Cortical inhibitory circuits in eye-movement generation

Remotely controlled drug release in deep brain regions of non-human primates

Many areas of science and medicine would benefit from selective release of drugs in specific regions. Nanoparticle drug carriers activated by focused ultrasound-remotely applied, depth-penetrating energy-may provide such selective interventions. Here, we developed stable, ultrasound-responsive nanoparticles that can be used to release drugs effectively and safely in non-human primates. The nanoparticles were used to release propofol in deep brain visual regions. The release reversibly modulated the subjects' visual choice behavior and was specific to the targeted region and to the released drug. Gadolinium-enhanced MR imaging suggested an intact blood-brain barrier. Blood draws showed normal clinical chemistry and hematology. In summary, this study provides a safe and effective approach to release drugs on demand in selected deep brain regions at levels sufficient to modulate behavior.

Remotely controlled drug release in deep brain regions of non-human primates

Many areas of science and medicine would benefit from selective release of drugs in specific regions of interest. Nanoparticle drug carriers activated by focused ultrasound-remotely applied, depth-penetrating energy-may provide such selective interventions. Here, we developed stable, ultrasound-responsive nanoparticles that can be used to release drugs effectively and safely in non-human primates. The nanoparticles were used to release propofol in deep brain visual regions. The release reversibly modulated the subjects' visual choice behavior and was specific to the targeted region and to the released drug. Gadolinium-enhanced MRI imaging suggested an intact blood-brain barrier. Blood draws showed normal clinical chemistry and hematology. In summary, this study provides a safe and effective approach to release drugs on demand in selected deep brain regions at levels sufficient to modulate behavior.

Current State of Potential Mechanisms Supporting Low Intensity Focused Ultrasound for Neuromodulation

Low intensity focused ultrasound (LIFU) has been gaining traction as a non-invasive neuromodulation technology due to its superior spatial specificity relative to transcranial electrical/magnetic stimulation. Despite a growing literature of LIFU-induced behavioral modifications, the mechanisms of action supporting LIFU's parameter-dependent excitatory and suppressive effects are not fully understood. This review provides a comprehensive introduction to the underlying mechanics of both acoustic energy and neuronal membranes, defining the primary variables for a subsequent review of the field's proposed mechanisms supporting LIFU's neuromodulatory effects. An exhaustive review of the empirical literature was also conducted and studies were grouped based on the sonication parameters used and behavioral effects observed, with the goal of linking empirical findings to the proposed theoretical mechanisms and evaluating which model best fits the existing data. A neuronal intramembrane cavitation excitation model, which accounts for differential effects as a function of cell-type, emerged as a possible explanation for the range of excitatory effects found in the literature. The suppressive and other findings need additional theoretical mechanisms and these theoretical mechanisms need to have established relationships to sonication parameters.

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

Premature saccades: A detailed physiological analysis

OBJECTIVE

Premature saccades (PSs) are those made with latencies too short for the direction and amplitude to be specifically programmed. We sought to determine the minimum latency needed to establish accurate direction and amplitude, and observed what occurs when saccades are launched before this minimum latency.

METHODS

In Experiment 1, 249 normal subjects performed the gap saccade task with horizontal targets. In Experiment 2, 28 normal subjects performed the gap saccade task with the targets placed in eight directions. In Experiment 3, 38 normal subjects, 49 patients with Parkinson's disease (PD), and 10 patients with spinocerebellar degeneration (SCD) performed the gap saccade task with horizontal targets.

RESULTS

In Experiment 1, it took 100 ms to accurately establish saccade amplitudes and directions. In Experiment 2, however, the latencies needed for accurate amplitude and direction establishment were both approximately 150 ms. In Experiment 3, the frequencies of PSs in patients with PD and SCD were lower than those of normal subjects.

CONCLUSIONS

The saccade amplitudes and directions are determined simultaneously, 100-150 ms after target presentation. PSs may result from prediction of the oncoming target direction or latent saccade activities in the superior colliculus.

SIGNIFICANCE

Saccade direction and amplitude are determined simultaneously.

Remote, brain region-specific control of choice behavior with ultrasonic waves

The ability to modulate neural activity in specific brain circuits remotely and systematically could revolutionize studies of brain function and treatments of brain disorders. Sound waves of high frequencies (ultrasound) have shown promise in this respect, combining the ability to modulate neuronal activity with sharp spatial focus. Here, we show that the approach can have potent effects on choice behavior. Brief, low-intensity ultrasound pulses delivered noninvasively into specific brain regions of macaque monkeys influenced their decisions regarding which target to choose. The effects were substantial, leading to around a 2:1 bias in choices compared to the default balanced proportion. The effect presence and polarity was controlled by the specific target region. These results represent a critical step towards the ability to influence choice behavior noninvasively, enabling systematic investigations and treatments of brain circuits underlying disorders of choice.

Parietal Cortex Regulates Visual Salience and Salience-Driven Behavior

Unique stimuli stand out. Despite an abundance of competing sensory stimuli, the detection of the most salient ones occurs without effort, and that detection contributes to the guidance of adaptive behavior. Neurons sensitive to the salience of visual stimuli are widespread throughout the primate visual system and are thought to shape the selection of visual targets. However, a neural source of salience remains elusive. In an attempt to identify a source of visual salience, we reversibly inactivated parietal cortex and simultaneously recorded salience signals in prefrontal cortex. Inactivation of parietal cortex not only caused pronounced and selective reductions of salience signals in prefrontal cortex but also diminished the influence of salience on visually guided behavior. These observations demonstrate a causal role of parietal cortex in regulating salience signals within the brain and in controlling salience-driven behavior.

Influence of Expected Reward on Temporal Order Judgment

Perceptual decision-making has been shown to be influenced by reward expected from alternative options or actions, but the underlying neural mechanisms are currently unknown. More specifically, it is debated whether reward effects are mediated through changes in sensory processing, later stages of decision-making, or both. To address this question, we conducted two experiments in which human participants made saccades to what they perceived to be either the first or second of two visually identical but asynchronously presented targets while we manipulated expected reward from correct and incorrect responses on each trial. By comparing reward-induced bias in target selection (i.e., reward bias) during the two experiments, we determined whether reward caused changes in sensory or decision-making processes. We found similar reward biases in the two experiments indicating that reward information mainly influenced later stages of decision-making. Moreover, the observed reward biases were independent of the individual's sensitivity to sensory signals. This suggests that reward effects were determined heuristically via modulation of decision-making processes instead of sensory processing. To further explain our findings and uncover plausible neural mechanisms, we simulated our experiments with a cortical network model and tested alternative mechanisms for how reward could exert its influence. We found that our experimental observations are more compatible with reward-dependent input to the output layer of the decision circuit. Together, our results suggest that, during a temporal judgment task, reward exerts its influence via changing later stages of decision-making (i.e., response bias) rather than early sensory processing (i.e., perceptual bias).

Recovery of contralesional saccade choice and reaction time deficits after a unilateral endothelin-1-induced lesion in the macaque caudal prefrontal cortex

The caudal primate prefrontal cortex (PFC) is involved in target selection and visually guided saccades through both covert attention and overt orienting eye movements. Unilateral damage to the caudal PFC often leads to decreased awareness of a contralesional target alone, referred to as “neglect,” or when it is presented simultaneously with an ipsilesional target, referred to as “extinction.” In the current study, we examined whether deficits in contralesional target selection were due to contralesional oculomotor deficits, such as slower reaction times. We experimentally induced a focal ischemic lesion in the right caudal PFC of 4 male macaque monkeys using the vasoconstrictor endothelin-1 and measured saccade choice and reaction times on double-stimulus free-choice tasks and single-stimulus trials before and after the lesion. We found that 1) endothelin-1-induced lesions in the caudal PFC produced contralesional target selection deficits that varied in severity and duration based on lesion volume and location; 2) contralesional neglect-like deficits were transient and recovered by week 4 postlesion; 3) contralesional extinction-like deficits were longer lasting and recovered by weeks 8–16 postlesion; 4) contralesional reaction time returned to baseline well before the contralesional choice deficit had recovered; and 5) neither the mean reaction times nor the reaction time distributions could account for the degree of contralesional extinction on the free-choice task throughout recovery. These findings demonstrate that the saccade choice bias observed after a right caudal PFC lesion is not exclusively due to contralesional motor deficits, but instead reflects a combination of impaired motor and attentional processing.

NEW & NOTEWORTHY Unilateral damage to the caudal prefrontal cortex in macaque monkeys results in impaired contralesional target selection during the simultaneous presentation of an ipsilesional target. We show that the recovery of contralesional target selection cannot be explained by the recovery of prolonged contralesional saccadic reaction times alone. This indicates that an impairment in contralesional attentional processing contributes to the magnitude of the saccade choice bias in the weeks following a unilateral caudal prefrontal cortex lesion.

Optimal Readout of Correlated Neural Activity in a Decision-Making Circuit

The neural correlates of decision making have been extensively studied with tasks involving a choice between two alternatives that is guided by visual cues. While a large body of work argues for a role of the lateral intraparietal (LIP) region of cortex in these tasks, this role may be confounded by the interaction between LIP and other regions, including medial temporal (MT) cortex. Here, we describe a simplified linear model of decision making that is adapted to two tasks: a motion discrimination and a categorization task. We show that the distinct contribution of MT and LIP may indeed be confounded in these tasks. In particular, we argue that the motion discrimination task relies on a straightforward visuomotor mapping, which leads to redundant information between MT and LIP. The categorization task requires a more complex mapping between visual information and decision behavior, and therefore does not lead to redundancy between MT and LIP. Going further, the model predicts that noise correlations within LIP should be greater in the categorization compared to the motion discrimination task due to the presence of shared inputs from MT. The impact of these correlations on task performance is examined by analytically deriving error estimates of an optimal linear readout for shared and unique inputs. Taken together, results clarify the contribution of MT and LIP to decision making and help characterize the role of noise correlations in these regions.

Dissecting patterns of preparatory activity in the frontal eye fields during pursuit target selection

We investigated the composition of preparatory activity of frontal eye field (FEF) neurons in monkeys performing a pursuit target selection task. In response to the orthogonal motion of a large and a small reward target, monkeys initiated pursuit biased toward the direction of large reward target motion. FEF neurons exhibited robust preparatory activity preceding movement initiation in this task. Preparatory activity consisted of two components, ramping activity that was constant across target selection conditions, and a flat offset in firing rates that signaled the target selection condition. Ramping activity accounted for 50% of the variance in the preparatory activity and was linked most strongly, on a trial-by-trial basis, to pursuit eye movement latency rather than to its direction or gain. The offset in firing rates that discriminated target selection conditions accounted for 25% of the variance in the preparatory activity and was commensurate with a winner-take-all representation, signaling the direction of large reward target motion rather than a representation that matched the parameters of the upcoming movement. These offer new insights into the role that the frontal eye fields play in target selection and pursuit control. They show that preparatory activity in the FEF signals more strongly when to move rather than where or how to move and suggest that structures outside the FEF augment its contributions to the target selection process.NEW & NOTEWORTHY We used the smooth eye movement pursuit system to link between patterns of preparatory activity in the frontal eye fields and movement during a target selection task. The dominant pattern was a ramping signal that did not discriminate between selection conditions and was linked, on trial-by-trial basis, to movement latency. A weaker pattern was composed of a constant signal that discriminated between selection conditions but was only weakly linked to the movement parameters.

Two subdivisions of macaque LIP process visual-oculomotor information differently

Although the cerebral cortex is thought to be composed of functionally distinct areas, the actual parcellation of area and assignment of function are still highly controversial. An example is the much-studied lateral intraparietal cortex (LIP). Despite the general agreement that LIP plays an important role in visual-oculomotor transformation, it remains unclear whether the area is primary sensory- or motor-related (the attention-intention debate). Although LIP has been considered as a functionally unitary area, its dorsal (LIPd) and ventral (LIPv) parts differ in local morphology and long-distance connectivity. In particular, LIPv has much stronger connections with two oculomotor centers, the frontal eye field and the deep layers of the superior colliculus, than does LIPd. Such anatomical distinctions imply that compared with LIPd, LIPv might be more involved in oculomotor processing. We tested this hypothesis physiologically with a memory saccade task and a gap saccade task. We found that LIP neurons with persistent memory activities in memory saccade are primarily provoked either by visual stimulation (vision-related) or by both visual and saccadic events (vision-saccade-related) in gap saccade. The distribution changes from predominantly vision-related to predominantly vision-saccade-related as the recording depth increases along the dorsal-ventral dimension. Consistently, the simultaneously recorded local field potential also changes from visual evoked to saccade evoked. Finally, local injection of muscimol (GABA agonist) in LIPv, but not in LIPd, dramatically decreases the proportion of express saccades. With these results, we conclude that LIPd and LIPv are more involved in visual and visual-saccadic processing, respectively.

Unilateral deactivation of macaque dorsolateral prefrontal cortex induces biases in stimulus selection

Following unilateral brain injury, patients are often unable to detect a stimulus presented in the contralesional field when another is presented simultaneously ipsilesionally. This phenomenon has been referred to as extinction and has been conceptualized as a deficit in selective attention. Although most commonly observed following damage to posterior parietal areas, extinction has been observed following lesions of prefrontal cortex (PFC) in both humans and nonhuman primates. To date, most studies in nonhuman primates have examined lesions of multiple PFC subregions, including the frontal eye fields (FEF). Theoretical accounts of attentional disturbances from human patients, however, also implicate other PFC areas, including the middle frontal gyrus. Here, we investigated the effects of deactivating PFC areas anterior to the FEF on stimulus selection using a free-choice task. Macaque monkeys were presented with two peripheral stimuli appearing either simultaneously, or at varying stimulus onset asynchronies, and their performance was evaluated during unilateral cryogenic deactivation of part of dorsolateral prefrontal cortex or the cortex lining the caudal principal sulcus, the likely homologue of the human middle frontal gyrus. A decreased proportion of saccades was made to stimuli presented in the hemifield contralateral to the deactivated PFC. We also observed increases in reaction times to contralateral stimuli and decreases for stimuli presented in the hemifield ipsilateral to the deactivated hemisphere. In both cases, these results were greatest when both PFC subregions were deactivated. These findings demonstrate that selection biases result from PFC deactivation and support a role of dorsolateral prefrontal subregions anterior to FEF in stimulus selection.

Saccade abnormalities associated with focal cerebral lesions - How cortical and basal ganglia commands shape saccades in humans

OBJECTIVE

To study saccade abnormalities associated with focal cerebral lesions, including the cerebral cortex and basal ganglia (BG).

METHODS

We studied the latency and amplitude of reflexive and voluntary saccades in 37 patients with focal lesions of the frontal and parietal cortices and BG (caudate and putamen), and 51 age-matched controls, along with the ability to inhibit unwanted reflexive saccades.

RESULTS

Latencies of reflexive saccades were prolonged in patients with parietal lesions involving the parietal eye field (PEF), whereas their amplitude was decreased with parietal or putaminal lesions. In contrast, latency of voluntary saccades was prolonged and their success rate reduced with frontal lesions including the frontal eye field (FEF) or its outflow tract as well as the dorsolateral/medial prefrontal cortex, and caudate lesions, whereas their amplitude was decreased with parietal lesions. Inhibitory control of reflexive saccades was impaired with frontal, caudate and, less prominently, parietal lesions.

CONCLUSIONS

PEF is important in triggering reflexive saccades, also determining their amplitude. Whereas FEF and the caudate emit commands for initiating voluntary saccades, their amplitude is mainly determined by PEF. Commands not only from FEF and dorsolateral/medial prefrontal cortex but also from the caudate and PEF serve to inhibit unnecessary reflexive saccades.

SIGNIFICANCE

The findings suggested how cortical and BG commands shape reflexive and voluntary saccades in humans.

Visual attention: Linking prefrontal sources to neuronal and behavioral correlates

Attention is a means of flexibly selecting and enhancing a subset of sensory input based on the current behavioral goals. Numerous signatures of attention have been identified throughout the brain, and now experimenters are seeking to determine which of these signatures are causally related to the behavioral benefits of attention, and the source of these modulations within the brain. Here, we review the neural signatures of attention throughout the brain, their theoretical benefits for visual processing, and their experimental correlations with behavioral performance. We discuss the importance of measuring cue benefits as a way to distinguish between impairments on an attention task, which may instead be visual or motor impairments, and true attentional deficits. We examine evidence for various areas proposed as sources of attentional modulation within the brain, with a focus on the prefrontal cortex. Lastly, we look at studies that aim to link sources of attention to its neuronal signatures elsewhere in the brain.

Anomalous use of context during task preparation in schizophrenia: a magnetoencephalography study

BACKGROUND

Impaired ability to use contextual information to optimally prepare for tasks contributes to performance deficits in schizophrenia. We used magnetoencephalography and an antisaccade task to investigate the neural basis of this deficit.

METHODS

In schizophrenia patients and healthy control participants, we examined the difference in preparatory activation to cues indicating an impending antisaccade or prosaccade. We analyzed activation for correct trials only and focused on the network for volitional ocular motor control-frontal eye field (FEF), dorsal anterior cingulate cortex (dACC), and the ventrolateral and dorsolateral prefrontal cortex (VLPFC, DLPFC).

RESULTS

Compared with control subjects, patients made more antisaccade errors and showed reduced differential preparatory activation in the dACC and increased differential preparatory activation in the VLPFC. In patients only, antisaccade error rates correlated with preparatory activation in the FEF, DLPFC, and VLPFC.

CONCLUSIONS

In schizophrenia, reduced differential preparatory activation of the dACC may reflect reduced signaling of the need for control. Greater preparatory activation in the VLPFC and the correlations of error rate with FEF, DLPFC, and VLPFC activation may reflect that patients who are more error prone require stronger activation in these regions for correct performance. These findings provide the first evidence of abnormal task preparation, distinct from response generation, during volitional saccades in schizophrenia. We conclude that schizophrenia patients are impaired in using task cues to modulate cognitive control and that this contributes to deficits inhibiting prepotent but contextually inappropriate responses and to behavior that is stimulus bound and error prone rather than flexibly guided by context.

Dissociable dopaminergic control of saccadic target selection and its implications for reward modulation

To investigate mechanisms by which reward modulates target selection, we studied the behavioral effects of perturbing dopaminergic activity within the frontal eye field (FEF) of monkeys performing a saccadic choice task and simulated the effects using a plausible cortical network. We found that manipulation of FEF activity either by blocking D1 receptors (D1Rs) or by stimulating D2 receptors (D2Rs) increased the tendency to choose targets in the response field of the affected site. However, the D1R manipulation decreased the tendency to repeat choices on subsequent trials, whereas the D2R manipulation increased that tendency. Moreover, the amount of shift in target selection resulting from the two manipulations correlated in opposite ways with the baseline stochasticity of choice behavior. Our network simulation results suggest that D1Rs influence target selection mainly through their effects on the strength of inputs to the FEF and on recurrent connectivity, whereas D2Rs influence the excitability of FEF output neurons. Altogether, these results reveal dissociable dopaminergic mechanisms influencing target selection and suggest how reward can influence adaptive choice behavior via prefrontal dopamine.

Visual and auditory cue integration for the generation of saccadic eye movements in monkeys and lever pressing in humans

This study examined how effectively visual and auditory cues can be integrated in the brain for the generation of motor responses. The latencies with which saccadic eye movements are produced in humans and monkeys form, under certain conditions, a bimodal distribution, the first mode of which has been termed express saccades. In humans, a much higher percentage of express saccades is generated when both visual and auditory cues are provided compared with the single presentation of these cues [H. C. Hughes et al. (1994) J. Exp. Psychol. Hum. Percept. Perform., 20, 131-153]. In this study, we addressed two questions: first, do monkeys also integrate visual and auditory cues for express saccade generation as do humans and second, does such integration take place in humans when, instead of eye movements, the task is to press levers with fingers? Our results show that (i) in monkeys, as in humans, the combined visual and auditory cues generate a much higher percentage of express saccades than do singly presented cues and (ii) the latencies with which levers are pressed by humans are shorter when both visual and auditory cues are provided compared with the presentation of single cues, but the distribution in all cases is unimodal; response latencies in the express range seen in the execution of saccadic eye movements are not obtained with lever pressing.

Functional imaging reveals rapid reorganization of cortical activity after parietal inactivation in monkeys

Impairments of spatial awareness and decision making occur frequently as a consequence of parietal lesions. Here we used event-related functional MRI (fMRI) in monkeys to investigate rapid reorganization of spatial networks during reversible pharmacological inactivation of the lateral intraparietal area (LIP), which plays a role in the selection of eye movement targets. We measured fMRI activity in control and inactivation sessions while monkeys performed memory saccades to either instructed or autonomously chosen spatial locations. Inactivation caused a reduction of contralesional choices. Inactivation effects on fMRI activity were anatomically and functionally specific and mainly consisted of: (i) activity reduction in the upper bank of the superior temporal sulcus (temporal parietal occipital area) for single contralesional targets, especially in the inactivated hemisphere; and (ii) activity increase accompanying contralesional choices between bilateral targets in several frontal and parieto-temporal areas in both hemispheres. There was no overactivation for ipsilesional targets or choices in the intact hemisphere. Task-specific effects of LIP inactivation on blood oxygen level-dependent activity in the temporal parietal occipital area underline the importance of the superior temporal sulcus for spatial processing. Furthermore, our results agree only partially with the influential interhemispheric competition model of spatial neglect and suggest an additional component of interhemispheric cooperation in the compensation of neglect deficits.

Disconnectivity of the cortical ocular motor control network in autism spectrum disorders

Response inhibition, or the suppression of prepotent but contextually inappropriate behaviors, is essential to adaptive, flexible responding. Individuals with autism spectrum disorders (ASD) consistently show deficient response inhibition during antisaccades. In our prior functional MRI study, impaired antisaccade performance was accompanied by reduced functional connectivity between the frontal eye field (FEF) and dorsal anterior cingulate cortex (dACC), regions critical to volitional ocular motor control. Here we employed magnetoencephalography (MEG) to examine the spectral characteristics of this reduced connectivity. We focused on coherence between FEF and dACC during the preparatory period of antisaccade and prosaccade trials, which occurs after the presentation of the task cue and before the imperative stimulus. We found significant group differences in alpha band mediated coherence. Specifically, neurotypical participants showed significant alpha band coherence between the right inferior FEF and right dACC and between the left superior FEF and bilateral dACC across antisaccade, prosaccade, and fixation conditions. Relative to the neurotypical group, ASD participants showed reduced coherence between these regions in all three conditions. Moreover, while neurotypical participants showed increased coherence between the right inferior FEF and the right dACC in preparation for an antisaccade compared to a prosaccade or fixation, ASD participants failed to show a similar increase in preparation for the more demanding antisaccade. These findings demonstrate reduced long-range functional connectivity in ASD, specifically in the alpha band. The failure in the ASD group to increase alpha band coherence with increasing task demand may reflect deficient top-down recruitment of additional neural resources in preparation to perform a difficult task.

New methods devised specify the size and color of the spots monkeys see when striate cortex (area V1) is electrically stimulated

Creating a prosthetic device for the blind is a central future task. Our research examines the feasibility of producing a prosthetic device based on electrical stimulation of primary visual cortex (area V1), an area that remains intact for many years after loss of vision attributable to damage to the eyes. As an initial step in this effort, we believe that the research should be carried out in animals, as it has been in the creation of the highly successful cochlear implant. We chose the rhesus monkey, whose visual system is similar to that of man. We trained monkeys on two tasks to assess the size, contrast, and color of the percepts created when single sites in area V1 are stimulated through microelectrodes. Here, we report that electrical stimulation within the central 5° of the visual field representation creates a small spot that is between 9 and 26 min of arc in diameter and has a contrast ranging between 2.6% and 10%. The dot generated by the stimulation in the majority of cases was darker than the background viewed by the animal and was composed of a variety of low-contrast colors. These findings can be used as inputs to models of electrical stimulation in area V1. On the basis of these findings, we derive what kinds of images would be expected when implanted arrays of electrodes are stimulated through a camera attached to the head whose images are converted into electrical stimulation using appropriate algorithms.

Control of visual cortical signals by prefrontal dopamine

The prefrontal cortex (PFC) is thought to modulate sensory signals in posterior cortices during top-down attention^1,2, yet little is known about the underlying neural circuitry. Experimental and clinical evidence suggest that prefrontal dopamine plays an important role in cognitive functions³, acting predominantly through D1 receptors (D1Rs). Here we show that dopamine D1Rs mediate prefrontal control of signals within visual cortex. We pharmacologically altered D1R-mediated activity within the frontal eye field (FEF) of the PFC and measured its effects on the responses of neurons within visual cortex. This manipulation was sufficient to enhance the response magnitude, orientation selectivity and response reliability of neurons in area V4 to an extent comparable with the known effects of top-down attention. The observed enhancement in V4 signals was restricted to neurons with response fields (RFs) overlapping the part of visual space affected by the D1R manipulation. Altering D1R or D2R-mediated FEF activity increased saccadic target selection, but the D2R manipulation did not enhance V4 signals. Our results identify a role of D1Rs in mediating the control of visual cortical signals by the PFC and demonstrate how processing within sensory areas can be altered in mental disorders involving prefrontal dopamine.

Pulvinar inactivation disrupts selection of movement plans

The coordinated movement of the eyes and hands under visual guidance is an essential part of goal-directed behavior. Several cortical areas known to be involved in this process exchange projections with the dorsal aspect of the thalamic pulvinar nucleus, suggesting that this structure may play a central role in visuomotor behavior. Here, we used reversible inactivation to investigate the role of the dorsal pulvinar in the selection and execution of visually guided manual and saccadic eye movements in macaque monkeys. We found that unilateral pulvinar inactivation resulted in a spatial neglect syndrome accompanied by visuomotor deficits including optic ataxia during visually guided limb movements. Monkeys were severely disrupted in their visually guided behavior regarding space contralateral to the side of the injection in several domains, including the following: (1) target selection in both manual and oculomotor tasks, (2) limb usage in a manual retrieval task, and (3) spontaneous visual exploration. In addition, saccades into the ipsilesional field had abnormally short latencies and tended to overshoot their mark. None of the deficits could be explained by a visual field defect or primary motor deficit. These findings highlight the importance of the dorsal aspect of the pulvinar nucleus as a critical hub for spatial attention and selection of visually guided actions.

More GABA, less distraction: a neurochemical predictor of motor decision speed

People vary markedly in the efficiency with which they can resolve competitive action decisions, even simple ones such as shifting gaze to one stimulus rather than another. We found that an individual's ability to rapidly resolve such competition is predicted by the concentration of GABA, the main inhibitory neurotransmitter, in a region of frontal cortex that is relevant for eye movements, but not in a control region (occipital cortex).

Target Selection by the Frontal Cortex during Coordinated Saccadic and Smooth Pursuit Eye Movements

Oculomotor tracking of moving objects is an important component of visually based cognition and planning. Such tracking is achieved by a combination of saccades and smooth-pursuit eye movements. In particular, the saccadic and smooth-pursuit systems interact to often choose the same target, and to maximize its visibility through time. How do multiple brain regions interact, including frontal cortical areas, to decide the choice of a target among several competing moving stimuli? How is target selection information that is created by a bias (e.g., electrical stimulation) transferred from one movement system to another? These saccade–pursuit interactions are clarified by a new computational neural model, which describes interactions between motion processing areas: the middle temporal area, the middle superior temporal area, the frontal pursuit area, and the dorsal lateral pontine nucleus; saccade specification, selection, and planning areas: the lateral intraparietal area, the frontal eye fields, the substantia nigra pars reticulata, and the superior colliculus; the saccadic generator in the brain stem; and the cerebellum. Model simulations explain a broad range of neuroanatomical and neurophysiological data. These results are in contrast with the simplest parallel model with no interactions between saccades and pursuit other than common-target selection and recruitment of shared motoneurons. Actual tracking episodes in primates reveal multiple systematic deviations from predictions of the simplest parallel model, which are explained by the current model.

Modulation of presaccadic activity in the frontal eye field by the superior colliculus

A cascade of neuronal signals precedes each saccadic eye movement to targets in the visual scene. In the cerebral cortex, this neuronal processing culminates in the frontal eye field (FEF), where neurons have bursts of activity before the saccade. This presaccadic activity is typically considered to drive downstream activity in the intermediate layers of the superior colliculus (SC), which receives direct projections from FEF. Consequently, the FEF activity is thought to be determined solely by earlier cortical processing and unaffected by activity in the SC. Recent evidence of an ascending path from the SC to FEF raises the possibility, however, that presaccadic activity in the FEF may also depend on input from the SC. Here we tested this possibility by recording from single FEF neurons during the reversible inactivation of SC. Our results indicate that presaccadic activity in the FEF does not require SC input: we never observed a significant reduction in FEF presaccadic activity when the SC was inactivated. Unexpectedly, in a third of experiments, SC inactivation elicited a significant increase in FEF presaccadic activity. The passive visual response of FEF neurons, in contrast, was virtually unaffected by inactivation of the SC. These findings show that presaccadic activity in the FEF does not originate in the SC but nevertheless may be influenced by modulatory signals ascending from the SC.

Conditions that alter saccadic eye movement latencies and affect target choice to visual stimuli and to electrical stimulation of area V1 in the monkey

In this study, we examined procedures that alter saccadic latencies and target selection to visual stimuli and electrical stimulation of area V1 in the monkey. It has been shown that saccadic eye movement latencies to singly presented visual targets form a bimodal distribution when the fixation spot is turned off a number of milliseconds prior to the appearance of the target (the gap period); the first mode has been termed express saccades and the second regular saccades. When the termination of the fixation spot is coincident with the appearance of the target (0 ms gap), express saccades are rarely generated. We show here that a bimodal distribution of saccadic latencies can also be obtained when an array of visual stimuli is presented prior to the appearance of the visual target, provided the elements of the array overlap spatially with the visual target. The overall latency of the saccadic eye movements elicited by electrical stimulation of area V1 is significantly shortened both when a gap is introduced between the termination of the fixation spot and the stimulation and when an array is presented. However, under these conditions, the distribution of saccadic latencies is unimodal. When two visual targets are presented after the fixation spot, introducing a gap has no effect on which target is chosen. By contrast, when electrical stimulation is paired with a visual target, introducing a gap greatly increases the frequency with which the electrical stimulation site is chosen.

Effect of inactivation of the cortical frontal eye field on saccades generated in a choice response paradigm

Previous studies using muscimol inactivations in the frontal eye fields (FEFs) have shown that saccades generated by recall from working memory are eliminated by these lesions, whereas visually guided saccades are relatively spared. In these experiments, we made reversible inactivations in FEFs in alert macaque monkeys and examined the effect on saccades in a choice response task. Our task required monkeys to learn arbitrary pairings between colored stimuli and saccade direction. Following inactivations, the percentage of choice errors increased as a function of the number of alternative (NA) pairings. In contrast, the percentage of dysmetric saccades (saccades that landed in the correct quadrant but were inaccurate) did not vary with NA. Saccade latency increased postlesion but did not increase with NA. We also made simultaneous inactivations in both FEFs. The results following bilateral lesions showed approximately twice as many choice errors. We conclude that the FEFs are involved in the generation of saccades in choice response tasks. The dramatic effect of NA on choice errors, but the lack of an effect of NA on motor errors or response latency, suggests that two types of processing are interrupted by FEF lesions. The first involves the formation of a saccadic intention vector from associate memory inputs, and the second, the execution of the saccade from the intention vector. An alternative interpretation of the first result is that a role of the FEFs may be to suppress incorrect responses. The doubling of choice errors following bilateral FEF lesions suggests that the effect of unilateral lesions is not caused by a general inhibition of the lesioned side by the intact side.

Top down attentional deficits in macaques with lesions of lateral prefrontal cortex

Brain imaging, electrical stimulation, and neurophysiological studies have all implicated the prefrontal cortex (PFC) in the top-down control of attention. Specifically, feedback from PFC has been proposed to bias activity in visual cortex in favor of attended stimuli over irrelevant distracters. To identify which attentional functions are critically dependent on PFC, we removed PFC unilaterally in combination with transection of the corpus callosum and anterior commissure in two macaques. In such a preparation, the ipsilesional hemisphere is deprived of top-down feedback from PFC to visual cortex, and the contralesional hemisphere can serve as an intact normal control. Monkeys were trained to fixate a central cue and discriminate the orientation of a colored target grating presented among colored distracter gratings in either the hemifield affected by the PFC lesion or the normal control hemifield. Locations of the targets and distracters were varied, and the color of the central cue specified the color of the target on each trial. The behavioral response was a bar release, and thus attentional impairments could be distinguished from impaired oculomotor control. When the cue was held constant for many trials, task performance in the affected hemifield was nearly normal. However, the monkeys were severely impaired when the cue was switched frequently across trials. The monkeys were unimpaired in a pop-out task with changing targets that did not require top-down attentional control. The PFC thus appears to play a critical role in the ability to flexibly reallocate attention on the basis of changing task demands.

What Delay Fields Tell Us About Striate Cortex

It is well known that electrical activation of striate cortex (area V1) can disrupt visual behavior. Based on this knowledge, we discovered that electrical microstimulation of V1 in macaque monkeys delays saccadic eye movements when made to visual targets located in the receptive field of the stimulated neurons. This review discusses the following issues. First, the parameters that affect the delay of saccades by microstimulation of V1 are reviewed. Second, the excitability properties of the V1 elements mediating the delay are discussed. Third, the properties that determine the size and shape of the region of visual space affected by stimulation of V1 are described. This region is called a delay field. Fourth, whether the delay effect is mainly due to a disruption of the visual signal transmitted through V1 or whether it is a disturbance of the motor signal transmitted between V1 and the brain stem saccade generator is investigated. Fifth, the properties of delay fields are used to estimate the number of elements activated directly by electrical microstimulation of macaque V1. Sixth, these properties are used to make inferences about the characteristics of visual percepts induced by such stimulation. Seventh, the disruptive effects of V1 stimulation in monkeys and humans are compared. Eighth, a cortical mechanism to account for the disruptive effects of V1 stimulation is proposed. Finally, these effects are related to normal vision.

Effects of allopregnanolone on sedation in men, and in women on oral contraceptives

Allopregnanolone is a known GABA(A) receptor agonist not previously given to men, or to women using oral contraceptives (OC). The effects of metabolites of sex hormones on the GABA(A) receptor are different between men and women. OC are known to change GABA(A) receptor subunit composition. These factors might play a role in the differential effect of allopregnanolone in men and women, and in women with or without OC. To study the sedative effect of and sensitivity to allopregnanolone in men and in women with OC, nine healthy men (mean age 24.6 years) and nine healthy women on OC (mean age 21.8 years) were given three, increasing, intravenous dosages (0.015, 0.03, and 0.045 mg/kg) of allopregnanolone. Saccadic eye velocity (SEV), subjective ratings, and electroencephalography (EEG) were used to evaluate the response to allopregnanolone. Repeated blood samples for analyses of serum allopregnanolone levels were drawn throughout the study day. Allopregnanolone decreased SEV more in women than in men, and increased subjective ratings of 'sedation'. The results in women on OC are similar to earlier results in women without OC. Subjective ratings of 'contentedness' decreased in men but increased in women. Serum levels of allopregnanolone were more highly increased in men compared to women. Other pharmacokinetic parameters were not different between sexes. On the EEG, beta power increased in men. In conclusion, men and women on OC reacted differently to allopregnanolone.

Role of GABA A inhibition in modulation of pyramidal tract neuron activity during postural corrections

In a previous study we demonstrated that the activity of pyramidal tract neurons (PTNs) of the motor cortex is modulated in relation to postural corrections evoked by periodical tilts of the animal. The modulation included an increase in activity in one phase of the tilt cycle and a decrease in the other phase. It is known that the motor cortex contains a large population of inhibitory GABAergic neurons. How do these neurons participate in periodic modulation of PTNs? The goal of this study was to investigate the role of GABA(A) inhibitory neurons of the motor cortex in the modulation of postural-related PTN activity. Using extracellular electrodes with attached micropipettes, we recorded the activity of PTNs in cats maintaining balance on a tilting platform both before and after iontophoretic application of the GABA(A) receptor antagonists gabazine or bicuculline. The tilt-related activity of 93% of PTNs was affected by GABA(A) receptor antagonists. In 88% of cells, peak activity increased by 75 +/- 50% (mean +/- SD). In contrast, the trough activity changed by a much smaller value and almost as many neurons showed a decrease as showed an increase. In 73% of the neurons, the phase position of the peak activity did not change or changed by no more than 0.1 of a cycle. We conclude that the GABAergic system of the motor cortex reduces the posture-related responses of PTNs but has little role in determining their response timing.

Oral progesterone decreases saccadic eye velocity and increases sedation in women

The aim of this study was to investigate the neurophysiological and behavioural effects of a single dose of progesterone in women. Allopregnanolone is a metabolite of progesterone and a potent positive modulator of the GABA(A) receptor and produces sedative and anxiolytic effects. This study was designed to examine the effect of oral progesterone and the metabolite allopregnanolone in women. Women (n=15) in their follicular phase received oral progesterone (400mg) or placebo. Dependent measures included plasma levels of progesterone and allopregnanolone, saccadic eye velocity (SEV), subjective ratings (visual analogue scales), and reaction time. Administration of progesterone decreased SEV and increased sedation. This effect is probably due to enhanced GABA activity.

Extending lifetime of plastic changes in the human brain

The ability of the brain to adjust to changing environments and to recover from damage rests on its remarkable capacity to adapt through plastic changes of underlying neural networks. We show here with an eye movement paradigm that a lifetime of plastic changes can be extended to several hours by repeated applications of theta burst transcranial magnetic stimulation to the frontal eye field of the human cortex. The results suggest that repeated application of the same stimulation protocol consolidates short-lived plasticity into long-lasting changes.

Microstimulation of V1 delays visually guided saccades: a parametric evaluation of delay fields

Electrical microstimulation of macaque striate cortex (area V1) delays the execution of saccadic eye movements made to a visual target placed in the receptive field of the stimulated neurons. The region of visual space within which saccades are delayed is called a delay field. We examined the effects of changing the parameters of stimulation and target size on the size of a delay field. Rhesus monkeys were required to generate a saccadic eye movement to a punctate and white visual target presented within or outside the receptive field of the neurons under study. On 50% of trials, a train of stimulation consisting of 0.2-ms anode-first pulses was delivered to the neurons before the onset of the visual target. Stimulations were performed in the operculum at 0.9-2.0 mm below the cortical surface. It was found that increases in current (50-100 microA), pulse frequency (100-300 Hz), or train duration (75-300 ms) increased the size of a delay field and increases in target size (0.1 degrees -0.2 degrees of visual angle) decreased the size of a delay field. Delay fields varied in size between 0.1 and 0.6 degrees of visual angle. These results are related to the properties of phosphenes induced by electrical stimulation of V1 in humans and compared to the interference effects observed following transcranial magnetic stimulation of human V1.

Which visual pathways cause fixation-related inhibition?

Visual stimuli can both inhibit and activate motor mechanisms. In one well-known example, the latency of saccadic eye movements is prolonged in the presence of a fixation stimulus, relative to the case in which the fixation stimulus disappears before the target appears. This automatic sensory-motor effect, known as the gap effect or fixation-offset effect, has been associated with inhibitory connections within the superior colliculus (SC). Visual information is provided to the SC and other oculomotor areas, such as the frontal eye fields (FEF), mainly by the magnocellular geniculostriate pathway, and also by the retinotectal pathway. We tested whether signals in these pathways are necessary to create fixation-related inhibition, by using stimuli invisible to them. We found that such stimuli, visible only to short-wave-sensitive cones (S cones), do produce fixation-related inhibition (including when warning effects were equated). We also demonstrate that this fixation-related inhibition cannot be explained by residual activation of luminance pathways and must be caused by a route separate from that of luminance fixation signals. Thus there are at least two routes that cause fixation-related inhibition, and direct sensory input to the SC or FEF by the magnocellular or retinotectal pathways is not required. We discuss the implications that there may be both cortical and collicular mechanisms.

Microstimulation of V1 affects the detection of visual targets: manipulation of target contrast

Electrical microstimulation of the striate cortex (area V1) in monkeys delays the execution of saccadic eye movements generated to a visual target located in the receptive field of the stimulated neurons. We have argued that this effect is because of disruption of the visual signal transmitted along the geniculostriate pathway. The delivery of electrical stimulation to V1 evokes a punctate light or dark phosphene in human subjects. If electrical stimulation of V1 in monkeys evokes a light or dark phosphene, then one might expect that the delay effect might vary according to whether monkeys are required to detect a light or a dark visual target. For instance, if the stimulation is activating V1 elements coding for a light visual stimulus but not a dark visual stimulus then stimulation may delay saccades generated to a light target but not to a dark target. We tested this idea by having monkeys generate saccadic eye movements to light or dark visual targets immediately after the stimulation was delivered to V1. We found that the delay effect induced by stimulation varied with target contrast, but remained invariant to whether a bright or dark visual target was presented in the receptive field of the stimulated neurons. The significance of these results is discussed with regard to using monkeys to develop a visual prosthesis for the blind.

Functional architecture of spatial attention in the parietal cortex of the behaving monkey

Functional architectures facilitate orderly transmittal of representations between cortices, allow for local interactions between neurons, and ensure a uniform distribution of feature representations with respect to larger-scale topographies. We sought to correlate such topographies with internal cognitive states. A psychophysical task for which the monkey was required to detect a change in one of two identical peripheral expanding flow fields tested for spatial shifts of attention. The monkey was cued as to which flow would change with a small cue near the fixation points. Reaction time data indicate that the monkey's performance in the optic flow detection task depended on the location of the cue. Using optical imaging of intrinsic signals, we show that a monkey's internally generated locus of attention is correlated with an 800-860 microm patchy topological architecture across the cortical surface of the inferior parietal lobule. The attentional patches vary in location but are stable in spatial frequency. The patches are embedded in a larger-scale and stable representation of eye position. Trial-by-trial analysis of the images indicates that the organizational scheme with simultaneous stable and variable subcomponents occurs within the experiment of 1 d, as well as across days. This novel functional architecture is the first to be correlated with attentional mechanisms and could support a fine-scale functional architecture underlying hemispatial neglect, an attentional deficit caused by parietal lesions.

Region- and age-specific deficits in γ-aminobutyric acidergic neuron development in the telencephalon of theuPAR-/- mouse

We have previously shown that in adult mice with a null mutation in the urokinase-type plasminogen activator receptor (uPAR) gene, maintained on a C57BL/6J/129Sv background, there is a selective loss of GABAergic interneurons in anterior cingulate and parietal cortex, with the parvalbumin-expressing subpopulation preferentially affected. Here, we performed a more detailed anatomical analysis of uPAR(-/-) mutation on the congenic C57BL/6J background. With glutamic acid decarboxylase-67 and gamma-aminobutyric acid (GABA) immunostaining, there is a similar region-selective loss of cortical interneurons in the congenic uPAR(-/-) mice from the earliest age examined (P21). In contrast, the loss of parvalbumin-immunoreactive cells is observed only in adult cortex, and the extent of this loss is less than in the mixed background. Moreover, earlier in development, although there are normal numbers of parvalbumin cells in the uPAR(-/-) cortex, fewer cells coexpress GABA, suggesting that the parvalbumin subpopulation migrates appropriately to the cortex, but does not differentiate normally. Among the other forebrain regions examined, only the adult hippocampus shows a loss of GABAergic interneurons, although the somatostatin, rather than the parvalbumin, subpopulation contributes to this loss. The data suggest that uPAR function is necessary for the normal development of a subpopulation of GABAergic neurons in the telencephalon. It is likely that the late-onset parvalbumin phenotype is due to the effects of an altered local environment on selectively vulnerable neurons and that the extent of this loss is strain dependent. Thus, an interplay between complex genetic factors and the environment may influence the phenotypic impact of the uPAR mutation both pre- and postnatally.

Phosphene Induction and the Generation of Saccadic Eye Movements by Striate Cortex

The purpose of this review is to critically examine phosphene induction and saccadic eye movement generation by electrical microstimulation of striate cortex (area V1) in humans and monkeys. The following issues are addressed: 1) Properties of electrical stimulation as they pertain to the activation of V1 elements; 2) the induction of phosphenes in sighted and blind human subjects elicited by electrical stimulation using various stimulation parameters and electrode types; 3) the induction of phosphenes with electrical microstimulation of V1 in monkeys; 4) the generation of saccadic eye movements with electrical microstimulation of V1 in monkeys; and 5) the tasks involved for the development of a cortical visual prosthesis for the blind. In this review it is concluded that electrical microstimulation of area V1 in trained monkeys can be used to accelerate the development of an effective prosthetic device for the blind.

Neural mechanisms underlying target selection with saccadic eye movements

In exploring the visual scene we make about three saccadic eye movements per second. During each fixation, in addition to analyzing the object at which we are looking, a decision has to be made as to where to look next. Although we perform this task with the greatest of ease, the computations to perform the task are complex and involve numerous brain structures. We have applied several investigative tools that include single-cell recordings, microstimulation, pharmacological manipulations and lesions to learn more about the neural control of visually guided eye saccadic movements. Electrical stimulation of the superior colliculus (SC), areas V1 and V2, the lateral intraparietal sulcus (LIP), the frontal eye fields (FEF) and the medial eye fields (MEF) produces saccadic eye movements at low current levels. After ablation of the SC, electrical microstimulation of V1, V2, and LIP no longer elicits saccadic eye movements whereas stimulation of the FEF and MEF continues to be effective. Ablation of the SC but not of the FEF eliminates short-latency saccadic eye movements to visual targets called "express saccades," whereas lesions of the FEF selectively interfere with target selection. Bilateral removal of both the SC and the FEF causes major, long lasting deficits: all visually elicited saccadic eye movements are eliminated. In intact monkeys, subthreshold electrical microstimulation of the FEF and MEF as well as the lower layers of V1 and V2 and of some subregions of LIP greatly facilitates the choice of targets presented in the receptive fields of the stimulated neurons. By contrast, stimulation of the upper layers of V1 and V2 and other sub-regions of LIP produces a dramatic interference in target selection. Examination of the role of inhibitory circuits in eye-movement generation reveals that local infusion of muscimol, a GABA (gamma-aminobutyric acid) agonist, or bicuculline, a GABA antagonist, interferes with target selection in V1. On the other hand, infusion of bicuculline into the FEF produces facilitation in target choice and irrepressible saccades. It appears therefore that inhibitory circuits play a central role in visual analysis in V1 and in the generation of saccadic eye movements in the FEF. It is proposed that two major streams can be discerned in visually guided eye-movement control, the posterior from occipital and parietal cortex that reaches the brainstem via the SC and the anterior from the FEF and MEF that has direct access to the brainstem oculomotor centers.

Microstimulation of V1 input layers disrupts the selection and detection of visual targets by monkeys

Electrical microstimulation delivered to primary visual cortex (V1) concurrently with the presentation of visual targets interferes with the selection of these targets. To determine the source of this interference, we stimulated the visual input layers of V1 as rhesus monkeys generated saccadic eye movements to visual targets presented at and outside the receptive field of the stimulated neurons. Columns of cells in V1 innervated by the left and right eye are segregated according to eye dominance, such that cells within a column respond best to visual stimuli presented to the ocular dominant eye. Interference was maximal when targets were presented to the ocular dominant eye, moderate when presented to the ocular inferior eye, and negligible when presented to both eyes. Thus, electrical microstimulation of the visual input layers of V1 disrupts the flow of visual information along the geniculostriate pathway. Knowing how electrical stimulation of V1 affects visual behaviour is necessary when using monkeys to develop a visual prosthesis for the blind.

Deficits in saccade target selection after inactivation of superior colliculus

Saccades are rapid eye movements that orient gaze toward areas of interest in the visual scene. Neural activity correlated with saccade target selection has been identified in several brain regions, including the superior colliculus (SC), but it is not known whether the SC is directly involved in target selection, or whether the SC merely receives selection-related signals from cortex in preparation for the execution of eye movements. In monkeys, we used focal reversible inactivation to test the functional contributions of the SC to target selection during visual search, and found that inactivation resulted in clear deficits. When a target appeared in the inactivated field, saccades were often misdirected to distractor stimuli. Control tasks showed that this deficit was not caused by low-level visual or motor impairments. Our results indicate that, in addition to its well-established involvement in movement execution, the SC has an important functional role in target selection.