Visual motion integration for perception and pursuit

Oculometric biomarkers of visuomotor deficits in clinically asymptomatic patients with systemic lupus erythematosus undergoing long-term hydroxychloroquine treatment

Introduction

This study examines a set of oculomotor measurements, or "oculometric" biomarkers, as potential early indicators of visual and visuomotor deficits due to retinal toxicity in asymptomatic Systemic Lupus Erythematosus (SLE) patients on long-term hydroxychloroquine (HCQ) treatment. The aim is to identify subclinical functional impairments that are otherwise undetectable by standard clinical tests and to link them to structural retinal changes.

Methods

We measured oculomotor responses in a cohort of SLE patients on chronic HCQ therapy using a previously established behavioral task and analysis technique. We also examined the relationship between oculometrics, OCT measures of retinal thickness, and standard clinical perimetry measures of visual function in our patient group using Bivariate Pearson Correlation and a Linear Mixed-Effects Model (LMM).

Results

Significant visual and visuomotor deficits were found in 12 asymptomatic SLE patients on long-term HCQ therapy compared to a cohort of 17 age-matched healthy controls. Notably, six oculometrics were significantly different. The median initial pursuit acceleration was 22%, steady-state pursuit gain 16%, proportion smooth 7%, and target speed responsiveness 31% lower, while catch-up saccade amplitude was 46% and fixation error 46% larger. Excluding the two patients with diagnosed mild toxicity, four oculometrics, all but fixation error and proportion smooth, remained significantly impaired compared to controls. Across our population of 12 patients (24 retinae), we found that pursuit latency, initial acceleration, steady-state gain, and fixation error were linearly related to retinal thickness even when age was accounted for, while standard measures of clinical function (Mean Deviation and Pattern Standard Deviation) were not.

Discussion

Our data show that specific oculometrics are sensitive early biomarkers of functional deficits in SLE patients on HCQ that could be harnessed to assist in the early detection of HCQ-induced retinal toxicity and other visual pathologies, potentially providing early diagnostic value beyond standard visual field and OCT evaluations.

Tracking and perceiving diverse motion signals: Directional biases in human smooth pursuit and perception

Human smooth pursuit eye movements and motion perception behave similarly when observers track and judge the motion of simple objects, such as dots. But moving objects in our natural environment are complex and contain internal motion. We ask how pursuit and perception integrate the motion of objects with motion that is internal to the object. Observers (n = 20) tracked a moving random-dot kinematogram with their eyes and reported the object’s perceived direction. Objects moved horizontally with vertical shifts of 0, ±3, ±6, or ±9° and contained internal dots that were static or moved ±90° up/down. Results show that whereas pursuit direction was consistently biased in the direction of the internal dot motion, perceptual biases differed between observers. Interestingly, the perceptual bias was related to the magnitude of the pursuit bias (r = 0.75): perceptual and pursuit biases were directionally aligned in observers that showed a large pursuit bias, but went in opposite directions in observers with a smaller pursuit bias. Dissociations between perception and pursuit might reflect different functional demands of the two systems. Pursuit integrates all available motion signals in order to maximize the ability to monitor and collect information from the whole scene. Perception needs to recognize and classify visual information, thus segregating the target from its context. Ambiguity in whether internal motion is part of the scene or contributes to object motion might have resulted in individual differences in perception. The perception-pursuit correlation suggests shared early-stage motion processing or perception-pursuit interactions.

Changes in visual speed perception induced by anticipatory smooth eye movements

Expectations about forthcoming visual motion shaped by observers' experiences are known to induce anticipatory smooth eye movements (ASEM) and changes in visual perception. Previous studies have demonstrated discrete effects of expectations on the control of ASEM and perception. However, the tasks designed in these studies were not able to segregate the effects of expectations and execution of ASEM itself on perception. In the current study, we attempted to directly examine the effect of ASEM itself on visual speed perception using a two-alternative forced-choice task (2AFC task), in which observers were asked to track a pair of sequentially presented visual motion stimuli with their eyes and to judge whether the second stimulus (test stimulus) was faster or slower than the first (reference stimulus). Our results showed that observers' visual speed perception, quantified by a psychometric function, shifted according to ASEM velocity. This was the case, even though there was no difference in the steady-state eye velocity. Further analyses revealed that the observers' perceptual decisions could be explained by a difference in the magnitude of retinal slip velocity in the initial phase of ocular tracking when the reference and test stimuli were presented, rather than in the steady-state phase. Our results provide psychophysical evidence of the importance of initial ocular tracking in visual speed perception and the strong impact of ASEM.

Common and independent processing of visual motion perception and oculomotor response

Visual motion signals are used not only to drive motion perception but also to elicit oculomotor responses. A fundamental question is whether perceptual and oculomotor processing of motion signals shares a common mechanism. This study aimed to address this question using visual motion priming, in which the perceived direction of a directionally ambiguous stimulus is biased in the same (positive priming) or opposite (negative priming) direction as that of a priming stimulus. The priming effect depends on the duration of the priming stimulus. It is assumed that positive and negative priming are mediated by high- and low-level motion systems, respectively. Participants were asked to judge the perceived direction of a π-phase-shifted test grating after a smoothly drifting priming grating during varied durations. Their eye movements were measured while the test grating was presented. The perception and eye movements were discrepant under positive priming and correlated under negative priming on a trial-by-trial basis when an interstimulus interval was inserted between the priming and test stimuli, indicating that the eye movements were evoked by the test stimulus per se. These findings suggest that perceptual and oculomotor responses are induced by a common mechanism at a low level of motion processing but by independent mechanisms at a high level of motion processing.

Differential saccade-pursuit coordination under sleep loss and low-dose alcohol

Introduction

Ocular tracking of a moving object requires tight coordination between smooth pursuit and saccadic eye movements. Normally, pursuit drives gaze velocity to closely match target velocity, with residual position offsets corrected by catch-up saccades. However, how/if common stressors affect this coordination is largely unknown. This study seeks to elucidate the effects of acute and chronic sleep loss, and low-dose alcohol, on saccade-pursuit coordination, as well as that of caffeine.

Methods

We used an ocular tracking paradigm to assess three metrics of tracking (pursuit gain, saccade rate, saccade amplitude) and to compute "ground lost" (from reductions in steady-state pursuit gain) and "ground recouped" (from increases in steady-state saccade rate and/or amplitude). We emphasize that these are measures of relative changes in positional offsets, and not absolute offset from the fovea.

Results

Under low-dose alcohol and acute sleep loss, ground lost was similarly large. However, under the former, it was nearly completely recouped by saccades, whereas under the latter, compensation was at best partial. Under chronic sleep restriction and acute sleep loss with a caffeine countermeasure, the pursuit deficit was dramatically smaller, yet saccadic behavior remained altered from baseline. In particular, saccadic rate remained significantly elevated, despite the fact that ground lost was minimal.

Discussion

This constellation of findings demonstrates differential impacts on saccade-pursuit coordination with low-dose alcohol impacting only pursuit, likely through extrastriate cortical pathways, while acute sleep loss not only disrupts pursuit but also undermines saccadic compensation, likely through midbrain/brainstem pathways. Furthermore, while chronic sleep loss and caffeine-mitigated acute sleep loss show little residual pursuit deficit, consistent with uncompromised cortical visual processing, they nonetheless show an elevated saccade rate, suggesting residual midbrain and/or brainstem impacts.

Micro-pursuit: A class of fixational eye movements correlating with smooth, predictable, small-scale target trajectories

Humans generate ocular pursuit movements when a moving target is tracked throughout the visual field. In this article, we show that pursuit can be generated and measured at small amplitudes, at the scale of fixational eye movements, and tag these eye movements as micro-pursuits. During micro-pursuits, gaze direction correlates with a target's smooth, predictable target trajectory. We measure similarity between gaze and target trajectories using a so-called maximally projected correlation and provide results in three experimental data sets. A first observation of micro-pursuit is provided in an implicit pursuit task, where observers were tasked to maintain their gaze fixed on a static cross at the center of screen, while reporting changes in perception of an ambiguous, moving (Necker) cube. We then provide two experimental paradigms and their corresponding data sets: a first replicating micro-pursuits in an explicit pursuit task, where observers had to follow a moving fixation cross (Cross), and a second with an unambiguous square (Square). Individual and group analyses provide evidence that micro-pursuits exist in both the Necker and Cross experiments but not in the Square experiment. The interexperiment analysis results suggest that the manipulation of stimulus target motion, task, and/or the nature of the stimulus may play a role in the generation of micro-pursuits.

Saccadic intrusion recognition intelligent algorithms based on different eye movement date classification methods

The saccadic intrusion recognition algorithm proposed by Tokuda doesn’t take into account individual differences and also, since the accuracy is not high enough, three aspects in this regard have been vastly improved. Firstly, the algorithm is tailored to deal with three types of missing data such as low confidence, not on the screen, and time missing, which improves the data fault tolerance of the algorithm. Secondly, the improved algorithm refers to the E&K algorithm, utilizing the ratio of saccade speed to overall speed. Then, the adaptive speed threshold has been determined accurately, which improves the sensitivity of the algorithm. Finally, the algorithm adds the upper limit of the amplitude for identifying regular saccades and filters out rapid retrospectives with large amplitude. A combination of all three minor improvements boosts the overall accuracy of the algorithm. In addition, the N-back task experiment provided data, which has been processed by the improved algorithm, and aided in arriving at a conclusion consistent with the previous ones. The experimental data compares the effect of the improved algorithm and DBSCAN in identifying fixation points. Furthermore, the results demonstrate the improved algorithm being significantly better than the DBSCAN algorithm in regard to sequence and sensitivity of data points.

Dose-dependent sensorimotor impairment in human ocular tracking after acute low-dose alcohol administration

Key points

Oculomotor behaviours are commonly used to evaluate sensorimotor disruption due to ethanol (EtOH).

The current study demonstrates the dose‐dependent impairment in oculomotor and ocular behaviours across a range of ultra‐low BACs (<0.035%).

Processing of target speed and direction, as well as pursuit eye movements, are significantly impaired at 0.015% BAC, suggesting impaired neural activity within brain regions associated with the visual processing of motion.

Catch‐up saccades during steady visual tracking of the moving target compensate for the reduced vigour of smooth eye movements that occurs with the ingestion of low‐dose alcohol.

Saccade dynamics start to become ‘sluggish’ at as low as 0.035% BAC.

Pupillary light responses appear unaffected at BAC levels up to 0.065%.

Abstract

Changes in oculomotor behaviours are often used as metrics of sensorimotor disruption due to ethanol (EtOH); however, previous studies have focused on deficits at blood‐alcohol concentrations (BACs) above about 0.04%. We investigated the dose dependence of the impairment in oculomotor and ocular behaviours caused by EtOH administration across a range of ultra‐low BACs (≤0.035%). We took repeated measures of oculomotor and ocular performance from sixteen participants, both pre‐ and post‐EtOH administration. To assess the neurological impacts across a wide range of brain areas and pathways, our protocol measured 21 largely independent performance metrics extracted from a range of behavioural responses ranging from ocular tracking of radial step‐ramp stimuli, to eccentric gaze holding, to pupillary responses evoked by light flashes. Our results show significant impairment of pursuit and visual motion processing at 0.015% BAC, reflecting degraded neural processing within extrastriate cortical pathways. However, catch‐up saccades largely compensate for the tracking displacement shortfall caused by low pursuit gain, although there still is significant residual retinal slip and thus degraded dynamic acuity. Furthermore, although saccades are more frequent, their dynamics are more sluggish (i.e. show lower peak velocities) starting at BAC levels as low as 0.035%. Small effects in eccentric gaze holding and no effect in pupillary response dynamics were observed at levels below 0.07%, showing the higher sensitivity of the pursuit response to very low levels of blood alcohol, under the conditions of our study.

Oculomotor behaviours are commonly used to evaluate sensorimotor disruption due to ethanol (EtOH).

The current study demonstrates the dose‐dependent impairment in oculomotor and ocular behaviours across a range of ultra‐low BACs (<0.035%).

Processing of target speed and direction, as well as pursuit eye movements, are significantly impaired at 0.015% BAC, suggesting impaired neural activity within brain regions associated with the visual processing of motion.

Catch‐up saccades during steady visual tracking of the moving target compensate for the reduced vigour of smooth eye movements that occurs with the ingestion of low‐dose alcohol.

Saccade dynamics start to become ‘sluggish’ at as low as 0.035% BAC.

Pupillary light responses appear unaffected at BAC levels up to 0.065%.

Distinct pattern of oculomotor impairment associated with acute sleep loss and circadian misalignment

Key points

Inadequate sleep and irregular work schedules have not only adverse consequences for individual health and well‐being, but also enormous economic and safety implications for society as a whole.

This study demonstrates that visual motion processing and coordinated eye movements are significantly impaired when performed after sleep loss and during the biological night, and thus may be contributing to human error and accidents.

Because affected individuals are often unaware of their sensorimotor and cognitive deficits, there is a critical need for non‐invasive, objective indicators of mild, yet potentially unsafe, impairment due to disrupted sleep or biological rhythms.

Our findings show that a set of eye‐movement measures can be used to provide sensitive and reliable indicators of such mild neural impairments.

Abstract

Sleep loss and circadian misalignment have long been known to impair human cognitive and motor performance with significant societal and health consequences. It is well known that human reaction time to a visual cue is impaired following sleep loss and circadian misalignment, but it has remained unclear how more complex visuomotor control behaviour is altered under these conditions. In this study, we measured 14 parameters of the voluntary ocular tracking response of 12 human participants (six females) to systematically examine the effects of sleep loss and circadian misalignment using a constant routine 24‐h acute sleep‐deprivation paradigm. The combination of state‐of‐the‐art oculometric and sleep‐research methodologies allowed us to document, for the first time, large changes in many components of pursuit, saccades and visual motion processing as a function of time awake and circadian phase. Further, we observed a pattern of impairment across our set of oculometric measures that is qualitatively different from that observed previously with other mild neural impairments. We conclude that dynamic vision and visuomotor control exhibit a distinct pattern of impairment linked with time awake and circadian phase. Therefore, a sufficiently broad set of oculometric measures could provide a sensitive and specific behavioural biomarker of acute sleep loss and circadian misalignment. We foresee potential applications of such oculometric biomarkers assisting in the assessment of readiness‐to‐perform higher risk tasks and in the characterization of sub‐clinical neural impairment in the face of a multiplicity of potential risk factors, including disrupted sleep and circadian rhythms.

Inadequate sleep and irregular work schedules have not only adverse consequences for individual health and well‐being, but also enormous economic and safety implications for society as a whole.

This study demonstrates that visual motion processing and coordinated eye movements are significantly impaired when performed after sleep loss and during the biological night, and thus may be contributing to human error and accidents.

Because affected individuals are often unaware of their sensorimotor and cognitive deficits, there is a critical need for non‐invasive, objective indicators of mild, yet potentially unsafe, impairment due to disrupted sleep or biological rhythms.

Our findings show that a set of eye‐movement measures can be used to provide sensitive and reliable indicators of such mild neural impairments.

Reinforcement effects in anticipatory smooth eye movements

When predictive information about target motion is available, anticipatory smooth pursuit eye movements (aSPEM) are consistently generated before target appearance, thereby reducing the typical sensorimotor delay between target motion onset and foveation. By manipulating the probability for target motion direction, we were able to bias the direction and mean velocity of aSPEM. This suggests that motion-direction expectancy has a strong effect on the initiation of anticipatory movements. To further understand the nature of anticipatory smooth eye movements, we investigated different effects of reinforcement on aSPEM. In a first experiment, the reinforcement was contingent to a particular anticipatory behavior. A monetary reward was associated to a criterion-matching anticipatory velocity as estimated online during the gap before target motion onset. Our results showed a small but significant effect of behavior-contingent monetary reward on aSPEM. In a second experiment, the proportion of rewarded trials was manipulated across motion directions (right vs. left) independently from participants' behavior. Our results indicate that a bias in expected reward does not systematically affect anticipatory eye movements. Overall, these findings strengthen the notion that anticipatory eye movements can be considered as an operant behavior (similar to visually guided ones), whereas the expectancy for a noncontingent reward cannot efficiently bias them.

Motion Extrapolation for Eye Movements Predicts Perceived Motion-Induced Position Shifts

Transmission delays in the nervous system pose challenges for the accurate localization of moving objects as the brain must rely on outdated information to determine their position in space. Acting effectively in the present requires that the brain compensates not only for the time lost in the transmission and processing of sensory information, but also for the expected time that will be spent preparing and executing motor programs. Failure to account for these delays will result in the mislocalization and mistargeting of moving objects. In the visuomotor system, where sensory and motor processes are tightly coupled, this predicts that the perceived position of an object should be related to the latency of saccadic eye movements aimed at it. Here we use the flash-grab effect, a mislocalization of briefly flashed stimuli in the direction of a reversing moving background, to induce shifts of perceived visual position in human observers (male and female). We find a linear relationship between saccade latency and perceived position shift, challenging the classic dissociation between "vision for action" and "vision for perception" for tasks of this kind and showing that oculomotor position representations are either shared with or tightly coupled to perceptual position representations. Altogether, we show that the visual system uses both the spatial and temporal characteristics of an upcoming saccade to localize visual objects for both action and perception.SIGNIFICANCE STATEMENT Accurately localizing moving objects is a computational challenge for the brain due to the inevitable delays that result from neural transmission. To solve this, the brain might implement motion extrapolation, predicting where an object ought to be at the present moment. Here, we use the flash-grab effect to induce perceptual position shifts and show that the latency of imminent saccades predicts the perceived position of the objects they target. This counterintuitive finding is important because it not only shows that motion extrapolation mechanisms indeed work to reduce the behavioral impact of neural transmission delays in the human brain, but also that these mechanisms are closely matched in the perceptual and oculomotor systems.

Visual Perception from Object Scanning as Revealed by Electrooculography

We the human beings are blessed by the nature to become well competent for performing highly precise and copious visual processes with how ever a restricted field of view. Howbeit, this process of visual perception is, to a great extent, controlled by the saccades or more commonly the eye movements. The positioning and accommodation of eyes allows an image to be placed (or fixed) in the fovea centralis of the eyes but although we do so to fix our gaze at a particular object, our eyes continuously move. Even though these fixational eye movements includes magnitude that should make them visible to us yet we remain oblivious to them. Microsacades, drifts and tremors that occurs frequently during fixational eye movements, contribute largely to the visual perception. We use saccades several times per second to move the fovea between points of interest and build an understanding of our visual environment.

The Control of Voluntary Eye Movements: New Perspectives

Primates use two types of voluntary eye movements to track objects of interest: pursuit and saccades. Traditionally, these two eye movements have been viewed as distinct systems that are driven automatically by low-level visual inputs. However, two sets of findings argue for a new perspective on the control of voluntary eye movements. First, recent experiments have shown that pursuit and saccades are not controlled by entirely different neural pathways but are controlled by similar networks of cortical and subcortical regions and, in some cases, by the same neurons. Second, pursuit and saccades are not automatic responses to retinal inputs but are regulated by a process of target selection that involves a basic form of decision making. The selection process itself is guided by a variety of complex processes, including attention, perception, memory, and expectation. Together, these findings indicate that pursuit and saccades share a similar functional architecture. These points of similarity may hold the key for understanding how neural circuits negotiate the links between the many higher order functions that can influence behavior and the singular and coordinated motor actions that follow.

Motion dependence of smooth pursuit eye movements in the marmoset

Smooth pursuit eye movements stabilize slow-moving objects on the retina by matching eye velocity with target velocity. Two critical components are required to generate smooth pursuit: first, because it is a voluntary eye movement, the subject must select a target to pursue to engage the tracking system; and second, generating smooth pursuit requires a moving stimulus. We examined whether this behavior also exists in the common marmoset, a New World primate that is increasingly attracting attention as a genetic model for mental disease and systems neuroscience. We measured smooth pursuit in two marmosets, previously trained to perform fixation tasks, using the standard Rashbass step-ramp pursuit paradigm. We first measured the aspects of visual motion that drive pursuit eye movements. Smooth eye movements were in the same direction as target motion, indicating that pursuit was driven by target movement rather than by displacement. Both the open-loop acceleration and closed-loop eye velocity exhibited a linear relationship with target velocity for slow-moving targets, but this relationship declined for higher speeds. We next examined whether marmoset pursuit eye movements depend on an active engagement of the pursuit system by measuring smooth eye movements evoked by small perturbations of motion from fixation or during pursuit. Pursuit eye movements were much larger during pursuit than from fixation, indicating that pursuit is actively gated. Several practical advantages of the marmoset brain, including the accessibility of the middle temporal (MT) area and frontal eye fields at the cortical surface, merit its utilization for studying pursuit movements.

Motion integration for ocular pursuit does not hinder perceptual segregation of moving objects

When confronted with a complex moving stimulus, the brain can integrate local element velocities to obtain a single motion signal, or segregate the elements to maintain awareness of their identities. The integrated motion signal can drive smooth-pursuit eye movements (Heinen and Watamaniuk, 1998), whereas the segregated signal guides attentive tracking of individual elements in multiple-object tracking tasks (MOT; Pylyshyn and Storm, 1988). It is evident that these processes can occur simultaneously, because we can effortlessly pursue ambulating creatures while inspecting disjoint moving features, such as arms and legs, but the underlying mechanism is unknown. Here, we provide evidence that separate neural circuits perform the mathematically opposed operations of integration and segregation, by demonstrating with a dual-task paradigm that the two processes do not share attentional resources. Human observers attentively tracked a subset of target elements composing a small MOT stimulus, while pursuing it ocularly as it translated across a computer display. Integration of the multidot stimulus yielded optimal pursuit. Importantly, performing MOT while pursuing the stimulus did not degrade performance on either task compared with when each was performed alone, indicating that they did not share attention. A control experiment showed that pursuit was not driven by integration of only the nontargets, leaving the MOT targets free for segregation. Nor was a predictive strategy used to pursue the stimulus, because sudden changes in its global velocity were accurately followed. The results suggest that separate neural mechanisms can simultaneously segregate and integrate the same motion signals.

Deficits in predictive smooth pursuit after mild traumatic brain injury

Given that even mild traumatic brain injury (TBI) may produce extensive diffuse axonal injury (DAI), we hypothesized that mild TBI patients would show deficits in predictive smooth pursuit eye movements (SPEM), associated with impaired cognitive functions, as these processes are dependent on common white matter connectivity between multiple cerebral and cerebellar regions. The ability to predict target trajectories during SPEM was investigated in 21 mild TBI patients using a periodic sinusoidal paradigm. Compared to 26 control subjects, TBI patients demonstrated decreased target prediction. TBI patients also showed increased eye position error and variability of eye position, which correlated with decreased target prediction. In all subjects, average target prediction, eye position error and eye position variability correlated with scores related to attention and executive function on the California Verbal Learning Test (CVLT-II). However, there were no differences between TBI and control groups in average eye gain or intra-individual eye gain variability, or in performance on the Wechsler Abbreviated Scale of Intelligence (WASI), suggesting that the observed deficits did not result from general oculomotor impairment or reduced IQ. The correlation between SPEM performance and CVLT-II scores suggests that predictive SPEM may be a sensitive assay of cognitive functioning, including attention and executive function. This is the first report to our knowledge that TBI patients show impaired predictive SPEM and eye position variability, and that these impairments correlate with cognitive deficits.

Object Motion Computation for the Initiation of Smooth Pursuit Eye Movements in Humans

Pursuing an object with smooth eye movements requires an accurate estimate of its two-dimensional (2D) trajectory. This 2D motion computation requires that different local motion measurements are extracted and combined to recover the global object-motion direction and speed. Several combination rules have been proposed such as vector averaging (VA), intersection of constraints (IOC), or 2D feature tracking (2DFT). To examine this computation, we investigated the time course of smooth pursuit eye movements driven by simple objects of different shapes. For type II diamond (where the direction of true object motion is dramatically different from the vector average of the 1-dimensional edge motions, i.e., VA ≠ IOC = 2DFT), the ocular tracking is initiated in the vector average direction. Over a period of less than 300 ms, the eye-tracking direction converges on the true object motion. The reduction of the tracking error starts before the closing of the oculomotor loop. For type I diamonds (where the direction of true object motion is identical to the vector average direction, i.e., VA = IOC = 2DFT), there is no such bias. We quantified this effect by calculating the direction error between responses to types I and II and measuring its maximum value and time constant. At low contrast and high speeds, the initial bias in tracking direction is larger and takes longer to converge onto the actual object-motion direction. This effect is attenuated with the introduction of more 2D information to the extent that it was totally obliterated with a texture-filled type II diamond. These results suggest a flexible 2D computation for motion integration, which combines all available one-dimensional (edge) and 2D (feature) motion information to refine the estimate of object-motion direction over time.

From 1D to 2D via 3D: dynamics of surface motion segmentation for ocular tracking in primates

In primates, tracking eye movements help vision by stabilising onto the retinas the images of a moving object of interest. This sensorimotor transformation involves several stages of motion processing, from the local measurement of one-dimensional luminance changes up to the integration of first and higher-order local motion cues into a global two-dimensional motion immune to antagonistic motions arising from the surrounding. The dynamics of this surface motion segmentation is reflected into the various components of the tracking responses and its underlying neural mechanisms can be correlated with behaviour at both single-cell and population levels. I review a series of behavioural studies which demonstrate that the neural representation driving eye movements evolves over time from a fast vector average of the outputs of linear and non-linear spatio-temporal filtering to a progressive and slower accurate solution for global motion. Because of the sensitivity of earliest ocular following to binocular disparity, antagonistic visual motion from surfaces located at different depths are filtered out. Thus, global motion integration is restricted within the depth plane of the object to be tracked. Similar dynamics were found at the level of monkey extra-striate areas MT and MST and I suggest that several parallel pathways along the motion stream are involved albeit with different latencies to build-up this accurate surface motion representation. After 200-300 ms, most of the computational problems of early motion processing (aperture problem, motion integration, motion segmentation) are solved and the eye velocity matches the global object velocity to maintain a clear and steady retinal image.

Expansion of Direction Space around the Cardinal Axes Revealed by Smooth Pursuit Eye Movements

It is well established that perceptual direction discrimination shows an oblique effect; thresholds are higher for motion along diagonal directions than for motion along cardinal directions. Here, we compare simultaneous direction judgments and pursuit responses for the same motion stimuli and find that both pursuit and perceptual thresholds show similar anisotropies. The pursuit oblique effect is robust under a wide range of experimental manipulations, being largely resistant to changes in trajectory (radial versus tangential motion), speed (10 versus 25 deg/s), directional uncertainty (blocked versus randomly interleaved), and cognitive state (tracking alone versus concurrent tracking and perceptual tasks). Our data show that the pursuit oblique effect is caused by an effective expansion of direction space surrounding the cardinal directions and the requisite compression of space for other directions. This expansion suggests that the directions around the cardinal directions are in some way overrepresented in the visual cortical pathways that drive both smooth pursuit and perception.

Recasting the Smooth Pursuit Eye Movement System

Primates use a combination of smooth pursuit and saccadic eye movements to stabilize the retinal image of selected objects within the high-acuity region near the fovea. Pursuit has traditionally been viewed as a relatively automatic behavior, driven by visual motion signals and mediated by pathways that connect visual areas in the cerebral cortex to motor regions in the cerebellum. However, recent findings indicate that this view needs to be reconsidered. Rather than being controlled primarily by areas in extrastriate cortex specialized for processing visual motion, pursuit involves an extended network of cortical areas, and, of these, the pursuit-related region in the frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation. These recent findings suggest that the pursuit system has a functional architecture very similar to that of the saccadic system. This viewpoint provides a new perspective on the processing steps that occur as descending control signals interact with circuits in the brain stem and cerebellum responsible for gating and executing voluntary eye movements. Although the traditional view describes pursuit and saccades as two distinct neural systems, it may be more accurate to consider the two movements as different outcomes from a shared cascade of sensory–motor functions.

Evidence for object permanence in the smooth-pursuit eye movements of monkeys

We recorded the smooth-pursuit eye movements of monkeys in response to targets that were extinguished (blinked) for 200 ms in mid-trajectory. Eye velocity declined considerably during the target blinks, even when the blinks were completely predictable in time and space. Eye velocity declined whether blinks were presented during steady-state pursuit of a constant-velocity target, during initiation of pursuit before target velocity was reached, or during eye accelerations induced by a change in target velocity. When a physical occluder covered the trajectory of the target during blinks, creating the impression that the target moved behind it, the decline in eye velocity was reduced or abolished. If the target was occluded once the eye had reached target velocity, pursuit was only slightly poorer than normal, uninterrupted pursuit. In contrast, if the target was occluded during the initiation of pursuit, while the eye was accelerating toward target velocity, pursuit during occlusion was very different from normal pursuit. Eye velocity remained relatively stable during target occlusion, showing much less acceleration than normal pursuit and much less of a decline than was produced by a target blink. Anticipatory or predictive eye acceleration was typically observed just prior to the reappearance of the target. Computer simulations show that these results are best understood by assuming that a mechanism of eye-velocity memory remains engaged during target occlusion but is disengaged during target blinks.

Effects of learning on smooth pursuit during transient disappearance of a visual target

Previous research has demonstrated learning in the pursuit system, but it is unclear whether these effects are the result of changes in visual or motor processing. The ability to maintain smooth pursuit during the transient disappearance of a visual target provides a way to assess pursuit properties in the absence of visual inputs. To study the long-term effects of learning on nonvisual signals for pursuit, we used an operant conditioning procedure. By providing a reinforcing auditory stimulus during periods of accurate tracking, we increased the pursuit velocity gain during target blanking from 0.59 in the baseline session to 0.89 after 8 to 10 daily sessions of training. Learning also reduced the occurrence of saccades. The learned effects generalized to untrained target velocities and persisted in the presence of a textured visual background. In a yoked-control group, the reinforcer was independent of the subjects' responses, and the velocity gain remained unchanged (from 0.6 to 0.63, respectively, before and after training). In a control group that received no reinforcer, gain increased slightly after repetition of the task (from 0.63 to 0.71, respectively, before and after training). Using a model of pursuit, we show that these effects of learning can be simulated by modifying the gain of an extra-retinal signal. Our results demonstrate that learned contingencies can increase eye velocity in the absence of visual signals and support the view that pursuit is regulated by extra-retinal signals that can undergo long-term plasticity.

Commentary: smooth pursuit eye movements: from low-level to high-level vision

If an object of great interest moves in our environment, we are able to elicit smooth pursuit eye movements that keep the image of the moving object stationary on our fovea. The processing of visual motion underlying the execution of smooth pursuit eye movements is very similar to the processing underlying the perception of visual motion. During initiation of smooth pursuit, an averaging across all available motion information occurs. Cognitive factors including attention, prediction and learning are able to influence the execution of smooth pursuit. The pursuit target trajectory in space is represented in the discharge rates of neurons in the posterior parietal cortex of rhesus monkeys.

Non-target influences on the initiation of smooth pursuit

Smooth pursuit is usually regarded as a relatively stereotyped oculomotor response in which the early part of the response reflects primarily the properties of the visual stimulus and cortical motion processing. We have investigated pursuit initiation in human subjects using the gap paradigm to alter fixation conditions, and single stationary distractors to alter visual context. The results suggest that a number of processes, distinct from motion processing, are involved in pursuit initiation. The processes which are modified by gaps and distractors are closely related and interact with each other. They may be shared with the saccade system.

Children's pursuit eye movements: a developmental study

We examined the pursuit eye movements of adults and three groups of children 4-6, 8-10, 12-16 years of age. The first experiment compared tracking performance of a partially occluded target with that of a fully visible target. The second experiment examined pursuit abilities of children using a non-cognitive source of information for motion, i.e., proprioception. In this experiment, we compared the ability to track one's own strobe-illuminated finger with the tracking of the experimenter's finger. In the first experiment, only children 4-6 years of age had difficulty inhibiting the tendency to look towards the visible portion of the partially occluded target. They also had significantly fewer epochs of pursuit relative to teenagers and adults. The older children's pursuit eye movements (8-10) were neither significantly different from the youngest nor from the two older groups. In the second experiment, all participants pursued their own finger better than the experimenter's finger, but the youngest children had significantly fewer epochs of pursuit relative to adults. Pursuit of a partially occluded target and incorporation of proprioceptive signals to drive smooth pursuit eye movements are abilities present at four years of age that continue to develop with increasing age.

From following edges to pursuing objects

Primates can generate accurate, smooth eye-movement responses to moving target objects of arbitrary shape and size, even in the presence of complex backgrounds and/or the extraneous motion of non-target objects. Most previous studies of pursuit have simply used a spot moving over a featureless background as the target and have thus neglected critical issues associated with the general problem of recovering object motion. Visual psychophysicists and theoreticians have shown that, for arbitrary objects with multiple features at multiple orientations, object-motion estimation for perception is a complex, multi-staged, time-consuming process. To examine the temporal evolution of the motion signal driving pursuit, we recorded the tracking eye movements of human observers to moving line-figure diamonds. We found that pursuit is initially biased in the direction of the vector average of the motions of the diamond's line segments and gradually converges to the true object-motion direction with a time constant of approximately 90 ms. Furthermore, transient blanking of the target during steady-state pursuit induces a decrease in tracking speed, which, unlike pursuit initiation, is subsequently corrected without an initial direction bias. These results are inconsistent with current models in which pursuit is driven by retinal-slip error correction. They demonstrate that pursuit models must be revised to include a more complete visual afferent pathway, which computes, and to some extent latches on to, an accurate estimate of object direction over the first hundred milliseconds or so of motion.

Time course of amodal completion revealed by a shape discrimination task

We measured the extent of amodal completion as a function of stimulus duration over the range of 15-210 msec, for both moving and stationary stimuli. Completion was assessed using a performance-based measure; a shape discrimination task that is easy if the stimulus is amodally completed and difficult if it is not. Specifically, participants judged whether an upright rectangle was longer horizontally or vertically, when the rectangle was unoccluded, occluded at its corners by four negative-contrast squares, or occluded at its corners by four zero-contrast squares. In the zero-contrast condition, amodal completion did not occur because there were no occlusion cues; in the unoccluded condition, the entire figure was present. Thus, comparing performance in the negative-contrast condition to these two extremes provided a quantitative measure of amodal completion. This measure revealed a rapid but measurable time course for amodal completion. Moving and stationary stimuli took the same amount of time to be completed (approximately 75 msec), but moving stimuli had slightly stronger completion at long durations.

Pursuit eye movements to second-order motion targets

We studied smooth-pursuit eye movements elicited by first- and second-order motion stimuli. Stimuli were random dot fields whose contrast was modulated by a Gaussian window with a space constant of 0.5 degrees. For the first-order stimuli, the random dots simply moved across the screen at the same speed as the window; for the second-order stimuli the window moved across stationary or randomly flickering dots. Additional stimuli which combined first- and second-order motion cues were used to determine the degree and type of interaction found between the two types of motion stimuli. Measurements were made at slow (1 degrees/s) and moderate (6 degrees/s) target speeds. At a velocity of 1 degrees/s the initiation, transition, and steady-state phases of smooth pursuit in response to second-order motion targets are severely affected when compared with the smooth pursuit of first-order motion targets. At a velocity of 6 degrees/s there is a small but significant deficit in steady-state pursuit of second-order motion targets but not much effect on pursuit initiation.