Shared decision signal explains performance and timing of pursuit and saccadic eye movements

The effect of impaired velocity signals on goal-directed eye and hand movements

Information about position and velocity is essential to predict where moving targets will be in the future, and to accurately move towards them. But how are the two signals combined over time to complete goal-directed movements? We show that when velocity information is impaired due to using second-order motion stimuli, saccades directed towards moving targets land at positions where targets were ~ 100 ms before saccade initiation, but hand movements are accurate. Importantly, the longer latencies of hand movements allow for additional time to process the sensory information available. When increasing the period of time one sees the moving target before making the saccade, saccades become accurate. In line with that, hand movements with short latencies show higher curvature, indicating corrections based on an update of incoming sensory information. These results suggest that movements are controlled by an independent and evolving combination of sensory information about the target's position and velocity.

A change in perspective: The interaction of saccadic and pursuit eye movements in oculomotor control and perception

Due to the close relationship between oculomotor behavior and visual processing, eye movements have been studied in many different areas of research over the last few decades. While these studies have brought interesting insights, specialization within each research area comes at the potential cost of a narrow and isolated view of the oculomotor system. In this review, we want to expand this perspective by looking at the interactions between the two most important types of voluntary eye movements: saccades and pursuit. Recent evidence indicates multiple interactions and shared signals at the behavioral and neurophysiological level for oculomotor control and for visual perception during pursuit and saccades. Oculomotor control seems to be based on shared position- and velocity-related information, which leads to multiple behavioral interactions and synergies. The distinction between position- and velocity-related information seems to be also present at the neurophysiological level. In addition, visual perception seems to be based on shared efferent signals about upcoming eye positions and velocities, which are to some degree independent of the actual oculomotor response. This review suggests an interactive perspective on the oculomotor system, based mainly on different types of sensory input, and less so on separate subsystems for saccadic or pursuit eye movements.

Micropursuit and the control of attention and eye movements in dynamic environments

It is more challenging to plan eye movements during perceptual tasks performed in dynamic displays than in static displays. Decisions about the timing of saccades become more critical, and decisions must also involve smooth eye movements, as well as saccades. The present study examined eye movements when judging which of two moving discs would arrive first, or collide, at a common meeting point. Perceptual discrimination after training was precise (Weber fractions < 6%). Strategies reflected a combined contribution of saccades and smooth eye movements. The preferred strategy was to look near the meeting point when strategies were freely chosen. When strategies were assigned, looking near the meeting point produced better performance than switching between the discs. Smooth eye movements were engaged in two ways: (a) low-velocity smooth eye movements correlated with the motion of each disc (micropursuit) were found while the line of sight remained between the discs; and (b) spontaneous smooth pursuit of the pair of discs occurred after the perceptual report, when the discs moved as a pair along a common path. The results show clear preferences and advantages for those eye movement strategies during dynamic perceptual tasks that require minimal management or effort. In addition, smooth eye movements, whose involvement during perceptual tasks within dynamic displays may have previously escaped notice, provide useful indictors of the strategies used to select information and distribute attention during the performance of dynamic perceptual tasks.

Visual motion integration of bidirectional transparent motion in mouse opto-locomotor reflexes

Visual motion perception depends on readout of direction selective sensors. We investigated in mice whether the response to bidirectional transparent motion, activating oppositely tuned sensors, reflects integration (averaging) or winner-take-all (mutual inhibition) mechanisms. We measured whole body opto-locomotor reflexes (OLRs) to bidirectional oppositely moving random dot patterns (leftward and rightward) and compared the response to predictions based on responses to unidirectional motion (leftward or rightward). In addition, responses were compared to stimulation with stationary patterns. When comparing OLRs to bidirectional and unidirectional conditions, we found that the OLR to bidirectional motion best fits an averaging model. These results reflect integration mechanisms in neural responses to contradicting sensory evidence as has been documented for other sensory and motor domains.

Predicted Position Error Triggers Catch-Up Saccades during Sustained Smooth Pursuit

For humans, visual tracking of moving stimuli often triggers catch-up saccades during smooth pursuit. The switch between these continuous and discrete eye movements is a trade-off between tolerating sustained position error (PE) when no saccade is triggered or a transient loss of vision during the saccade due to saccadic suppression. de Brouwer et al. (2002b) demonstrated that catch-up saccades were less likely to occur when the target re-crosses the fovea within 40-180 ms. To date, there is no mechanistic explanation for how the trigger decision is made by the brain. Recently, we proposed a stochastic decision model for saccade triggering during visual tracking (Coutinho et al., 2018) that relies on a probabilistic estimate of predicted PE (PEpred). Informed by model predictions, we hypothesized that saccade trigger time length and variability will increase when pre-saccadic predicted errors are small or visual uncertainty is high (e.g., for blurred targets). Data collected from human participants performing a double step-ramp task showed that large pre-saccadic PEpred (>10°) produced short saccade trigger times regardless of the level of uncertainty while saccade trigger times preceded by small PEpred (<10°) significantly increased in length and variability, and more so for blurred targets. Our model also predicted increased signal-dependent noise (SDN) as retinal slip (RS) increases; in our data, this resulted in longer saccade trigger times and more smooth trials without saccades. In summary, our data supports our hypothesized predicted error-based decision process for coordinating saccades during smooth pursuit.

Dissociation of pursuit target selection from saccade execution

Pursuit and saccades almost always select the same target. Is this the results of a common selection process or does smooth pursuit obligatorily follow the stimulus targeted by saccades? To address this question, we used microstimulation of the primate superior colliculus (SC) to redirect the eyes from a selected pursuit target to a distracter moving in the opposite direction. During each trial, monkeys pursued a horizontally moving array of colored target stimuli. In half of the trials, this target array was accompanied by a distracter array moving horizontally in the opposite direction, offset by the vertical amplitude of the stimulation-evoked saccade. We stimulated the SC during maintained pursuit on half of the trials, and measured pursuit eye velocity during the 50-ms interval immediately following the stimulation-evoked saccade to the distracter array. Saccades evoked by SC stimulation did not alter pursuit target selection. Pursuit velocity on average changed by less than 10% of that expected if the monkey had completely switched targets. Moreover, the same changes in velocity occurred when there was no distracter, indicating that even these small changes in pursuit velocity were a direct effect of the evoked saccade, not partial selection of the distracter. These results show that motor execution of saccades is not sufficient to select a pursuit target, and support the idea that the coordination of pursuit and saccades is accomplished by a shared target selection process.

Superior colliculus inactivation alters the weighted integration of visual stimuli

The primate superior colliculus (SC) is important for the winner-take-all selection of targets for orienting movements. Such selection takes time, however, and the earliest motor responses typically are guided by a weighted vector average of the visual stimuli, before the winner-take-all selection of a single target. We tested whether SC activity plays a role in this initial stage of orienting by inactivating the SC in two macaques (Macaca mulatta) with local muscimol injections. After SC inactivation, initial orienting responses still followed a vector average, but the contribution of the visual stimulus inside the affected field was decreased, and the contribution of the stimulus outside the affected field was increased. These results demonstrate that the SC plays an important role in the weighted integration of visual signals for orienting, in addition to its role in the winner-take-all selection of the target.

Testing a simplified method for measuring velocity integration in saccades using a manipulation of target contrast

A growing number of studies in vision research employ analyses of how perturbations in visual stimuli influence behavior on single trials. Recently, we have developed a method along such lines to assess the time course over which object velocity information is extracted on a trial-by-trial basis in order to produce an accurate intercepting saccade to a moving target. Here, we present a simplified version of this methodology, and use it to investigate how changes in stimulus contrast affect the temporal velocity integration window used when generating saccades to moving targets. Observers generated saccades to one of two moving targets which were presented at high (80%) or low (7.5%) contrast. In 50% of trials, target velocity stepped up or down after a variable interval after the saccadic go signal. The extent to which the saccade endpoint can be accounted for as a weighted combination of the pre- or post-step velocities allows for identification of the temporal velocity integration window. Our results show that the temporal integration window takes longer to peak in the low when compared to high contrast condition. By enabling the assessment of how information such as changes in velocity can be used in the programming of a saccadic eye movement on single trials, this study describes and tests a novel methodology with which to look at the internal processing mechanisms that transform sensory visual inputs into oculomotor outputs.

Inactivation of primate superior colliculus biases target choice for smooth pursuit, saccades, and button press responses

In addition to its well-known role in the control of saccades, the primate superior colliculus (SC) has been implicated in the processes of target choice for overt orienting movements and for covert spatial attention. We focally inactivated the SC, by muscimol injection, while monkeys selected the target of a smooth pursuit, saccade, or button press response from two competing stimuli. The choice stimuli were placed so that one appeared within and the other appeared outside the affected visual field. SC inactivation biased the subject to choose stimuli out of the affected visual field for all three types of responses, although the effects on target choice were significantly smaller for button presses. Inactivation caused no changes in the selection of single stimuli within or out of the affected visual field, indicating the choice bias was not caused by deficits in response execution. The inactivation-induced bias for smooth pursuit and button press responses indicates SC activity is important for selecting the target, independent of any role in saccade preparation.

Coordination of smooth pursuit and saccade target selection in monkeys

The coordination of saccadic and smooth pursuit eye movements in macaque monkeys was investigated using a target selection paradigm with two moving targets crossing at a center fixation point. A task in which monkeys selected a target based on its color was used to test the hypothesis that common neural signals underlie target selection for pursuit and saccades, as well as testing whether target selection signals are available to the saccade and pursuit systems simultaneously or sequentially. Several combinations of target color, speed, and direction were used. In all cases, smooth pursuit was highly selective for the rewarded target before any saccade occurred. On >80% of the trials, the saccade was directed toward the same target as both pre- and postsaccadic pursuit. The results favor a model in which a shared target selection signal is simultaneously available to both the saccade and pursuit systems, rather than a sequential model.

Saccades and pursuit: two outcomes of a single sensorimotor process

Saccades and smooth pursuit eye movements are two different modes of oculomotor control. Saccades are primarily directed toward stationary targets whereas smooth pursuit is elicited to track moving targets. In recent years, behavioural and neurophysiological data demonstrated that both types of eye movements work in synergy for visual tracking. This suggests that saccades and pursuit are two outcomes of a single sensorimotor process that aims at orienting the visual axis.

Behavioral analysis of predictive saccade tracking as studied by countermanding

The ability to make predictive saccadic eye movements is dependent on neural signals that anticipate the onset of a visual target. We used a novel paradigm-based on the saccade-countermanding task-as a tool to investigate rhythm saccade pacing and to provide information on the mechanisms of predictive timing. In particular, we examined the ability of normal subjects to stop a sequence of periodically paced eye movements when cued by a stop signal that was presented at different times with respect to the last target of the sequence (stop signal delay, SSD). The timing of the stop signal affected the ability to stop the saccadic sequence (make a saccade to a central target rather than to the peripheral alternating targets) in different ways, depending on the preceding tracking behavior. For the same SSD, subjects cancelled fewer trials during predictive tracking (promoted by tracking targets alternating at a fast pacing rate, 1.0 Hz) than during reactive tracking (tracking alternating targets at a low pacing rate, 0.2 Hz). In addition, on non-canceled trials, there was an increase in the delay of the corrective saccade to the central target with increasing SSD for pacing at 0.2 Hz, but the timing of the corrective saccade remained near constant for 1.0 Hz pacing. In examining the timing between movements, we estimate that the repetitive GO process that drives the saccades during predictive tracking begins earlier and has a shorter duration than the repetitive GO process during reactive tracking. These behavioral results provide further insight into the initiation process of predictive responses. In particular, the reduced reaction time and the corresponding short duration of the predictive process may result from a faster accumulation of neuronal discharge to a relatively fixed threshold.

Brain and behavior: a task-dependent eye movement study

Recent electrophysiological and behavioral studies have found similarities in the neurology of pursuit and saccadic eye movements. In a previous study on eye movements using closely matched paradigms for pursuit and saccades, we revealed that both exhibit bimodal distributions of latency to predictable (PRD) and randomized (RND) stimuli; however, the latency to each type of stimulus was different, and there was more segregation of latencies in saccades than pursuit (Burke MR, Barnes GR. 2006. Quantitative differences in smooth pursuit and saccadic eye movements in humans. Exp Brain Res. 175(4):596-608). To investigate the brain areas involved in these tasks, and to search for correlates of behavior, we used functional magnetic resonance imaging during equivalent PRD and RND target presentations. In the contrast pursuit > saccades, which reflects velocity-dependent versus position-dependent activities, respectively, we found higher activation in the dorsolateral prefrontal cortex (DLPFC) for pursuit and in the frontopolar region for saccades. In the contrast RND > PRD, which principally reflects activation related to visually driven versus memory-driven responses, respectively, we found a higher sustained level of activation in the frontal eye fields during visually guided eye movements. The reverse contrast revealed higher activity for the memory-guided responses in the supplementary eye fields and the superior parietal lobe. In addition, we found learning-related activation during the PRD condition in visual area V5, the DLPFC, and the cerebellum.

Effects of unilateral ocular motor nerve palsies on smooth pursuit eye movements in adult patients

AIM

The aim of this study was to investigate the influence of single ocular muscle weakness on smooth pursuit eye movements.

METHODS

Infrared video recordings of horizontal and vertical eye movements were obtained from 14 adult patients with either unilateral abducens nerve palsy or trochlear nerve palsy. During the recordings, subsequent series of horizontal, vertical and oblique ramp stimuli of 10 degrees/s constant target velocity and +/-10 degrees amplitude were presented under monocular viewing conditions.

RESULTS

In both forms of ocular nerve palsies, similar changes of pursuit eye movements were observed in the pulling plane of the paretic muscles. The movements of the covered paretic eye showed the lowest amplitude and gain values as well as the lowest numbers of catch-up saccades. The highest amplitude and gain values were calculated from the movements of the covered sound eye. The highest numbers of saccades, however, were produced by the fixating paretic eye.

CONCLUSIONS

We conclude that the fixating paretic eye compensates for the paresis by raising the pursuit gain and the number of catch-up saccades. In the covered paretic eye, however, monocular adaptation is connected with a symmetric low pursuit gain and a reduced number of saccades in the pulling plane of the paretic muscle.

Saccades exert spatial control of motion processing for smooth pursuit eye movements

Saccades modulate the relationship between visual motion and smooth eye movement. Before a saccade, pursuit eye movements reflect a vector average of motion across the visual field. After a saccade, pursuit primarily reflects the motion of the target closest to the endpoint of the saccade. We tested the hypothesis that the saccade produces a spatial weighting of motion around the endpoint of the saccade. Using a moving pursuit stimulus that stepped to a new spatial location just before a targeting saccade, we controlled the distance between the endpoint of the saccade and the position of the moving target. We demonstrate that the smooth eye velocity following the targeting saccade weights the presaccadic visual motion inputs by the distance from their location in space to the endpoint of the saccade, defining the extent of a spatiotemporal filter for driving the eyes. The center of the filter is located at the endpoint of the saccade in space, not at the position of the fovea. The filter is stable in the face of a distracter target, is present for saccades to stationary and moving targets, and affects both the speed and direction of the postsaccadic eye movement. The spatial filter can explain the target-selecting gain change in postsaccadic pursuit, and has intriguing parallels to the process by which perceptual decisions about a restricted region of space are enhanced by attention. The effect of the spatial saccade plan on the pursuit response to a given retinal motion describes the dynamics of a coordinate transformation.

Quantitative differences in smooth pursuit and saccadic eye movements

Recently it has been suggested that smooth pursuit (SP) and saccadic (SAC) eye movements share many common brain substrates in the planning and control of eye movements (Krauzlis in J Neurophysiol 91:591-603, 2004). Evidence is mounting that these two types of eye movements may also share similar mechanisms used to drive both reactive and predictive eye movement responses (Missal and Keller in J Neurophysiol 88:1880-1892, 2002, Keller and Missal in Ann NY Acad Sci 1004:29-39, 2003). The objective of this study was to quantify these similarities by establishing whether the behavioural response properties of human eye movements to predictive (PRD) and randomized (RND) conditions are quantitatively similar for both SP and SAC in directly comparable paradigms. Two previous studies have attempted to evaluate the coordination and motor preparation time of SP and saccadic eye movements (Erkelens in Vis Res 46:163-170, 2006; Joiner and Shelhamer in Exp Brain Res, Epub ahead of print, 2006). However, no previous study has quantitatively evaluated PRD and RND conditions to discretely presented SP and SAC tasks. We used simple SAC and SP paradigms in blocks of PRD and RND presentations, with eye movements monitored throughout using an IR-limbus eye-tracking system (Skalar). Twelve normal subjects (aged between 20 and 39 years) participated in the study which took place over two recording sessions, on two separate days. Data were analysed for two main comparable descriptive statistics: latency and eye velocity/displacement gain. The results presented here support the notion that SP and SAC share common brain substrates/mechanisms in the generation of responses to PRD and RND visual targets but differ in the movement preparation time.

Pursuit and saccadic tracking exhibit a similar dependence on movement preparation time

Data from previous human and primate studies on saccadic and smooth pursuit eye movements suggest that there are shared internal inputs (for example, perception, attention, expectation, and memory) for the initiation of the two types of movements. Additional reports examining the effect of preparation time on movement responses have shown that when ample time is allowed subjects usually generate long-latency "reactive" responses. When the time allowed to prepare a movement is short, however, subjects respond with reduced latency and often anticipate the stimulus ("predictive" response). Based on these findings, we believe that the shared internal inputs at early stages of movement preparation may result in saccade and pursuit eye movements demonstrating the same dependence on preparation time despite acting through different neural pathways further downstream. Previously we demonstrated a behavioral "phase transition" when normal subjects tracked alternating targets with saccades. When preparation time was long (low-frequency pacing) subjects made reactive saccades (latency approximately 180 ms). As preparation time monotonically decreased (pacing frequency increased), there was an abrupt transition to a predictive response (latency <100 ms). In the present study we show that a similar transition exists in smooth pursuit tracking and that the point of transition between the two behaviors is the same for both systems. In other words, the same behavior (reactive versus predictive) is selected when pursuit and saccade tracking are tested under the same time constraints. This provides further evidence that the two types of movements are different motor outcomes of a common decision process.