Stimulus conditions that enhance anticipatory slow eye movements

Interactions between circuit architecture and plasticity in a closed-loop cerebellar system

Determining the sites and directions of plasticity underlying changes in neural activity and behavior is critical for understanding mechanisms of learning. Identifying such plasticity from neural recording data can be challenging due to feedback pathways that impede reasoning about cause and effect. We studied interactions between feedback, neural activity, and plasticity in the context of a closed-loop motor learning task for which there is disagreement about the loci and directions of plasticity: vestibulo-ocular reflex learning. We constructed a set of circuit models that differed in the strength of their recurrent feedback, from no feedback to very strong feedback. Despite these differences, each model successfully fit a large set of neural and behavioral data. However, the patterns of plasticity predicted by the models fundamentally differed, with the direction of plasticity at a key site changing from depression to potentiation as feedback strength increased. Guided by our analysis, we suggest how such models can be experimentally disambiguated. Our results address a long-standing debate regarding cerebellum-dependent motor learning, suggesting a reconciliation in which learning-related changes in the strength of synaptic inputs to Purkinje cells are compatible with seemingly oppositely directed changes in Purkinje cell spiking activity. More broadly, these results demonstrate how changes in neural activity over learning can appear to contradict the sign of the underlying plasticity when either internal feedback or feedback through the environment is present.

Bayesian approaches to smooth pursuit of random dot kinematograms: effects of varying RDK noise and the predictability of RDK direction

Smooth pursuit eye movements respond on the basis of both immediate and anticipated target motion, where anticipations may be derived from either memory or perceptual cues. To study the combined influence of both immediate sensory motion and anticipation, subjects pursued clear or noisy random dot kinematograms (RDKs) whose mean directions were chosen from Gaussian distributions with SDs = 10° (narrow prior) or 45° (wide prior). Pursuit directions were consistent with Bayesian theory in that transitions over time from dependence on the prior to near total dependence on immediate sensory motion (likelihood) took longer with the noisier RDKs and with the narrower, more reliable, prior. Results were fit to Bayesian models in which parameters representing the variability of the likelihood either were or were not constrained to be the same for both priors. The unconstrained model provided a statistically better fit, with the influence of the prior in the constrained model smaller than predicted from strict reliability-based weighting of prior and likelihood. Factors that may have contributed to this outcome include prior variability different from nominal values, low-level sensorimotor learning with the narrow prior, or departures of pursuit from strict adherence to reliability-based weighting. Although modifications of, or alternatives to, the normative Bayesian model will be required, these results, along with previous studies, suggest that Bayesian approaches are a promising framework to understand how pursuit combines immediate sensory motion, past history, and informative perceptual cues to accurately track the target motion that is most likely to occur in the immediate future.NEW & NOTEWORTHY Smooth pursuit eye movements respond on the basis of anticipated, as well as immediate, target motions. Bayesian models using reliability-based weighting of previous (prior) and immediate target motions (likelihood) accounted for many, but not all, aspects of pursuit of clear and noisy random dot kinematograms with different levels of predictability. Bayesian approaches may solve the long-standing problem of how pursuit combines immediate sensory motion and anticipation of future motion to configure an effective response.

On the relation between anticipatory ocular torsion and anticipatory smooth pursuit

Humans and other animals move their eyes in anticipation to compensate for sensorimotor delays. Such anticipatory eye movements can be driven by the expectation of a future visual object or event. Here we investigate whether such anticipatory responses extend to ocular torsion, the eyes’ rotation about the line of sight. We recorded three-dimensional eye position in head-fixed healthy human adults who tracked a rotating dot pattern moving horizontally across a computer screen. This kind of stimulus triggers smooth pursuit with a horizontal and torsional component. In three experiments, we elicited expectation of stimulus rotation by repeatedly showing the same rotation (Experiment 1), or by using different types of higher-level symbolic cues indicating the rotation of the upcoming target (Experiments 2 and 3). Across all experiments, results reveal reliable anticipatory horizontal smooth pursuit. However, anticipatory torsion was only elicited by stimulus repetition, but not by symbolic cues. In summary, torsion can be made in anticipation of an upcoming visual event only when low-level motion signals are accumulated by repetition. Higher-level cognitive mechanisms related to a symbolic cue reliably evoke anticipatory pursuit but did not modulate torsion. These findings indicate that anticipatory torsion and anticipatory pursuit are at least partly decoupled and might be controlled separately.

Predictive Smooth Pursuit Eye Movements

Smooth pursuit eye movements maintain the line of sight on smoothly moving targets. Although often studied as a response to sensory motion, pursuit anticipates changes in motion trajectories, thus reducing harmful consequences due to sensorimotor processing delays. Evidence for predictive pursuit includes (a) anticipatory smooth eye movements (ASEM) in the direction of expected future target motion that can be evoked by perceptual cues or by memory for recent motion, (b) pursuit during periods of target occlusion, and (c) improved accuracy of pursuit with self-generated or biologically realistic target motions. Predictive pursuit has been linked to neural activity in the frontal cortex and in sensory motion areas. As behavioral and neural evidence for predictive pursuit grows and statistically based models augment or replace linear systems approaches, pursuit is being regarded less as a reaction to immediate sensory motion and more as a predictive response, with retinal motion serving as one of a number of contributing cues.

A Subconscious Interaction between Fixation and Anticipatory Pursuit

Ocular smooth pursuit and fixation are typically viewed as separate systems, yet there is evidence that the brainstem fixation system inhibits pursuit. Here we present behavioral evidence that the fixation system modulates pursuit behavior outside of conscious awareness. Human observers (male and female) either pursued a small spot that translated across a screen, or fixated it as it remained stationary. As shown previously, pursuit trials potentiated the oculomotor system, producing anticipatory eye velocity on the next trial before the target moved that mimicked the stimulus-driven velocity. Randomly interleaving fixation trials reduced anticipatory pursuit, suggesting that a potentiated fixation system interacted with pursuit to suppress eye velocity in upcoming pursuit trials. The reduction was not due to passive decay of the potentiated pursuit signal because interleaving "blank" trials in which no target appeared did not reduce anticipatory pursuit. Interspersed short fixation trials reduced anticipation on long pursuit trials, suggesting that fixation potentiation was stronger than pursuit potentiation. Furthermore, adding more pursuit trials to a block did not restore anticipatory pursuit, suggesting that fixation potentiation was not overridden by certainty of an imminent pursuit trial but rather was immune to conscious intervention. To directly test whether cognition can override fixation suppression, we alternated pursuit and fixation trials to perfectly specify trial identity. Still, anticipatory pursuit did not rise above that observed with an equal number of random fixation trials. The results suggest that potentiated fixation circuitry interacts with pursuit circuitry at a subconscious level to inhibit pursuit.SIGNIFICANCE STATEMENT When an object moves, we view it with smooth pursuit eye movements. When an object is stationary, we view it with fixational eye movements. Pursuit and fixation are historically regarded as controlled by different neural circuitry, and alternating between invoking them is thought to be guided by a conscious decision. However, our results show that pursuit is actively suppressed by prior fixation of a stationary object. This suppression is involuntary, and cannot be avoided even if observers are certain that the object will move. The results suggest that the neural fixation circuitry is potentiated by engaging stationary objects, and interacts with pursuit outside of conscious awareness.

Stopping smooth pursuit

If a visual object of interest suddenly starts to move, we will try to follow it with a smooth movement of the eyes. This smooth pursuit response aims to reduce image motion on the retina that could blur visual perception. In recent years, our knowledge of the neural control of smooth pursuit initiation has sharply increased. However, stopping smooth pursuit eye movements is less well understood and will be discussed in this paper. The most straightforward way to study smooth pursuit stopping is by interrupting image motion on the retina. This causes eye velocity to decay exponentially towards zero. However, smooth pursuit stopping is not a passive response, as shown by behavioural and electrophysiological evidence. Moreover, smooth pursuit stopping is particularly influenced by active prediction of the upcoming end of the target. Here, we suggest that a particular class of inhibitory neurons of the brainstem, the omnipause neurons, could play a central role in pursuit stopping. Furthermore, the role of supplementary eye fields of the frontal cortex in smooth pursuit stopping is also discussed.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Influence of predictability on control of extra-retinal components of smooth pursuit during prolonged 2D tracking

We compared pursuit responses to 2D target motion in three separate conditions: predictable, randomised and randomised with timing cues. The target moved on a continuous quadrilateral path in which right-angle direction changes allowed anticipatory eye acceleration and deceleration in orthogonal axes to be assessed. Results indicated that whether the timing of direction changes was random or predictable, anticipatory acceleration, initiated by extra-retinal mechanisms, occurred in the new direction at approximately the same time as anticipatory deceleration in the terminating direction, but deceleration was of greater magnitude than acceleration. When path duration was randomised within a range of durations, the timing of acceleration and deceleration was almost constant irrespective of actual ramp duration but was dependent on the mean duration of the range. When ramp duration was predictable both deceleration and acceleration increased, the latter allowing peak velocity to be attained earlier than when randomised. When timing cues were given at a fixed time prior to direction change in randomised stimuli, this also resulted in higher anticipatory acceleration/deceleration. When both duration and velocity of sequential ramps were randomised, deceleration was dependent on target velocity, but acceleration remained constant. Altogether these findings show that although acceleration and deceleration in orthogonal axes occur almost simultaneously and are similarly affected by predictability, control of their magnitude is relatively independent. We suggest that deceleration and acceleration result from the switching off and on, respectively, of retinal and extra-retinal oculomotor components prior to direction change, with dynamics dependent on predictability of stimulus magnitude and timing.

Anticipatory smooth eye movements in autism spectrum disorder

Smooth pursuit eye movements are important for vision because they maintain the line of sight on targets that move smoothly within the visual field. Smooth pursuit is driven by neural representations of motion, including a surprisingly strong influence of high-level signals representing expected motion. We studied anticipatory smooth eye movements (defined as smooth eye movements in the direction of expected future motion) produced by salient visual cues in a group of high-functioning observers with Autism Spectrum Disorder (ASD), a condition that has been associated with difficulties in either generating predictions, or translating predictions into effective motor commands. Eye movements were recorded while participants pursued the motion of a disc that moved within an outline drawing of an inverted Y-shaped tube. The cue to the motion path was a visual barrier that blocked the untraveled branch (right or left) of the tube. ASD participants showed strong anticipatory smooth eye movements whose velocity was the same as that of a group of neurotypical participants. Anticipatory smooth eye movements appeared on the very first cued trial, indicating that trial-by-trial learning was not responsible for the responses. These results are significant because they show that anticipatory capacities are intact in high-functioning ASD in cases where the cue to the motion path is highly salient and unambiguous. Once the ability to generate anticipatory pursuit is demonstrated, the study of the anticipatory responses with a variety of types of cues provides a window into the perceptual or cognitive processes that underlie the interpretation of events in natural environments or social situations.

Cognitive processes involved in smooth pursuit eye movements: behavioral evidence, neural substrate and clinical correlation

Smooth-pursuit eye movements allow primates to track moving objects. Efficient pursuit requires appropriate target selection and predictive compensation for inherent processing delays. Prediction depends on expectation of future object motion, storage of motion information and use of extra-retinal mechanisms in addition to visual feedback. We present behavioral evidence of how cognitive processes are involved in predictive pursuit in normal humans and then describe neuronal responses in monkeys and behavioral responses in patients using a new technique to test these cognitive controls. The new technique examines the neural substrate of working memory and movement preparation for predictive pursuit by using a memory-based task in macaque monkeys trained to pursue (go) or not pursue (no-go) according to a go/no-go cue, in a direction based on memory of a previously presented visual motion display. Single-unit task-related neuronal activity was examined in medial superior temporal cortex (MST), supplementary eye fields (SEF), caudal frontal eye fields (FEF), cerebellar dorsal vermis lobules VI–VII, caudal fastigial nuclei (cFN), and floccular region. Neuronal activity reflecting working memory of visual motion direction and go/no-go selection was found predominantly in SEF, cerebellar dorsal vermis and cFN, whereas movement preparation related signals were found predominantly in caudal FEF and the same cerebellar areas. Chemical inactivation produced effects consistent with differences in signals represented in each area. When applied to patients with Parkinson's disease (PD), the task revealed deficits in movement preparation but not working memory. In contrast, patients with frontal cortical or cerebellar dysfunction had high error rates, suggesting impaired working memory. We show how neuronal activity may be explained by models of retinal and extra-retinal interaction in target selection and predictive control and thus aid understanding of underlying pathophysiology.

Dramatic impairment of prediction due to frontal lobe degeneration

Prediction is essential for motor function in everyday life. For instance, predictive mechanisms improve the perception of a moving target by increasing eye speed anticipatively, thus reducing motion blur on the retina. Subregions of the frontal lobes play a key role in eye movements in general and in smooth pursuit in particular, but their precise function is not firmly established. Here, the role of frontal lobes in the timing of predictive action is demonstrated by studying predictive smooth pursuit during transient blanking of a moving target in mild frontotemporal lobar degeneration (FTLD) and Alzheimer's disease (AD) patients. While control subjects and AD patients predictively reaccelerated their eyes before the predicted time of target reappearance, FTLD patients did not. The difference was so dramatic (classification accuracy >90%) that it could even lead to the definition of a new biomarker. In contrast, anticipatory eye movements triggered by the disappearance of the fixation point were still present before target motion onset in FTLD patients and visually guided pursuit was normal in both patient groups compared with controls. Therefore, FTLD patients were only impaired when the predicted timing of an external event was required to elicit an action. These results argue in favor of a role of the frontal lobes in predictive movement timing.

The role of prediction and anticipation on age-related effects on smooth pursuit eye movements

Externally guided sensory-motor processes deteriorate with increasing age. Internally guided, for example, predictive, behavior usually helps to overcome sensory-motor delays. We studied whether predictive components of visuomotor transformation decline with age. We investigated smooth pursuit eye movements (SPEM) of 45 healthy subjects with paradigms of different degrees of predictability with respect to target motion onset, type (smoothed triangular, ramp stimulation), and direction by blanking the target at various intervals of the ramp stimulation. Using repetitive trials of SPEM stimulation, we could dissociate anticipatory and predictive components of extraretinal smooth pursuit behavior. The main results suggest that basic motor parameters decline with increasing age, whereas both anticipation and prediction of target motion did not change with age. We suggest that the elderly maintain their capability of using prediction in the immediate control of motor behavior, which might be a way to compensate for age-related delays in sensory-motor transformation, even in the absence of sensory signals.

Individual differences in impulsivity predict anticipatory eye movements

Impulsivity is the tendency to act without forethought. It is a personality trait commonly used in the diagnosis of many psychiatric diseases. In clinical practice, impulsivity is estimated using written questionnaires. However, answers to questions might be subject to personal biases and misinterpretations. In order to alleviate this problem, eye movements could be used to study differences in decision processes related to impulsivity. Therefore, we investigated correlations between impulsivity scores obtained with a questionnaire in healthy subjects and characteristics of their anticipatory eye movements in a simple smooth pursuit task. Healthy subjects were asked to answer the UPPS questionnaire (Urgency Premeditation Perseverance and Sensation seeking Impulsive Behavior scale), which distinguishes four independent dimensions of impulsivity: Urgency, lack of Premeditation, lack of Perseverance, and Sensation seeking. The same subjects took part in an oculomotor task that consisted of pursuing a target that moved in a predictable direction. This task reliably evoked anticipatory saccades and smooth eye movements. We found that eye movement characteristics such as latency and velocity were significantly correlated with UPPS scores. The specific correlations between distinct UPPS factors and oculomotor anticipation parameters support the validity of the UPPS construct and corroborate neurobiological explanations for impulsivity. We suggest that the oculomotor approach of impulsivity put forth in the present study could help bridge the gap between psychiatry and physiology.

Eye movements: the past 25 years

This article reviews the past 25 years of research on eye movements (1986-2011). Emphasis is on three oculomotor behaviors: gaze control, smooth pursuit and saccades, and on their interactions with vision. Focus over the past 25 years has remained on the fundamental and classical questions: What are the mechanisms that keep gaze stable with either stationary or moving targets? How does the motion of the image on the retina affect vision? Where do we look - and why - when performing a complex task? How can the world appear clear and stable despite continual movements of the eyes? The past 25 years of investigation of these questions has seen progress and transformations at all levels due to new approaches (behavioral, neural and theoretical) aimed at studying how eye movements cope with real-world visual and cognitive demands. The work has led to a better understanding of how prediction, learning and attention work with sensory signals to contribute to the effective operation of eye movements in visually rich environments.

Influence of previous target motion on anticipatory pursuit deceleration

During visual pursuit of a moving target, expected changes in its trajectory often evoke anticipatory smooth pursuit responses. In the present study, we investigated characteristics of anticipatory smooth pursuit decelerations before a change or the end of a target trajectory. Healthy humans had to pursue with the eyes a target moving along a circular path that predictably or unpredictably reversed direction and then retraced its movement back to the starting position. We found that anticipatory eye decelerations were often evoked in temporal expectation of target reversal and of the end of the trajectory. The latency of anticipatory decelerations initiated before target reversal was variable, had poor temporal accuracy and depended on the history of previous trials. Anticipations of the end of the trajectory were more accurate, more precise and were not influenced by previous trials. In this case, subjects probably based their estimate of the end of the trajectory on the duration just experienced before target motion reversal. These results suggest that anticipatory eye decelerations are based on the characteristics of the current or preceding trials depending on the most reliable information available.

Causality attribution biases oculomotor responses

When viewing one object move after being struck by another, humans perceive that the action of the first object "caused" the motion of the second, not that the two events occurred independently. Although established as a perceptual and linguistic concept, it is not yet known whether the notion of causality exists as a fundamental, preattentional "Gestalt" that can influence predictive motor processes. Therefore, eye movements of human observers were measured while viewing a display in which a launcher impacted a tool to trigger the motion of a second "reaction" target. The reaction target could move either in the direction predicted by transfer of momentum after the collision ("causal") or in a different direction ("noncausal"), with equal probability. Control trials were also performed with identical target motion, either with a 100 ms time delay between the collision and reactive motion, or without the interposed tool. Subjects made significantly more predictive movements (smooth pursuit and saccades) in the causal direction during standard trials, and smooth pursuit latencies were also shorter overall. These trends were reduced or absent in control trials. In addition, pursuit latencies in the noncausal direction were longer during standard trials than during control trials. The results show that causal context has a strong influence on predictive movements.

Predictive disjunctive pursuit of virtual images perceived to move in depth

Human and nonhuman primates predictively use smooth pursuit and saccades to track visual targets that move in a fronto-parallel plane. This behaviour is believed to be facilitated by short-term memory of the target motion and/or an efference copy of the subject's motor effort. Subjects in our experiments tracked dichoptically viewed targets that appeared to move vertically, right or left and towards the subject. The virtual image of the target was tracked using disjunctive smooth pursuit and saccades. To reveal predictive tracking, targets were blanked 100 ms after the onset of motion for intervals of 800 ms. During the blanked interval, subjects initiated pursuit and predictively tracked the unseen virtual image using memory guided eye movements. Our data are consistent with recent electrophysiological studies that describe cells that encode target or eye movements in depth when a target is briefly blanked but pursuit is maintained. However, predictive pursuit of a virtual target with disjunctive eye movements poses a challenge for understanding how a short-term memory store might encode the desired eye movement, its coordinate frame, and how it is transformed into motor commands.

Evidence for a link between the extra-retinal component of random-onset pursuit and the anticipatory pursuit of predictable object motion

During pursuit of moving targets that temporarily disappear, residual smooth eye movements represent the internal (extra-retinal) component of pursuit. However, this response is dependent on expectation of target reappearance. By comparing responses with and without such expectation during early random-onset pursuit, we examined the temporal development of the extra-retinal component and compared it with anticipatory pursuit, another form of internally driven response. In an initial task (mid-ramp extinction), a moving, random-velocity target was initially visible for 100 or 150 ms but then extinguished for 600 ms before reappearing and continuing to move. Responses comprised an initial visually driven rapid rise in eye velocity, followed by a secondary slower increase during extinction. In a second task (short ramp), with identical initial target presentation but no expectation of target reappearance, the initial rapid rise in eye velocity was followed by decay toward zero. The expectation-dependent difference between responses to these tasks increased in velocity during extinction much more slowly than the initial, visually driven component. In a third task (initial extinction), the moving target was extinguished at motion onset but reappeared 600 ms later. Repetition of identical stimuli evoked anticipatory pursuit triggered by initial target offset. Temporal development and scaling of this anticipatory response, which was based on remembered velocity from prior stimuli, was remarkably similar to and covaried with the difference between mid-ramp extinction and short ramp tasks. Results suggest a common mechanism is responsible for anticipatory pursuit and the extra-retinal component of random-onset pursuit, a finding that is consistent with a previously developed model of pursuit.

Neuronal bases of directional expectation and anticipatory pursuit

Expectation of upcoming events is an essential cognitive function on which anticipatory actions are based. The neuronal basis of this prospective representation is poorly understood. We trained rhesus monkeys in a smooth-pursuit task in which the direction of upcoming target motion was indicated using a color cue. Under these conditions, directional expectation frequently evoked anticipatory smooth movements. We found that the activity of a population of neurons in the supplementary eye fields encoded the expected future direction of the target. Neuronal activity increased after presentation of the cue, indicating future target motion in the preferred direction. Neuronal activity either remained unaltered or was reduced if the antipreferred direction was cued. In addition, approximately 30% of these neurons were more active during trials with anticipatory pursuit in the preferred direction than during trials when monkeys did not anticipate target motion onset. This subset of recorded neurons encoded the direction of the subsequent anticipatory pursuit. We hypothesize that the neural representation of directional expectation could be conceptualized as a competitive interaction between pools of neurons representing likely future events, with the winner of this competition determining the direction of the subsequent anticipatory movement. Similar mechanisms could drive prediction before movement initiation in other motor domains.

Predicting 2D Target Velocity Cannot Help 2D Motion Integration for Smooth Pursuit Initiation

Smooth pursuit eye movements reflect the temporal dynamics of bidimensional (2D) visual motion integration. When tracking a single, tilted line, initial pursuit direction is biased toward unidimensional (1D) edge motion signals, which are orthogonal to the line orientation. Over 200 ms, tracking direction is slowly corrected to finally match the 2D object motion during steady-state pursuit. We now show that repetition of line orientation and/or motion direction does not eliminate the transient tracking direction error nor change the time course of pursuit correction. Nonetheless, multiple successive presentations of a single orientation/direction condition elicit robust anticipatory pursuit eye movements that always go in the 2D object motion direction not the 1D edge motion direction. These results demonstrate that predictive signals about target motion cannot be used for an efficient integration of ambiguous velocity signals at pursuit initiation.

Influence of Cognitive Expectation on the Initiation of Anticipatory and Visual Pursuit Eye Movements in the Rhesus Monkey

A classic paradigm to study anticipatory pursuit consists in training monkeys to look at a target that appears in the center of a visual display, disappears during a short “gap” period, then reappears and immediately starts to move. To determine the role of prior directional information on anticipatory pursuit eye movements, we trained rhesus monkeys to associate the color of a centrally presented visual cue with the direction of an upcoming target motion. In a first experiment, a gap period occurred randomly in 50% of the trials. Consequently, two possible choices of timing of target motion onset were given to subjects to guide their anticipatory responses. In a second experiment, a gap period occurred during each trial and only a single choice of timing of target motion onset was given to subjects. We found that monkeys used the learned association between the color of the cue and the direction of future target motion to voluntarily initiate anticipatory pursuit movements in the appropriate direction. Anticipatory movements could be classified in two distinct populations: early and late movements. Early movements were most frequent when prior directional information was provided and when two choices of timing of target motion onset were given. The latency of visual pursuit was shortened and its velocity was larger when prior directional information was provided. We conclude that cognitive expectation of future target motion plays a dominant role in determining characteristics of anticipatory pursuit in the monkey.

The occluded onset pursuit paradigm: prolonging anticipatory smooth pursuit in the absence of visual feedback

Humans can produce anticipatory smooth pursuit (ASP) for a few hundred ms prior to the appearance of a moving target. Once visual feedback is available, however, it is difficult to distinguish ASP from the visually-driven response with which it merges. Here we have developed a paradigm that extends the anticipatory period to show unequivocally how ASP can evolve over periods of up to 600 ms before being influenced by visual feedback. ASP was evoked by repeated presentation of constant velocity (ramp) stimuli preceded by auditory cues. The target was occluded during the initial part of the ramp, so that when it became visible it had already moved to an eccentric position. The occlusion period (T occ) varied from 0 to 500 ms in 100 ms increments; for each period ramps were presented in blocks of 8 with velocity held constant at 8, 16, 24 or 32 degrees/s. Eye displacement trajectories showed that subjects attempted to match the unseen target trajectory with a mixture of saccades and smooth pursuit. The smooth component was initiated progressively earlier in relation to target appearance as T occ increased, leading to progressively higher ASP gains by the time the target became visible. This prolongation of ASP throughout the occlusion period reveals the underlying internal drive that produces it, a drive that under normal circumstances quickly becomes masked by visual feedback.

Anticipatory movement timing using prediction and external cues

Animals often make anticipatory movements to compensate for slow reaction times. Anticipatory movements can be timed using external, sensory cues, or by an internal prediction of when an event will occur. However, it is unknown whether external or internal cues dominate the anticipatory response when both are present. Smooth pursuit eye movements are generated by a motor system heavily influenced by anticipation. We measured pursuit to determine how its timing was influenced when both a predictable event and a visual cue were present. Monkeys tracked a moving target that appeared at a constant time relative to the onset of a fixation point. At a randomized time before target onset, the fixation point disappeared, creating a temporal "gap" that cued impending target motion. We found that the gap onset cue and prediction of target onset together determined pursuit initiation time. We also investigated whether prediction could override the gap onset cue or vice versa by manipulating target onset and, hence, the duration of time that the animal had to estimate to predict it. When target motion began earlier, the pursuit system relied more on prediction to trigger a movement, whereas the cue was more often used when the target moved later. Pursuit latency in previous trials partially accounted for this behavior. The results suggest that neither internal nor external factors dominate to control the anticipatory response and that the relative contributions vary with stimulus conditions. A model in which neuronal anticipation and fixation signals interact can explain the results.

Anticipatory VOR Suppression Induced by Visual and Nonvisual Stimuli in Humans

We compared the predictive behavior of smooth pursuit (SP) and suppression of the vestibuloocular reflex (VOR) in humans by examining anticipatory smooth eye movements, a phenomenon that arises after repeated presentations of sudden target movement preceded by an auditory warning cue. We investigated whether anticipatory smooth eye movements also occur prior to cued head motion, particularly when subjects expect interaction between the VOR and either real or imagined head-fixed targets. Subjects were presented with horizontal motion stimuli consisting of a visual target alone (SP), head motion in darkness (VOR), or head motion in the presence of a real or imagined head-fixed target (HFT and IHFT, respectively). Stimulus sequences were delivered as single cycles of a velocity sinusoid (frequency: 0.5 or 1.0 Hz) that were either cued (a sound cue 400 ms earlier) or noncued. For SP, anticipatory smooth eye movements developed over repeated trials in the cued, but not the noncued, condition. In the VOR condition, no such anticipatory eye movements were observed even when cued. In contrast, anticipatory responses were observed under cued, but not noncued, HFT and IHFT conditions, as for SP. Anticipatory HFT responses increased in proportion to the velocity of preceding stimuli. In general, anticipatory gaze responses were similar in cued SP, HFT, and IHFT conditions and were appropriate for expected target motion in space. Anticipatory responses may represent the output of a central mechanism for smooth-eye-movement generation that operates during predictive SP as well as VOR modulations that are linked with SP even in the absence of real visual targets.

Supplementary Eye Fields Stimulation Facilitates Anticipatory Pursuit

Anticipatory movements are motor responses occurring before likely sensory events in contrast to reflexive actions. Anticipatory movements are necessary to compensate for delays present in sensory and motor systems. Smooth pursuit eye movements are often used as a paradigmatic example for the study of anticipation. However, the neural control of anticipatory pursuit is unknown. A previous study suggested that the supplementary eye fields (SEFs) could play a role in the guidance of smooth pursuit to predictable target motion. In this study, we favored anticipatory responses in monkeys by making the parameters of target motion highly predictable and electrically stimulated the SEF before and during this behavior. Stimulation sites were restricted to regions of the SEF where saccades could not be evoked at the same low currents. We found that electrical microstimulation in the SEF increased the velocity of anticipatory pursuit movements and decreased their latency. These effects will be referred to as anticipatory pursuit facilitation. The degree of facilitation was the largest if the stimulation train was delivered near the end of the fixation period, before the moment when anticipatory pursuit usually begins. No anticipatory smooth eye movements could be evoked during fixation without an expectation of target motion. These results suggest that the SEF pursuit area might be involved in the process of guiding anticipatory pursuit.

Predictive Smooth Ocular Pursuit During the Transient Disappearance of a Visual Target

When a moving target disappears and there is a complete absence of visual feedback signals, eye velocity decays rapidly but often recovers to previous levels if there is an expectation the target will reappear further along its trajectory Given that eye velocity cannot be maintained under such circumstances, the anticipatory recovery may function to minimize the developing velocity error. When there is a change in target velocity during a transient, any recovery should ideally be scaled and hence predictive of the expected target velocity at reappearance. This study confirmed that subjects did not maintain eye velocity close to target velocity for the duration of the inter-stimulus interval (ISI). The majority of subjects exhibited an initial reduction in eye velocity followed by a scaled recovery prior to target reappearance. Eye velocity during the ISI was, therefore, predictive of the expected change in target velocity. These behavioral data were simulated using a model in which gain applied to the visuomotor drive is reduced after the loss of visual feedback and then modulated depending on subject’s expectation regarding the target’s future trajectory.

The volitional inhibition of anticipatory ocular pursuit using a stop signal

Unlike limb movements, smooth pursuit eye movements cannot normally be performed in the absence of a target. However, when subjects have a high expectancy of an imminent target appearance, the situation changes, and anticipatory smooth pursuit (ASP) tends to precede target onset by several hundred milliseconds. The velocity of this ASP is scaled predictively according to expected target velocity. And when an upcoming target is unexpectedly altered, or fails to appear, ASP continues regardless for approximately 150-200 ms before modification by visual feedback begins [J. Neurophysiol., 84 (2000) 2340]. These and other observations led to the earlier suggestion that ASP might be ballistic, being pre-programmed from start to finish. Two experiments with different timing parameters were therefore performed to test this hypothesis using a version of Logan's [Psychol. Rev., 91 (1984) 295] stop signal task. The aim was to test whether ASP could be stopped at will, and if so, whether the time taken to stop varied as a function of the time since ASP onset. Results showed that in response to a stop signal, ASP can be inhibited at any point in its trajectory, and for the majority of subjects in experiment 1, and all the subjects in experiment 2, with a latency that does not change significantly with target speed or time since ASP onset. These results provide the first demonstration that anticipatory movements can be stopped volitionally in response to a stop signal. Possible cognitive and neurophysiological mechanisms underlying this process are discussed.

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.

Interaction between smooth anticipation and saccades during ocular orientation in darkness

A saccade triggered during sustained smooth pursuit is programmed using retinal information about the relative position and velocity of the target with respect to the eye. Thus the smooth pursuit and saccadic systems are coordinated by using common retinal inputs. Yet, in the absence of retinal information about the relative motion of the eye with respect to the target, the question arises whether the smooth and saccadic systems are still able to be coordinated possibly by using extraretinal information to account for the saccadic and smooth eye movements. To address this question, we flashed a target during smooth anticipatory eye movements in darkness, and the subjects were asked to orient their visual axis to the remembered location of the flash. We observed multiple orientation saccades (typically 2-3) toward the memorized location of the flash. The first orienting saccade was programmed using only the position error at the moment of the flash, and the smooth eye movement was ignored. However, subsequent saccades executed in darkness compensated gradually for the smooth eye displacement (mean compensation congruent with 70%). This behavior revealed a 400-ms delay in the time course of orientation for the compensation of the ongoing smooth eye displacement. We conclude that extraretinal information about the smooth motor command is available to the saccadic system in the absence of visual input. There is a 400-ms delay for smooth movement integration, saccade programming and execution.

The role of expectancy and volition in smooth pursuit eye movements

The most important factor allowing the generation of pursuit eye movements prior to target onset is confidence in the likelihood of imminent target appearance. We show how these anticipatory pursuit responses are essentially ballistic motor primitives and how the signal that drives them in normally defined by stored information concerning target speed, duration and direction. But we also show how static cues may be used to grade the level of these motor primitives 'on-line'. We further demonstrate that, when concatenated, these graded motor primitives can be rapidly combined to form predictive smooth movement trajectories in response to complex multi-ramp sequences.

Anticipatory control of hand and eye movements in humans during oculo-manual tracking

Anticipatory activity of hand and eye has been examined during oculo-manual tracking of a constant velocity visual target with a hand cursor. Both target and cursor were presented briefly (< 480 ms), but repeatedly, at regular inter-stimulus intervals (ISI). In Expt 1, the build-up of hand and eye responses was examined for target velocities varying from 10-40 deg x s(-1) with an ISI of 2.4 s. The velocity 100 ms after target onset (i.e. prior to visual feedback) for both hand and eye (V100) progressively increased over the first four presentations but then attained a steady state (SS). SS V100 values for eye and hand increased in proportion to target velocity and were thus predictive of forthcoming movement. Hand velocity exceeded eye velocity but both exhibited similar anticipatory trajectories. In Expt 2, target velocity was constant (40 deg x s(-1)) but ISI varied from 0.48-3.74 s. Subjects made anticipatory eye movements for all ISIs but hand movements were often reactive at the longest ISI. If the target failed to appear as expected, subjects initiated predictive hand and eye responses with timing appropriate for the prevailing ISI. In Expt 3, predictive responses were compared with responses to randomised presentation. Peak hand velocity was greater in the randomised mode than in the predictive condition, whereas the converse was true for peak eye velocity. This difference is discussed in terms of the mechanisms of positional error correction in hand and eye. Results provide evidence of similar anticipatory mechanisms in hand and eye, using storage of velocity and timing to achieve rapid prediction of target motion.

An fMRI study of anticipation and learning of smooth pursuit eye movements in humans

We studied the neural substrate of anticipation and learning of smooth pursuit eye movements in humans using fMRI. Both predictable and non-predictable eye movements, compared to baseline, activated a common network previously associated with oculomotor function. The temporal dynamics of activity in a subset of these areas suggested a strong correlation with type of condition. Specifically, differential decreases in activity were seen in dorsolateral prefrontal cortex and the intraparietal sulcus during the predictable condition. During the non-predictable condition the same areas exhibited evidence of high levels of activity that further increased throughout the condition. In contrast, differential increases associated with the predictable condition were seen in anterior cingulate and preSMA cortex regions. These changes in activity mirror the time course of the short-term learning of eye movements seen behaviourally, and are congruent with learning-related changes that have been reported for other motor paradigms.

Ocular pursuit responses to repeated, single-cycle sinusoids reveal behavior compatible with predictive pursuit

The link between anticipatory smooth eye movements and prediction in sinusoidal pursuit was investigated by presentation of series of identical, single-cycle, sinusoidal target motion stimuli. Stimuli occurred at randomized intervals (1.2-2.8 s) but were preceded by an audio warning cue 480 ms before each presentation. Cycle period (T) varied from 0.64 to 2.56 s and target displacement from 4 to 20 degrees in separate series. For T </= 1.28 s, responses to the first stimulus of each series exhibited a time delay across the whole cycle (mean = 121 ms for T = 0.8 s). But, in the second and subsequent (steady-state) presentations, anticipatory movements, proportional to target velocity, were made and time delay was significantly reduced (mean = 43 ms for T = 0.8 s). Steady-state time delays were comparable to those evoked during continuous sinusoidal pursuit and less than pursuit reaction time. Even when subjects did not follow the target in the first presentation, they responded to the second presentation with reduced time delay. Throughout the experiments, three types of catch trial (A-C) were introduced. In A, the target failed to appear as expected after the warning cue. Anticipatory smooth movements were initiated, reaching a peak velocity proportional to prior target velocity around 200 ms after expected target onset. In B, the target stopped midway through the cycle. Even if the target remained on and was stationary, the eye movement continued to be driven away from the stationary target with a velocity similar to that of prior responses, reaching a peak velocity that was again proportional to expected target velocity after >/=205 ms. In C, the amplitude of the single sinusoid was unexpectedly increased or decreased. When it decreased, eye velocity throughout the first half-cycle of the response was close to that executed in response to prior stimuli of higher velocity and did not return to an appropriate level for 382-549 ms. Conversely, when amplitude increased, eye velocity remained inappropriately low for the first half-cycle. Results of A and C indicate that subjects are able to use velocity information stored from prior presentations to initiate an oculomotor drive that predominates over visual feedback for the first half-cycle. Results of B indicate that the second part of the cycle is also preprogrammed because it continued despite efforts to suppress it by fixation. The results suggest that initial retinal velocity error information can be sampled, stored, and subsequently replayed as a bi-directional anticipatory pattern of movement that reduces temporal delay and could account for predictive control during sinusoidal pursuit.

Context-dependent smooth eye movements evoked by stationary visual stimuli in trained monkeys

The appearance of a stationary but irrelevant cue triggers a smooth eye movement away from the position of the cue in monkeys that have been trained extensively to smoothly track the motion of moving targets while not making saccades to the stationary cue. We have analyzed the parameters that regulate the size of the cue-evoked smooth eye movement and examined whether presentation of the cue changes the initiation of pursuit for subsequent steps of target velocity. Cues evoked smooth eye movements in blocks of target motions that required smooth pursuit to moving targets, but evoked much smaller smooth eye movements in blocks that required saccades to stationary targets. The direction of the cue-evoked eye movement was always opposite to the position of the cue and did not depend on whether subsequent target motion was toward or away from the position of fixation. The latency of the cue-evoked smooth eye movement was near 100 ms and was slightly longer than the latency of pursuit for target motion away from the position of fixation. The size of the cue-evoked smooth eye movement was as large as 10 degrees /s and decreased as functions of the eccentricity of the cue and the illumination of the experimental room. To study the initiation of pursuit in the wake of the cues, we used bilateral cues at equal eccentricities to the right and left of the position of fixation. These evoked smaller eye velocities that were consistent with vector averaging of the responses to each cue. In the wake of bilateral cues, the initiation of pursuit was enhanced for target motion away from the position of fixation, but not for target motion toward the position of fixation. We suggest that the cue-evoked smooth eye movement is related to a previously postulated on-line gain control for pursuit, and that it is a side-effect of sudden activation of the gain-controlling element.

Deficits of smooth pursuit initiation in patients with degenerative cerebellar lesions

It is well known that cerebellar dysfunction can lead to an impairment of eye velocity during sustained pursuit tracking of continuously moving visual target. We have now studied the initiation of smooth pursuit eye movements towards predictable and randomized visual step-ramp stimuli in six patients with degenerative cerebellar lesions and six age-matched healthy controls using the magnetic scleral search-coil technique. In comparison with the control subjects, the cerebellar patients showed a significant delay of pursuit onset, and their initial eye acceleration was significantly decreased. These cerebellar deficits of pursuit initiation were similarly found in response to both randomized and predictable step-ramps, suggesting that predictive input does not compensate for cerebellar deficits in the initiation period of smooth pursuit. When we compared initial saccades during smooth tracking of foveofugal and foveopetal step-ramps, the absolute position error of these saccades did not significantly differ between patients and controls. In fact, none of the patients showed any bias of the saccadic position error that was related to the direction or velocity of the ongoing target motion. This work presents further evidence that the effect of cerebellar degeneration is not limited to the impaired velocity gain of steady-state smooth pursuit. Instead, it prolongs the processing time required to initiate smooth pursuit and impairs the initial eye acceleration. These two deficits were not associated with an abnormal assessment of target velocity and they were not modified by predictive control mechanisms, suggesting that cerebellar deficits of smooth initiation are not primarily caused by abnormal information on target motion being relayed to the cerebellum.

Predictive smooth pursuit eye movements during identification of moving acuity targets

Repetitive, brief target ramp movements every few seconds lead to anticipatory acceleration before each ramp onset and anticipatory deceleration before ramp offset. We assessed whether identifying novel changes in the pursuit target would alter this pattern of anticipatory pursuit. Without target identification (TI), anticipatory acceleration increased when intervals between ramps were regular, rather than random. It increased further when, between ramps, the target was invisible rather than stationary and visible. Anticipatory deceleration increased when the target was expected to stop rather than disappear at ramp offset. For TI trials, the pursuit target changed briefly into a Landolt C acuity target that had to be identified. Compared to no TI, anticipatory acceleration decreased when a stationary C always appeared just before ramp onset. It increased when a moving C appeared just after ramp onset, but only when the target was invisible between ramps. Anticipatory deceleration was reduced when a moving C appeared just before ramp offset, but did not increase when a stationary C appeared just after ramp offset. The changes were significant, but of small magnitude, suggesting that predictive pursuit, especially with a visible target between ramps, cannot be greatly influenced by attempts to selectively improve acuity at a particular phase of the stimulus.

Kinematics of eye movements of cats to broadband acoustic targets

Operant conditioning was used to train cats with their heads immobilized to localize sound by directing their eyes to the location of the sources. The kinematics of those eye movements were studied and compared with eye movements to visual targets at the same locations. The main finding of this study is that eye movements to broadband long-duration acoustic targets have two components: an initial slow phase of variable duration and a fast, normal saccade. The slow component is characterized by a persistent, shallow velocity ramp, while the saccadic component of the response falls on the main sequence computed from eye movements to visual targets. The slow component was shorter before saccades to long-duration stimuli performed under the delayed-saccade task and practically absent before saccades to transient acoustic stimuli. The results suggest that the initial slow component is used by cats to deal with uncertainty associated with the location of long-duration broadband targets and that the input to the saccade integrator(s) is similar under both visual and acoustic conditions.

Predictive smooth pursuit of complex two-dimensional trajectories demonstrated by perturbation responses in monkeys

Two-dimensional sum-of-sines waveforms were pursued by the eye with very small phase delays compared with visual feedback delays estimated in the same monkeys. Processing delays in making smooth corrections averaged 90 msec after infrequent right-angle perturbations from a circular trajectory. These feedback delays were much larger than component phase delays during pursuit that averaged: 10 msec for sinusoids, 3 msec for circles, 20 msec for sum-of-two-sines trajectories, and 19 msec for sum-of-three-sines trajectories. This suggests that predictive control can play a strong role during tracking for a variety of simple and complex target trajectories.

Anticipatory smooth eye movements of high velocity triggered by large target steps: normal performance and effect of cerebellar degeneration

We studied frequencies and dynamic characteristics of anticipatory smooth eye movements (ASEM) in humans who were tracking step target movements of 20-70 deg amplitude. During presentation of periodic steps of constant amplitude healthy subjects showed frequent high velocity ASEM reaching maximal peak velocities of 5-40 deg/sec. There was no effect of ASEM on the frequency of anticipatory saccades. Randomization of target step amplitude or onset reduced the frequency of ASEM but did not completely abolish fast ASEM. In patients with cerebellar degeneration who exhibited impaired smooth pursuit, fast ASEM were absent and the number of slow ASEM was minimal. In conclusion, large sequential target steps can elicit much higher ASEM velocities than typically described in the literature. Similar to slow ASEM triggered by small steps, these fast ASEM do not require specific training and are not canceled by unpredictable step target motion. However, fast ASEM depend on the intact function of the cerebellum which gives further evidence of their generation by the smooth pursuit oculomotor subsystem.

The relationship of anticipatory smooth eye movement to smooth pursuit initiation

We measured anticipatory smooth eye movements and smooth pursuit initiation with predictable and unpredictable step-ramp stimuli in normal subjects. Subjects generated anticipatory eye motion before targets moved and during intervals when targets suddenly disappeared. Expectations of target trajectory modified pursuit acceleration and latency, demonstrating that pursuit initiation is not governed by visual inputs alone. Anticipatory smooth eye movements and predictive contributions to smooth pursuit had similar accelerations and velocities. Anticipation and pursuit initiation varied in parallel between subjects; anticipation was stronger in subjects who generated faster smooth pursuit. These findings imply that anticipatory and smooth pursuit eye movements are governed by a common mechanism.

Predictive smooth pursuit eye movements near abrupt changes in motion direction

The stimulus-response characteristics of predictive smooth pursuit eye movements near the time of predictable, abrupt changes in target motion direction were studied. Expectations about the speed and direction of target motion both before and after the direction change affected specific components of the predictive pursuit responses. We propose that, when the direction of target motion is expected to change, the cessation of motion in one direction and the initiation of motion in a new direction are separately anticipated and that predictive pursuit movements are summated responses to these two events.

Cognitive expectations, not habits, control anticipatory smooth oculomotor pursuit

Human smooth pursuit eye movements anticipate the future path of moving targets. Anticipatory pursuit is sometimes attributed to cognitive expectations about future motion and other times to the habitual repetition of previous pursuit responses. Expectations and habits were separated by having subjects smoothly pursue a target moving along a randomly-selected path that was either undisclosed to the subject before each trial or disclosed by means of auditory or visual cues. When the path was undisclosed, the direction of anticipatory smooth eye movements was determined by the direction of target motion in the previous trial. In the presence of cues-the critical condition for separating habits and expectations-effects of previous trials diminished and anticipatory smooth eye movements were primarily determined by the direction of motion the subject was told to expect. These results show a strong contribution of cognitive expectations which overrides persevering smooth oculomotor habits. Smooth pursuit eye movements are driven by a signal that combines the present target motion with the target motion expected to occur several hundred milliseconds into the future. The expected motion is based on a genuine cognitive prediction, not lower-level sensory or motor memories of past events.