality, we found that the neurons with the stronger directionality were those that showed the greatest similarity in the preferred direction of motion for both the planar and combined stimuli. In a second set of experiments, we overlapped planar motion and radial or circular motion to create transparent stimuli with the same motion components as the vector combination stimuli, but without the shifted centers of motion. We found that the neurons that responded most strongly to the planar motion when it was combined with radial or circular motion also responded best when the planar motion was overlapped by a transparent motion stimulus. We conclude that the responses of those neurons with stronger directional responses to both the motion of planar and vector combination stimuli are most readily understood as responding to the total planar motion in the stimulus, a planar motion mechanism. Other neurons that had weaker directional responses showed no such similarity in the preferred directions of planar motion in the vector combination and the transparent overlap stimuli and fit best with a mechanism dependent on the global motion pattern. We also found that neurons having significant responses to both radial and circular motion also responded to the spiral stimuli that result from a vector combination of radial and circular motion. The preferred planar-spiral vector combination stimulus was frequently the one containing that neurons' preferred direction of planar motion, which makes them similar to other MSTd neurons.

Activity of neurons in monkey superior colliculus during interrupted saccades

1. Recent studies of the monkey superior colliculus (SC) have identified several types of cells in the intermediate layers (including burst, buildup, and fixation neurons) and the sequence of changes in their activity during the generation of saccadic eye movements. On the basis of these observations, several hypotheses about the organization of the SC leading to saccade generation have placed the SC in a feedback loop controlling the amplitude and direction of the impending saccade. We tested these hypotheses about the organization of the SC by perturbing the system while recording the activity of neurons within the SC. 2. We applied a brief high-frequency train of electrical stimulation among the fixation cells in the rostral pole of the SC. This momentarily interrupted the saccade in midflight: after the initial eye acceleration, the eye velocity decreased (frequently to 0) and then again accelerated. Despite the break in the saccade, these interrupted saccades were of about the same amplitude as normal saccades. The postinterruption saccades were usually initiated immediately after the termination of stimulation and occurred regardless of whether the saccade target was visible or not. The velocity-amplitude relationship of the preinterruption component of the saccade fell slightly above the main sequence for control saccades of that amplitude, whereas postinterruption saccades fell near the main sequence. 3. Collicular burst neurons are silent during fixation and discharge a robust burst of action potentials for saccades to a restricted region of the visual field that define a closed movement field. During the stimulation-induced saccadic interruption, these burst neurons all showed a pause in their high-frequency discharge. During an interrupted saccade to a visual target, the typical saccade-related burst was broken into two parts: the first part of the burst began before the initial preinterruption saccade; the second burst began before the postinterruption saccade. 4. We quantified three aspects of the resumption of activity of burst neurons following saccade interruption: 1) the total number of spikes in the pre- and postinterruption bursts, was very similar to the total number of spikes in the control saccade burst; 2) the increase in total duration of the burst (preinterruption period + interruption + postinterruption period) was highly correlated with the increase in total saccade duration (preinterruption saccade + interruption + postinterruption saccade); and 3) the time course of the postinterruption saccade and the resumed cell discharge both followed the same monotonic trajectory as the control saccade in most cells. 5. The same population of burst neurons was active for both the preinterruption and the postinterruption saccades, provided that the stimulation was brief enough to allow the postinterruption saccade to occur immediately. If the postinterruption saccade was delayed by > 100 ms, then burst neurons at a new and more rostral locus related to such smaller saccades became active in association with the smaller remaining saccade. We interpret this shift in active locations within the SC as a termination of the initial saccadic error command and the triggering of a new one. 6. Buildup neurons usually had two aspects to their discharge: a high-frequency burst for saccades of the optimal amplitude and direction (similar to burst neurons), and a low-frequency discharge for saccades of optimal direction whose amplitudes were equal to or greater than the optimal (different from burst neurons). The stimulation-induced interruption in saccade trajectory differentially affected these two components of buildup neuron discharge. The high-frequency burst component was affected in a manner very similar to the burst neurons.(ABSTRACT TRUNCATED)

Response of monkey MST neurons to optic flow stimuli with shifted centers of motion

Neurons in the dorsal region of the medial superior temporal area (MSTd) have previously been shown to respond to the expanding radial motion that occurs as an observer moves through the environment. In previous experiments, MSTd neurons were tested with radial and circular motion centered in the visual field. However, different directions of observer motion, relative to the direction of gaze, are accompanied by visual motion centered at different locations in the visual field. The present experiments investigated whether neurons that respond to radial and circular motion might respond differently when the center of motion was shifted to different regions of the visual field. About 90% of the 245 neurons studied responded differently when the center of motion was shifted away from the center of the field. The centers of motion preferred by each neuron were limited to one area of the visual field. All parts of the visual field were represented in the sample, with greater numbers of neurons preferring centers of motion closer to the center of the field. We hypothesize that each of the MSTd neurons has a center of motion field with a gradient of preferred centers of motion, and that there is an orderly arrangement of MSTd neurons with each region of the visual field being represented by a set of neurons. This arrangement creates the potential for graded responses from individual neurons for different directions of heading as an observer moves through the environment.

Saccade-related activity in monkey superior colliculus. I. Characteristics of burst and buildup cells

1. In the monkey superior colliculus (SC), the activity of most saccade-related neurons studied so far consists of a burst of activity in a population of cells at one place on the SC movement map. In contrast, recent experiments in the cat have described saccade-related activity as a slow increase in discharge before saccades followed by a hill of activity moving across the SC map. In order to explore this striking difference in the distribution of activity across the SC, we recorded from all saccade-related neurons that we encountered in microelectrode penetrations through the monkey SC and placed them in categories according to their activity during the generation of saccades. 2. When we considered the activity preceding the onset of the saccade, we could divide the cells into two categories. Cells with burst activity had a high-frequency discharge just before saccade onset but little activity between the signal to make a saccade and saccade onset. About two thirds of the saccade-related cells had only a burst of activity. Cells with a buildup of activity began to discharge at a low frequency after the signal to make a saccade and the discharge continued until generation of the saccade. About one third of the saccade-related cells studied had a buildup of activity, and about three fourths of these cells also gave a burst of activity with the saccade in addition to the slow buildup of activity. 3. The buildup of activity seemed to be more closely related to preparation to make a saccade than to the generation of the saccade. The buildup developed even in cases when no saccade occurred. 4. The falling phase of the discharge of these saccade-related cells stopped with the end of the saccade (a clipped discharge), shortly after the end of the saccade (partially clipped), or long after the end of the saccade (unclipped). 5. Some cells had closed movement fields in which saccades that were substantially smaller or larger than the optimal amplitude were not associated with increased activity. Other cells tended to have open-ended movement fields without any peripheral border; they were active for all saccades of optimal direction whose amplitudes were equal to or greater than a given amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Saccade-related activity in monkey superior colliculus. II. Spread of activity during saccades

1. In the companion paper we described two classes of cells in the monkey superior colliculus (SC) that were related to saccade generation, buildup cells and burst cells, which fell into two functional sublayers within the intermediate layers of the SC. Fixation cells in the rostral SC were deemed to be part of the buildup cell layer. The buildup cells had several characteristics in common with cells in the cat described as having a "hill of activity" moving across the SC, but the burst cells had no such characteristics. In this paper we further investigate whether there is evidence for such a moving hill of activity in the monkey by analyzing the spatial and temporal activity of cells across the SC during the generation of visually guided saccades. 2. We recorded the activity of single cells while the monkey made saccades of different amplitudes (0.5-60 degrees). We recorded cells from locations extending from the rostral to caudal SC in order to sample cells whose optimal amplitudes ranged from small to large saccades. This allowed us to see any shift of activity across the SC before, during, and after saccades. It also allowed us to determine the fraction of the SC that was active during the successive phases of saccade generation. 3. During active visual fixation, the fixation cells in the rostral pole of the buildup layer showed an increased discharge rate. From the population reconstruction, we estimate that the zone of active cells spanned the most rostral 0.72 mm in each SC. Assuming the SC is 5 mm in length, approximately 15% of the cells lying along the horizontal meridian in the buildup layer would be active during fixation. 4. At least 100 ms before the initiation of a saccade, long-lead activity began to appear in the buildup layer at the site on the SC motor map related to the next saccade. Fixation activity in the rostral poles simultaneously began to diminish, but the cells in the burst layer remained relatively silent. 5. Approximately 25 ms before saccade onset, the fixation cells ceased firing and both burst and buildup cells began to burst. The active zone in the burst layer was estimated to be approximately 1.4 mm diam, occupying roughly 28% of the SC along a line running from the rostral pole through the center of the initially active zone. The size of this active area among the burst cells was independent of saccade amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Superior colliculus cell types and models of saccade generation

Recent experiments on the cat and monkey have revealed several different cell types within the superior colliculus, including fixation, burst, and build up cells. During primate saccades, activity remains fixed at one location in burst cells, but spreads across the colliculus in build up cells. New models based on the activity of these cell types suggest their functional roles in saccade generation.

Fixation cells in monkey superior colliculus. I. Characteristics of cell discharge

1. We studied the role of the superior colliculus (SC) in the control of visual fixation by recording from cells in the rostral pole of the SC in awake monkeys that were trained to perform fixation and saccade tasks. 2. We identified a subset of neurons in three monkeys that we refer to as fixation cells. These cells increased their tonic discharge rate when the monkey actively fixated a visible target spot to obtain a reward. This sustained activity persisted when the visual stimulation of the target spot was momentarily removed but the monkey was required to continue fixation. 3. The fixation cells were in the rostral pole of the SC. As the electrode descended through the SC, we encountered visual cells with foveal and parafoveal receptive fields most superficially, saccade-related burst cells with parafoveal movement fields below these visual cells, and fixation cells below the burst cells. From this sequence in depth, the fixation cells appeared to be centered in the deeper reaches of the intermediate layers, and this was confirmed by small marking lesions identified histologically. 4. During saccades, the tonically active fixation cells showed a pause in their rate of discharge. The duration of this pause was correlated to the duration of the saccade. Many cells did not decrease their discharge rate for small-amplitude contraversive saccades. 5. The saccade-related pause in fixation cell discharge always began before the onset of the saccade. The mean time from pause onset to saccade onset for contraversive saccades and ipsiversive saccades was 36.2 and 33.0 ms, respectively. Most fixation cells were reactivated before the end of contraversive saccades. The mean time from saccade terminatioN to pause end was -2.6 ms for contraversive saccades and 9.9 ms for ipsiversive saccades. The end of the saccade-related pause in fixation cell discharge was more tightly correlated to saccade termination, than pause onset was to saccade onset. 6. After the saccade-related pause in discharge, many fixation cells showed an increased discharge rate exceeding that before the pause. This increased postsaccadic discharge rate persisted for several hundred milliseconds. 7. The discharge rate of fixation cells was not consistently altered when the monkey actively fixated targets requiring different orbital positions. 8. Fixation cells discharged during smooth pursuit eye movements as they did during fixation. They maintained a steady tonic discharge during pursuit at different speeds and in different directions, provided the monkey looked at the moving target.(ABSTRACT TRUNCATED AT 400 WORDS)

Fixation cells in monkey superior colliculus. II. Reversible activation and deactivation

1. We tested the hypothesis that a subset of neurons, which we have referred to as fixation cells, located within the rostral pole of the monkey superior colliculus (SC) controls the generation of saccadic eye movements. We altered the activity of these neurons with either electrical stimulation or GABAergic drugs. 2. An increase in the activity of fixation cells in the rostral SC, induced by a train of low-frequency electrical stimulation, delayed the initiation of saccades. With bilateral stimulation the monkey was able to make saccades only after stimulation ceased. 3. Pulses of stimulation delivered during the saccade produced an interruption of the saccade in midflight. The latency to the onset of this perturbation was as short as 12 ms. 4. Injection of the gamma-aminobutyric acid (GABA) antagonist bicuculline into the rostral pole of the SC, which decreases normal GABA inhibition and increases cell activity, increased the latency of saccades to both visual and remembered targets. 5. Injection of the GABA agonist muscimol into the rostral SC, which increases normal GABA inhibition and decreases activity, reduced the latency for saccades to visual targets. The monkey also had difficulty maintaining visual fixation and suppressing unwanted saccades. 6. After muscimol injections, monkeys frequently made very short-latency saccades forming a peak in the saccade latency histogram at < 100 ms. These saccades are similar to express saccades made by normal monkeys. This finding suggests that the fixation cells in the rostral SC are critical for controlling the frequency of express saccades. 7. These results support the hypothesis that fixation cells in the rostral SC inhibit the generation of saccadic eye movements and that they form part of a system of oculomotor control, that of visual fixation.

An illusory transformation of optic flow fields

We compared a human observer's ability to locate the focus of expansion (FOE) of a radial optic flow field when this flow field was either combined with, or overlapped by, planar motion. With combined stimuli, in which the FOE was displaced in the direction opposite to the planar motion, subjects accurately located the displaced FOE. With overlapping (transparent) stimuli and the FOE remaining in the center of the display, we found an illusory transformation of the radial pattern: the focus of expansion appeared to be shifted in the direction of the planar motion. The speed of both the planar and radial patterns of motion influenced the illusion. Presence or absence of visual fixation had little effect. We suggest that this illusion might provide a clue as to the way the brain processes planar and radial motion which might in turn be relevant to the interaction of the planar and radial motion components of optic flow fields.

Disparity sensitivity of neurons in monkey extrastriate area MST

We tested the disparity sensitivity of neurons from the medial superior temporal area (MST) in awake behaving monkeys. While the monkey looked at a fixation spot on a screen in front of it, random dot stimuli moved in the preferred direction of the cell under study, and the disparity of the dots made the stimuli appear to move in a frontoparallel plane in front of, on, or behind the screen. Over 90% of the 272 neurons studied were sensitive to the disparity of the visual stimulus. Of those disparity-sensitive cells, 95% were most responsive either to near stimuli (stimuli with crossed disparities appearing to move in front of the screen) or to far stimuli (stimuli with uncrossed disparities appearing to move behind the screen). In a smaller sample of the disparity-sensitive cells, we found cells whose preferred direction of stimulus motion reversed as the disparity of the stimulus reversed. For example, a cell that responded best to rightward motion for near stimuli responded best to leftward motion for far stimuli. We found that 40% of the disparity-sensitive cells had this disparity-dependent direction selectivity. This disparity-dependent direction selectivity was maintained over the entire range of speeds tested (6-56 degrees/sec). We tested whether the disparity sensitivity of the neurons indicated the distance of the stimulus from the screen where the monkey was fixating (relative depth) or the distance of the stimulus from the monkey (absolute depth) by having the monkey fixate at different depths in front of or behind the screen. For most MST neurons, the changes in vergence did not alter the disparity response, indicating that the disparity sensitivity of these neurons conveyed information on depth relative to the plane of fixation. We conclude that the disparity characteristics of cells in the dorsomedial MST are those expected of a system serving primarily coarse rather than fine stereopsis. The correlation between disparity selectivity and direction selectivity in these neurons, as well as their other properties, suggests a role in signaling the direction of self-motion of the observer through the environment.

Neuronal correlates of optic flow stimulation

Neurons in a region of monkey extrastriate cortex, MSTd, respond to the components of optic flow stimulation. Some of these neurons (single-component neurons) are selective for a single type of motion such as inward- or outward-radial motion. Other neurons respond to multiple types of rotation, for example, rightward planar, clockwise circular, and inward radial. Rather than forming discrete groups, we think these neurons represent a continuum covering the range from single-component sensitivity to multiple-component sensitivity. By combining the optic flow stimuli, we have also been able to recognize that such combinations alter the response of cells in the continuum to varying degrees. At this point, while our evidence is consistent with the hypothesis that cells in area MSTd contribute to the processing of optic flow stimuli, we do not know whether these neurons do in fact serve this function. As in all single-cell recording experiments, even those in awake animals performing tasks closer to real-world tasks than we have succeeded in emulating here, the activity of the cell in relationship to the visual stimulation is simply a correlate of the optic flow stimulation and may or may not contribute to the processing of optic flow stimulation upon which behavior depends. Further information on a number of characteristics of these cells might clarify their role. Information on such factors as whether heading in the environment is conveyed by individual neurons, or whether this property is more likely to be conveyed over a population of neurons, and the role of changes in the point of fixation of the eyes are critical points. Generation of behavior on the basis of the optic flow stimulation and determination that this behavior is modified by selective lesion of MSTd would also strengthen the argument that visual motion processing in this area is related to analyzing optic flow information.

Role of the rostral superior colliculus in active visual fixation and execution of express saccades

1. In the rostral pole of the monkey superior colliculus (SC) a subset of neurons (fixation cells) discharge tonically when a monkey actively fixates a target spot and pause during the execution of saccadic eye movements. 2. To test whether these fixation cells are necessary for the control of visual fixation and saccade suppression, we artificially inhibited them with a local injection of muscimol, an agonist of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). After injection of muscimol into the rostral pole of one SC, the monkey was less able to suppress the initiation of saccades. Many unwanted visually guided saccades were initiated less than 100 ms after onset of a peripheral visual stimulus and therefore fell into the range of express saccades. 3. We propose that fixation cells in the rostral SC form part of a fixation system that facilitates active visual fixation and suppresses the initiation of unwanted saccadic eye movements. Express saccades can only occur when activity in this fixation system is reduced.

Superior colliculus neurons mediate the dynamic characteristics of saccades

1. The locus of activity within the superior colliculus (SC) is related to the desired displacement of the eye. Current hypotheses suggest that the location of this locus of activity determines the amplitude of the saccade and that the level of activity at this locus determines eye velocity. We present evidence that suggests that, although the locus determines the amplitude of the saccade, the level of activity in the colliculus encodes dynamic motor error (the difference between desired and current eye displacement). 2. We categorized 86 neurons in the intermediate and deep layers of the superior colliculus of two rhesus monkeys by their activity in relation to the end of saccadic eye movements. In 36% of the cells (n = 31), activity was completely cut off by the end of the saccade (clipped cells). For 53% of cells (n = 46), the major burst of activity ceased by the end of the saccade, but activity continued for 30-100 ms after the end of the movement (partially clipped cells). The remaining 10% of the cells (n = 9) had no clear burst of activity (unclipped cells) but rather had activity that increased gradually before the saccade and then slowly decreased for up to 100 ms after the saccade. These categories were part of a continuum of cell types rather than discrete classes of cells. 3. We first determined whether this new categorization of cells revealed a special relation between the discharge of clipped and partially clipped cells and saccadic amplitude and peak velocity. As expected, we found a steady increase in spike count as saccadic amplitude increased up to the center of the movement field, and an increase in peak spike discharge as peak velocity increased up to a maximum radial eye velocity. Variability in the cell discharge was substantially greater than the variability of saccadic amplitude or peak velocity. We concluded that these single point or averaged measures did not reveal any new functional relationship of these cells. 4. We then examined the relationship of the temporal pattern of discharge of clipped and partially clipped cells to instantaneous changes in radial error and radial velocity. There was a monotonic decay in spike discharge with declining radial error. In contrast, there was a complex, multivalued relationship between spike discharge and radial velocity; collicular cells produced two different values of spike discharge for the same velocity, one during acceleration and the other during deceleration of the eye during a saccade.(ABSTRACT TRUNCATED AT 400 WORDS)

Recovery of function after lesions in the superior temporal sulcus in the monkey

1. Ibotenic acid lesions in the monkey's middle temporal area (MT) and the medial superior temporal area (MST) in the superior temporal sulcus (STS) have previously been shown to produce a deficit in initiation of smooth-pursuit eye movements to moving visual targets. The deficits, however, recovery within a few days. In the present experiments we investigated the factors that influence that recovery. 2. We tested two aspects of the monkey's ability to use motion information to acquire moving targets. We used eye-position error as a measure of the monkey's ability to make accurate initial saccades to the moving target. We measured eye speed within the first 100 ms after the saccade to evaluate the monkey's initial smooth pursuit. 3. We determined that pursuit recovery was not dependent specifically on the use of neurotoxic lesions. Although the rate of recovery was slightly altered by replacing the usual neurotoxin (ibotenic acid) with another neurotoxin [alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)] or with an electrolytic lesion, pursuit recovery still occurred within a period of days to weeks. 4. There was a relationship between the size and location of the lesion and the recovery time. The time to recovery for eye-position error and initial eye speed increased with the fraction of MT removed. Whether the rate of recovery and size of lesions within regions on the anterior bank were related was unresolved. 5. We found that a large AMPA lesion within the STS that removed all of MT and nearly all of MST drastically altered the rate of recovery. Recovery was incomplete more than 7 mo after the lesion. Even with this lesion, however, the monkey's ability to use motion information for pursuit was not completely eliminated. 6. The large lesion also included parts of areas V1, V2, V3, and V4, but analysis of the visual fields associated with this lesion indicated that these areas probably did not have a substantial effect on recovery. 7. We tested whether visual motion experience of the monkey after a lesion was necessary for recovery by limiting the monkey's experience either by using a mask or by using 4-Hz stroboscopic illumination. In one monkey the eye-position error component of pursuit was prolonged to greater than 2 wk, but recovery of eye speed was not. Reduced motion experience had little effect on recovery in the other two monkeys. These results suggest that such visual motion experience is not necessary for the recovery of pursuit.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity of MST neurons to optic flow stimuli. II. Mechanisms of response selectivity revealed by small-field stimuli

1. In these experiments we examined the receptive field mechanisms that support the optic flow field selective responses of neurons in the dorsomedial region of the medial superior temporal area (MSTd). Our experiments tested the predictions of two hypotheses of optic flow field selectivity. The direction mosaic hypothesis states that these receptive fields contain a set of planar direction-selective subfields that match the local directions of motion within optic flow fields. The vector field hypothesis states that these receptive fields are uniquely sensitive to distributed properties of planar, circular, or radial optic flow fields. 2. Experiments using large-field stimuli revealed that some neurons showed changes in optic flow field selectivity depending on the position of the stimulus in the receptive field; these are position-dependent responses. However, other neurons maintained the same optic flow field selectivities in spite of changes in stimulus position; these are position-invariant responses. We have used the position dependence or invariance of optic flow field selectivity as a way of testing the direction mosaic and vector field hypotheses. Position dependence is more consistent with the direction mosaic hypothesis, whereas position invariance is more consistent with the vector field hypothesis. 3. To test for position effects, we examined the optic flow field selectivity of small subfields within the large receptive fields of 160 MSTd neurons. First, we centered small-field optic flow stimuli of various sizes over the same position in the receptive field. Most MSTd neurons showed decreasing response amplitude with decreasing stimulus size but maintained optic flow field selectivity. 4. We then placed small-field stimuli at various positions within the large receptive field of these MSTd neurons. Position-invariant response selectivity was most prominent in single-component neurons, suggesting that they were more consistent with the vector field hypothesis. Position-dependent response selectivity was most prominent in triple-component neurons, suggesting that they were more consistent with the direction mosaic hypothesis. However, the variations in planar direction preference throughout the receptive field of these triple-component neurons were not consistent with a direction mosaic explanation of the large-field circular or radial selectivity observed. 5. Small-field position studies also demonstrated the existence of zones within the receptive field in which either direction-selective inhibitory or direction-selective excitatory responses predominated. The degree of overlap between these zones increased from nonselective to triple- to double- and finally to single-component neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity of MST neurons to optic flow stimuli. I. A continuum of response selectivity to large-field stimuli

1. Neurons in the dorsomedial region of the medial superior temporal area (MSTd) have large receptive fields that include the fovea, are directionally selective for moving visual stimuli, prefer the motion of large fields to small spots, and respond to rotating and expanding patterns of motion as well as frontal parallel planar motion. These characteristics suggested that these neurons might contribute to the analysis of the large-field optic flow stimulation generated as an observer moves through the visual environment. 2. We tested the response of MSTd neurons in two awake monkeys by systematically presenting a set of translational and rotational stimuli to each neuron. These 100 X 100 degrees stimuli were the motion components from which all optic flow fields are derived. 3. In 220 single neurons we found 23% that responded primarily to one component of motion (planar, circular, or radial), 34% that responded to two components (planocircular or planoradial, but never circuloradial), and 29% that responded to all three components. 4. The number of stimulus components to which a neuron responded was unrelated to the size or eccentricity of its receptive field. 5. Triple-, double-, and single-component neurons varied widely in the strength of their responses to the preferred components. Grouping these neurons together revealed that they did not form discrete classes but rather a continuum of response selectivity. 6. This continuum was apparent in other response characteristics. Direction selectivity was weakest in triple-component neurons, strongest in single-component neurons. Significant inhibitory responses were less frequent in triple-component neurons than in single-component neurons. 7. There was some indication that the neurons of similar component classes occupied adjacent regions within MSTd, but all combinations of component and direction selectivity were occasionally found in immediate juxtaposition. 8. Experiments on a subset of neurons showed that the speed of motion, the dot density, and the number of different speed planes in the display had little influence on these responses. 9. We conclude that the selective responses of many MSTd neurons to the rotational and translational components of optic flow make these neurons reasonable candidates for contributing to the analysis of optic flow fields.

Location of saccade-related neurons in the macaque superior colliculus

The locations of saccade-related neurons were studied in the superior colliculi of two adult rhesus monkeys (Macaca mulatta) by placing marking lesions at the sites of physiologically characterized cells and comparing these histologically identified sites with the collicular laminae and acetylcholinesterase (AChE)-rich patches. Three major conclusions were drawn on the basis of 39 histologically identified sites at which saccade-related neurons were recorded. First, saccade-related neurons were distributed from the ventral half of the optic layer through the deep gray layer, and were most concentrated in the intermediate gray and white layers. Second, there was a clear relationship between the discharge characteristics of these saccade-related neurons and the depths at which they were found. Neurons having presaccadic bursts, defined as clipped and partially-clipped, tended to be encountered more dorsally, and neurons that did not have bursts (unclipped) were encountered more ventrally. Although cells having different discharge characteristics seemed to be organized along a dorsoventral axis, there was no compelling evidence that these properties were specified by their laminar locations. Third, there was no clear correlation between the locations of saccade-related neurons and the distribution of individual AChE-rich patches. Saccade-related cells were found both in the caudal superior colliculus where patches were located and in the rostral superior colliculus where patches were not found; both within and between the two tiers of AChE-rich patches in the caudal superior colliculus; and both within and between individual AChE-rich patches. However, the depth-level at which saccade-related neurons occurred generally matched the region bounded by the two tiers of AChE-rich patches in the intermediate and deep layers, and the dorsal and ventral extent of saccade-related neurons was the same as that of the AChE-rich patches.

The role of disparity-sensitive cortical neurons in signalling the direction of self-motion

Movement of an observer through the environment generates motion on the retina. This optic flow provides information about the direction of self-motion, but only if it contains differential motion of elements at different depths. If the observer tracks a stationary object while moving in a direction different from his line of sight, the images of objects in the foreground and in the background move in opposite directions. We have found neurons in the cerebral cortex of monkeys that prefer one direction of motion when the disparity of a stimulus corresponds to foreground motion and prefer the opposite direction when the disparity corresponds to background motion. We propose that these neurons contribute a signal about the direction of self-motion.

Modulation of pursuit eye movements by stimulation of cortical areas MT and MST

1. Many cells in the superior temporal sulcus (STS) of the monkey that represent the foveal region of the visual field discharge during pursuit eye movements. Damage to these areas produces a deficit in the maintenance of pursuit eye movements when the target towards the side of the brain with the lesion. In the present experiments, we electrically stimulated these areas to better localize and understand the mechanisms underlying this directional pursuit deficit. 2. Monkeys were trained to pursue a moving target using a step-ramp task in which the target first stepped to an eccentric position and then moved smoothly across the screen. Trains of stimulation were applied after the monkey had begun to pursue the target to study stimulation effects of maintenance of pursuit. 3. Stimulation during pursuit frequently produced eye acceleration toward the side of the brain stimulated. Eye speed increased during pursuit toward the side stimulated and decreased during pursuit away from the side stimulated. This increase in velocity toward the side of the brain where stimulation presumably activated cells is consistent with the decrease in pursuit velocity toward the side of the brain after cells were removed by chemical lesions. 4. The increase or decrease in pursuit speed following stimulation produced a slip of the target on the retina. The pursuit system seemed to be insensitive to this slip during the period of stimulation, however, since the effect of stimulation during pursuit of a stabilized image (open-loop condition) was similar to that resulting from stimulation under normal pursuit conditions (closed-loop). This insensitivity to visual motion during stimulation suggests that the stimulation substitutes for that visual input. 5. The separation of eye and target position that resulted from stimulation did produce catch-up saccades. This provides added evidence that alteration of middle temporal area (MT) and medial superior temporal area (MST) modifies visual-motion but not visual-position information. 6. Stimulation that produced eye acceleration during pursuit produced only a slight effect during fixation of a stationary target. The effectiveness of the stimulation also increased as the speed of the pursuit increased between 5 and 25 degrees/s. These observations, which show that pursuit velocity altered the effect of stimulation, suggest that the stimulation acted on visual motion processing before information about the pursuit movement itself is incorporated. Since this stimulation produces directional pursuit effects, we hypothesize that the directional bias for pursuit originates in the visual signal conveyed to the pursuit system.(ABSTRACT TRUNCATED AT 400 WORDS)

A grid system and a microsyringe for single cell recording

The designs of two instruments are presented which have proven to be useful in single cell and chemical injection studies performed in awake monkeys. The first is a plastic grid that acts as a guide to produce parallel penetrations with either a microelectrode or microsyringe. The second is a syringe for injecting microliter quantities of a solution that also allows recording of neuronal activity.

Pursuit and optokinetic deficits following chemical lesions of cortical areas MT and MST

1. Previous experiments have shown that punctate chemical lesions within the middle temporal area (MT) of the superior temporal sulcus (STS) produce deficits in the initiation and maintenance of pursuit eye movements (10, 34). The present experiments were designed to test the effect of such chemical lesions in an area within the STS to which MT projects, the medial superior temporal area (MST). 2. We injected ibotenic acid into localized regions of MST, and we observed two deficits in pursuit eye movements, a retinotopic deficit and a directional deficit. 3. The retinotopic deficit in pursuit initiation was characterized by the monkey's inability to match eye speed to target speed or to adjust the amplitude of the saccade made to acquire the target to compensate for target motion. This deficit was related to the initiation of pursuit to targets moving in any direction in the visual field contralateral to the side of the brain with the lesion. This deficit was similar to the deficit we found following damage to extrafoveal MT except that the affected area of the visual field frequently extended throughout the entire contralateral visual field tested. 4. The directional deficit in pursuit maintenance was characterized by a failure to match eye speed to target speed once the fovea had been brought near the moving target. This deficit occurred only when the target was moving toward the side of the lesion, regardless of whether the target began to move in the ipsilateral or contralateral visual field. There was no deficit in the amplitude of saccades made to acquire the target, or in the amplitude of the catch-up saccades made to compensate for the slowed pursuit. The directional deficit is similar to the one we described previously following chemical lesions of the foveal representation in the STS. 5. Retinotopic deficits resulted from any of our injections in MST. Directional deficits resulted from lesions limited to subregions within MST, particularly lesions that invaded the floor of the STS and the posterior bank of the STS just lateral to MT. Extensive damage to the densely myelinated area of the anterior bank or to the posterior parietal area on the dorsal lip of the anterior bank produced minimal directional deficits. 6. We conclude that damage to visual motion processing in MST underlies the retinotopic pursuit deficit just as it does in MT. MST appears to be a sequential step in visual motion processing that occurs before all of the visual motion information is transmitted to the brainstem areas related to pursuit.(ABSTRACT TRUNCATED AT 400 WORDS)