Quantitative analysis of catch-up saccades during sustained pursuit

During visual tracking of a moving stimulus, primates orient their visual axis by combining two very different types of eye movements, smooth pursuit and saccades. The purpose of this paper was to investigate quantitatively the catch-up saccades occurring during sustained pursuit. We used a ramp-step-ramp paradigm to evoke catch-up saccades during sustained pursuit. In general, catch-up saccades followed the unexpected steps in position and velocity of the target. We observed catch-up saccades in the same direction as the smooth eye movement (forward saccades) as well as in the opposite direction (reverse saccades). We made a comparison of the main sequences of forward saccades, reverse saccades, and control saccades made to stationary targets. They were all three significantly different from each other and were fully compatible with the hypothesis that the smooth pursuit component is added to the saccadic component during catch-up saccades. A multiple linear regression analysis was performed on the saccadic component to find the parameters determining the amplitude of catch-up saccades. We found that both position error and retinal slip are taken into account in catch-up saccade programming to predict the future trajectory of the moving target. We also demonstrated that the saccadic system needs a minimum period of approximately 90 ms for taking into account changes in target trajectory. Finally, we reported a saturation (above 15 degrees /s) in the contribution of retinal slip to the amplitude of catch-up saccades.     

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.     

Adaptation of catch-up saccades during the initiation of smooth pursuit eye movements

Reduction of retinal speed and alignment of the line of sight are believed to be the respective primary functions of smooth pursuit and saccadic eye movements. As the eye muscles strength can change in the short-term, continuous adjustments of motor signals are required to achieve constant accuracy. While adaptation of saccade amplitude to systematic position errors has been extensively studied, we know less about the adaptive response to position errors during smooth pursuit initiation, when target motion has to be taken into account to program saccades, and when position errors at the saccade endpoint could also be corrected by increasing pursuit velocity. To study short-term adaptation (250 adaptation trials) of tracking eye movements, we introduced a position error during the first catch-up saccade made during the initiation of smooth pursuit—in a ramp-step-ramp paradigm. The target position was either shifted in the direction of the horizontally moving target (forward step), against it (backward step) or orthogonally to it (vertical step). Results indicate adaptation of catch-up saccade amplitude to back and forward steps. With vertical steps, saccades became oblique, by an inflexion of the early or late saccade trajectory. With a similar time course, post-saccadic pursuit velocity was increased in the step direction, adding further evidence that under some conditions pursuit and saccades can act synergistically to reduce position errors.     

Accuracies of saccades to moving targets during pursuit initiation and maintenance

The overall goals of the studies presented here were to compare (1) the accuracies of saccades to moving targets with either a novel or a known target motion, and (2) the relationships between the measures of target motion and saccadic amplitude during pursuit initiation and maintenance. Since resampling of position error just prior to saccade initiation can confound the interpretation of results, the target ramp was masked during the planning and execution of the saccade. The results suggest that saccades to moving targets were significantly more accurate if the target motion was known from the early part of the trial (e.g., during pursuit maintenance) than in the case of novel target motion (e.g., during pursuit initiation); both these types of saccades were more accuate than those when target motion information was not available. Using target velocity in space as a rough estimate of the magnitude of the extra-retinal signal during pursuit maintenance, the saccadic amplitude was significantly associated with the extra-retinal target motion information after accounting for the position error. In most subjects, this association was stronger than the one between retinal slip velocity and saccadic amplitude during pursuit initiation. The results were similar even when the smooth eye motion prior to the saccade was controlled. These results suggest that different sources of target motion information (retinal image velocity vs internal representation of previous target motion in space) are used in planning saccades during different stages of pursuit. The association between retinal slip velocity and saccadic amplitude is weak during initiation, thus explaining poor saccadic accuracy during this stage of pursuit.     

Saccadic and smooth pursuit eye movements: computational modeling of a common inhibitory mechanism in brainstem

The oculomotor system coordinates different types of eye movements in order to orient the visual axis, including saccade and smooth pursuit,. It was traditionally thought that the premotor pathways for these different eye movements are largely separate. In particular, a group of midline cells in the pons called omnipause neurons were considered to be part of only the saccadic system. Recent experimental findings have shown activity modulation of these brainstem premotor neurons during both kinds of eye movements. In this study, we propose a new computational model of the brainstem circuitry underlying the generation of saccades and smooth pursuit eye movements. Similar models have been developed earlier, but mainly looking at pure saccades. Here, we integrated recent neurophysiological findings on omnipause neuron activity during smooth pursuit. Our computational model can mimic some new experimental findings as the similarity of "eye velocity profile" with "omnipause neuron pattern of activity" in pursuit movement. We showed that pursuit neuron activity is augmented during catch-up saccades; this increment depends on the initial pursuit velocity in catch-up saccade onset. We conclude that saccadic and pursuit components of catch-up saccades are added to each other nonlinearly.     

A quantitative analysis of the correlations between eye movements and neural activity in the pretectum

The study of the saccadic system has focused mainly on neurons active before the beginning of saccades, in order to determine their contribution in movement planning and execution. However, most oculomotor structures contain also neurons whose activity starts only after the onset of saccades, the maximum of their activity sometimes occurring near saccade end. Their characteristics are still largely unknown. We investigated pretectal neurons with saccade-related activity in the alert cat during eye movements towards a moving target. They emitted a high-frequency burst of action potentials after the onset of saccades, irrespective of their direction, and will be referred to as "pretectal saccade-related neurons". The delay between saccade onset and cell activity varied from 17 to 66 ms on average. We found that burst parameters were correlated with the parameters of saccades; the peak eye velocity was correlated with the peak of the spike density function, the saccade amplitude with the number of spikes in the burst, and burst duration increased with saccade duration. The activity of six pretectal saccade-related neurons was studied during smooth pursuit at different velocities. A correlation was found between smooth pursuit velocity and mean firing rate. A minority of these neurons (2/6) were also visually responsive. Their visual activity was proportional to the difference between eye and target velocity during smooth pursuit (retinal slip). These results indicate that the activity of pretectal saccade-related neurons is correlated with the characteristics of eye movements. This finding is in agreement with the known anatomical projections from premotor regions of the saccadic system to the pretectum.     

Evidence for synergy between saccades and smooth pursuit during transient target disappearance

Visual tracking of moving objects requires prediction to compensate for visual delays and minimize mismatches between eye and target position and velocity. In everyday life, objects often disappear behind an occluder, and prediction is required to align eye and target at reappearance. Earlier studies investigating eye motion during target blanking showed that eye velocity first decayed after disappearance but was sustained or often recovered in a predictive way. Furthermore, saccades were directed toward the unseen target trajectory and therefore appeared to correct for position errors resulting from eye velocity decay. To investigate the synergy between smooth and saccadic eye movements, this study used a target blanking paradigm where both position and velocity of the target at reappearance could vary independently but were presented repeatedly to facilitate prediction. We found that eye velocity at target reappearance was only influenced by expected target velocity, whereas saccades responded to the expected change of target position at reappearance. Moreover, subjects exhibited on-line adaptation, on a trial-by-trial basis, between smooth and saccadic components; i.e., saccades compensated for variability of smooth eye displacement during the blanking period such that gaze at target reappearance was independent of the level of smooth eye displacement. We suggest these results indicate that information arising from efference copies of saccadic and smooth pursuit systems are combined with the goal of adjusting eye position at target reappearance. Based on prior experimental evidence, we hypothesize that this spatial remapping is carried out through interactions between a number of identified neurophysiological structures.     

Facilitation of smooth pursuit initiation by electrical stimulation in the supplementary eye fields.

The role of the supplementary eye fields (SEF) during smooth pursuit was investigated with electrical microstimulation. We found that stimulation in the SEF increased the acceleration and velocity of the eyes in the direction of target motion during smooth pursuit initiation but not during sustained pursuit. The increase in eye velocity during initiation will be referred to as pursuit facilitation and was observed at sites where saccades could not be evoked with the same stimulation parameters. On average, electrical stimulation increased eye velocity by approximately 20%. At most sites, the threshold for a significant facilitation was 50 microA with a stimulation frequency of 300 Hz. Facilitation of pursuit initiation depended on the timing of stimulation trains. The effect was most pronounced if the stimulation was delivered before smooth pursuit initiation. On average, eye velocity in stimulation trials increased linearly as a function of eye velocity in control trials, and this function had a slope greater than one, suggesting a multiplicative influence of the stimulation. Stimulation during a fixation task did not evoke smooth eye movements. The latency of catch-up saccades was increased during facilitation, but their accuracy was not affected. Saccades toward stationary targets were not affected by the stimulation. The results are further evidence that the SEF plays a role in smooth pursuit in addition to its known role in saccade planning and suggest that this role may be to control the gain of smooth pursuit during initiation. The covariance between pursuit facilitation and the timing of the catch-up saccade as a result of stimulation suggests that these different eye movements systems are coordinated to achieve a common goal.     

Common inhibitory mechanism for saccades and smooth-pursuit eye movements

The premotor pathways subserving saccades and smooth-pursuit eye movements are usually thought to be different. Indeed, saccade and smooth-pursuit eye movements have different dynamics and functions. In particular, a group of midline cells in the pons called omnipause neurons (OPNs) are considered to be part of the saccadic system only. It has been established that OPNs keep premotor neurons for saccades under constant inhibition during fixation periods. Saccades occur only when the activity of OPNs has completely stopped or paused. Accordingly, electrical stimulation in the region of OPNs inhibits premotor neurons and interrupts saccades. The premotor relay for smooth pursuit is thought to be organized differently and omnipause neurons are not supposed to be involved in smooth-pursuit eye movements. To investigate this supposition, OPNs were recorded during saccades and during smooth pursuit in the monkey (Macaca mulatta). Unexpectedly, we found that neuronal activity of OPNs decreased during smooth pursuit. The resulting activity reduction reached statistical significance in approximately 50% of OPNs recorded during pursuit of a target moving at 40 degrees /s. On average, activity was reduced by 34% but never completely stopped or paused. The onset of activity reduction coincided with the onset of smooth pursuit. The duration of activity reduction was correlated with pursuit duration and its intensity was correlated with eye velocity. Activity reduction was observed even in the absence of catch-up saccades that frequently occur during pursuit. Electrical microstimulation in the OPNs' area induced a strong deceleration of the eye during smooth pursuit. These results suggest that OPNs form an inhibitory mechanism that could control the time course of smooth pursuit. This inhibitory mechanism is part of the fixation system and is probably needed to avoid reflexive eye movements toward targets that are not purposefully selected. This study shows that saccades and smooth pursuit, although they are different kinds of eye movements, are controlled by the same inhibitory system.     

Smooth pursuit in schizophrenia: Abnormalities of open- and closed-loop responses

A sample of 29 schizophrenia patients and 27 nonpsychiatric subjects were tested on measures of open- and closed-loop smooth-pursuit performance. Rashbass step-ramps were used to measure pursuit latency and open-loop gain. Regular ramps were used to calculate frequency and amplitude of both catch-up saccades and square-wave jerks, frequency of anticipatory saccades, and steady-state gain. Schizophrenia patients demonstrated lower open-loop gain than did nonpsychiatric subjects, an effect that was accentuated at faster target velocities. They also showed reduced steady-state gain, but only to 30%s right-moving targets. There was no evidence of saccadic abnormalities during smooth pursuit among the schizophrenia patients. These patients generated fewer square-wave jerks than did nonpsychiatric subjects for 10% left-moving targets. These results suggest an abnormality of smooth-pursuit initiation among patients with schizophrenia.     

Direct evidence for a position input to the smooth pursuit system

When objects move in our environment, the orientation of the visual axis in space requires the coordination of two types of eye movements: saccades and smooth pursuit. The principal input to the saccadic system is position error, whereas it is velocity error for the smooth pursuit system. Recently, it has been shown that catch-up saccades to moving targets are triggered and programmed by using velocity error in addition to position error. Here, we show that, when a visual target is flashed during ongoing smooth pursuit, it evokes a smooth eye movement toward the flash. The velocity of this evoked smooth movement is proportional to the position error of the flash; it is neither influenced by the velocity of the ongoing smooth pursuit eye movement nor by the occurrence of a saccade, but the effect is absent if the flash is ignored by the subject. Furthermore, the response started around 85 ms after the flash presentation and decayed with an average time constant of 276 ms. Thus this is the first direct evidence of a position input to the smooth pursuit system. This study shows further evidence for a coupling between saccadic and smooth pursuit systems. It also suggests that there is an interaction between position and velocity error signals in the control of more complex movements.     

Predictive responses of periarcuate pursuit neurons to visual target motion

The smooth pursuit eye movement system uses retinal information about the image-slip-velocity of the target in order to match the eye-velocity-in-space (i.e., gaze-velocity) to the actual target velocity. To maintain the target image on the fovea during smooth gaze tracking, and to compensate for the long delays involved in processing visual motion information and/or eye velocity commands, the pursuit system must use prediction. We have shown recently that both retinal image-slip-velocity and gaze-velocity signals are coded in the discharge of single pursuit-related neurons in the simian periarcuate cortex. To understand how periarcuate pursuit neurons are involved in predictive smooth pursuit, we examined the discharge characteristics of these neurons in trained Japanese macaques. When a stationary target abruptly moved sinusoidally along the preferred direction at 0.5 Hz, the response delays of pursuit cells seen at the onset of target motion were compensated in succeeding cycles. The monkeys were also required to continue smooth pursuit of a sinusoidally moving target while it was blanked for about half of a cycle at 0.5 Hz. This blanking was applied before cell activity normally increased and before the target changed direction. Normalized mean gain of the cells' responses (re control value without blanking) decreased to 0.81(+/-0.67 SD), whereas normalized mean gain of the eye movement (eye gain) decreased to 0.65 (+/-0.16 SD). A majority (75%) of pursuit neurons discharged appropriately up to 500 ms after target blanking even though eye velocity decreased sharply, suggesting a dissociation of the activity of those pursuit neurons and eye velocity. To examine whether pursuit cell responses contain a predictive component that anticipates visual input, the monkeys were required to fixate a stationary target while a second test laser spot was moved sinusoidally. A majority (68%) of pursuit cells tested responded to the second target motion. When the second spot moved abruptly along the preferred direction, the response delays clearly seen at the onset of sinusoidal target motion were compensated in succeeding cycles. Blanking (400-600 ms) was also applied during sinusoidal motion at 1 Hz before the test spot changed its direction and before pursuit neurons normally increased their activity. Preferred directions were similar to those calculated for target motion (normalized mean gain=0.72). Similar responses were also evoked even if the second spot was flashed as it moved. Since the monkeys fixated the stationary spot well, such flashed stimuli should not induce significant retinal slip. These results taken together suggest that the prediction-related activity of periarcuate pursuit neurons contains extracted visual components that reflect direction and speed of the reconstructed target image, signals sufficient for estimating target motion. We suggest that many periarcuate pursuit neurons convey this information to generate appropriate smooth pursuit eye movements.     

The saccadic component of ocular pursuit is influenced by the predictability of the target motion in humans

Predictive control is an important aspect of the smooth pursuit eye movements: it has been shown that when the target motion is composed of a mixture of sinusoids of different frequencies it becomes unpredictable and there is a decline in gain for the lowest frequencies but not for the highest one. Using such a pseudo-random paradigm we studied the effect of predictability of the target motion on the saccadic component of pursuit. For both the saccadic and the smooth components of pursuit, we observed that the gains for the lowest frequencies were significantly lower than the gain for the highest frequency. Thus, predictability of the motion of a visual target seems to influence both the smooth pursuit component and the saccadic component of ocular pursuit in the same way.     

Eye movement and visual motion perception in schizophrenia I: Apparent motion evoked smooth pursuit eye movement reveals a hidden dysfunction in smooth pursuit eye movement in schizophrenia

To date, smooth pursuit eye movement in schizophrenia has only been investigated using a target stimulus in continuous motion. However, smooth pursuit can also be evoked by an oscillating jumping dot that appears to be in apparent motion and although there is no continuous motion on the retinal surface this apparently moving stimulus can effortlessly elicit smooth-pursuit eye movement. In the first of two experiments smooth pursuit eye movement was evoked by target stimuli in continuous (real) motion at seven target velocities from 5.0 to 35.0 deg/s, and in a second experiment it was measured in response to an oscillating jumping dot in apparent motion at eight target velocities from 5.0 to 25.0 deg/s in a group with mixed-symptoms in schizophrenia and in a control group. The results of Experiment 1 provided no evidence for a dysfunction in continuous motion evoked smooth pursuit eye movement in the group with schizophrenia. However, following the removal of saccadic eye movements in smooth pursuit, the group with schizophrenia showed significantly lower smooth pursuit eye velocity at target velocities from 20.0 to 35.0 deg/s. The results of Experiment 2 revealed that apparent motion evoked smooth pursuit eye velocity in the group with schizophrenia was significantly lower in comparison with normal observers at all target velocities up to 25.0 deg/s with the inclusion or exclusion of saccadic eye movements. The findings demonstrate that overall smooth pursuit eye movement evoked in response to a continuous (real) motion target in the group with schizophrenia may nevertheless contain a hidden temporal resolution and integration dysfunction that is revealed when smooth pursuit eye movement is evoked in response to an oscillating jumping dot in apparent motion. The findings also demonstrate that normal smooth pursuit eye movement in normal observers can be made to resemble the dysfunctional smooth pursuit eye movement that is naturally found in some people with schizophrenia by using a target stimulus in apparent motion.     

What triggers catch-up saccades during visual tracking?

When tracking moving visual stimuli, primates orient their visual axis by combining two kinds of eye movements, smooth pursuit and saccades, that have very different dynamics. Yet, the mechanisms that govern the decision to switch from one type of eye movement to the other are still poorly understood, even though they could bring a significant contribution to the understanding of how the CNS combines different kinds of control strategies to achieve a common motor and sensory goal. In this study, we investigated the oculomotor responses to a large range of different combinations of position error and velocity error during visual tracking of moving stimuli in humans. We found that the oculomotor system uses a prediction of the time at which the eye trajectory will cross the target, defined as the "eye crossing time" (T(XE)). The eye crossing time, which depends on both position error and velocity error, is the criterion used to switch between smooth and saccadic pursuit, i.e., to trigger catch-up saccades. On average, for T(XE) between 40 and 180 ms, no saccade is triggered and target tracking remains purely smooth. Conversely, when T(XE) becomes smaller than 40 ms or larger than 180 ms, a saccade is triggered after a short latency (around 125 ms).     

Role of arcuate frontal cortex of monkeys in smooth pursuit eye movements. I. Basic response properties to retinal image motion and position

Anatomical and physiological studies have shown that the "frontal pursuit area" (FPA) in the arcuate cortex of monkeys is involved in the control of smooth pursuit eye movements. To further analyze the signals carried by the FPA, we examined the activity of pursuit-related neurons recorded from a discrete region near the arcuate spur during a variety of oculomotor tasks. Pursuit neurons showed direction tuning with a wide range of preferred directions and a mean full width at half-maximum of 129 degrees. Analysis of latency using the "receiver operating characteristic" to compare responses to target motion in opposite directions showed that the directional response of 58% of FPA neurons led the initiation of pursuit, while 19% led by 25 ms or more. Analysis of neuronal responses during pursuit of a range of target velocities revealed that the sensitivity to eye velocity was larger during the initiation of pursuit than during the maintenance of pursuit, consistent with two components of firing related to image motion and eye motion. FPA neurons showed correlates of two behavioral features of pursuit documented in prior reports. 1) Eye acceleration at the initiation of pursuit declines as a function of the eccentricity of the moving target. FPA neurons show decreased firing at the initiation of pursuit in parallel with the decline in eye acceleration. This finding is consistent with prior suggestions that the FPA plays a role in modulating the gain of visual-motor transmission for pursuit. 2) A stationary eccentric cue evokes a smooth eye movement opposite in direction to the cue and enhances the pursuit evoked by subsequent target motions. Many pursuit neurons in the FPA showed weak, phasic visual responses for stationary targets and were tuned for the positions about 4 degrees eccentric on the side opposite to the preferred pursuit direction. However, few neurons (12%) responded during the preparation or execution of saccades. The responses to the stationary target could account for the behavioral effects of stationary, eccentric cues. Further analysis of the relationship between firing rate and retinal position error during pursuit in the preferred and opposite directions failed to provide evidence for a large contribution of image position to the firing of FPA neurons. We conclude that FPA processes information in terms of image and eye velocity and that it is functionally separate from the saccadic frontal eye fields, which processes information in terms of retinal image position.     

Properties of signals that determine the amplitude and direction of saccadic eye movements in monkeys

Monkeys were trained to make saccades to briefly flashed targets. We presented the flash during smooth pursuit of another target, so that there was a smooth change in eye position after the flash. We could then determine whether the flash-evoked saccades compensated for the intervening smooth eye movements to point the eyes at the position of the flash in space. We defined the "retinal error" as the vector from the position of the eye at the time of the flash to the position of the target. We defined "spatial error" as the vector from the position of the eye at the time of the saccade to the position of the flashed target in space. The direction of the saccade (in polar coordinates) was more highly correlated with the direction of the retinal error than with the direction of the spatial error. Saccade amplitude was also better correlated with the amplitude of the retinal error. We obtained the same results whether the flash was presented during pursuit with the head fixed or during pursuit with combined eye-head movements. Statistical analysis demonstrated that the direction of the saccade was determined only by the retinal error in two of the three monkeys. In the third monkey saccade direction was determined primarily by retinal error but had a consistent bias toward spatial error. The bias can be attributed to this monkey's earlier practice in which the flashed target was reilluminated so he could ultimately make a saccade to the correct position in space. These data suggest that the saccade generator does not normally use nonvisual feedback about smooth changes in eye or gaze position. In two monkeys we also provided sequential target flashes during pursuit with the second flash timed so that it occurred just before the first saccade. As above, the first saccade was appropriate for the retinal error provided by the first flash. The second saccade compensated for the first and pointed the eyes at the position of the second target in space. We conclude, as others have before (12, 21), that the saccade generator receives feedback about its own output, saccades. Our results require revision of existing models of the neural network that generates saccades. We suggest two models that retain the use of internal feedback suggested by others. We favor a model that accounts for our data by assuming that internal feedback originates directly from the output of the saccade generator and reports only saccadic changes in eye position.     

Single neuron activity in the dorsomedial frontal cortex during smooth pursuit eye movements

This report describes the behavior of neurons in the dorsomedial frontal cortex during smooth pursuit eye movements. Single neurons were recorded from monkeys while they tracked a small target that moved from the center of a screen outward. The firing rate of most cells was modulated during smooth pursuit eye movements, and often the activity peaked around pursuit initiation. Visual motion of the small target with the eyes fixed could activate pursuit neurons, but did not account for the total pursuit response. Neurons were also selective for the direction in which the animal was tracking, indicating that they were linked to the generation of the eye movements, and not to non-specific arousal effects. The results suggest that the dorsomedial frontal cortex participates in initiating smooth pursuit. It is proposed that the dorsomedial frontal cortex is part of a partial alternative path to the classic pursuit pathway that might be used to facilitate the initiation or control of eye movements beyond a simple reflexive response to retinal slip.     

Influences of hand movements on eye movements in tracking tasks in man

Summary We investigated horizontal smooth pursuit eye movements and hand movements in tracking tasks in order to find out whether hand movements influence eye movements and if so, in what ways. Externally controlled target movements were tracked either by the eyes alone or by the eyes and right hand together. Because a possible influence might depend on the stimulus, we used two classes of target movements: sinusoidal target movements (predictable target movements) and pseudo-random target movements (unpredictable target movements). Our data show that the eye movements contained only a few small saccades when sinusoidal target movements with frequencies higher than about 1 Hz were tracked by eyes and hand together. More and larger saccades were made when the same target movements were tracked by the eyes alone. The difference in smoothness of eye movements was highly significant between the two tracking conditions. Such a difference was not found during the tracking of a pseudorandom target motion. This suggests that the influence of hand movements is related to the predictability of the stimulus. In contrast to the gain of the smooth pursuit eye movements and the maximum of the cross-correlation function, the gain of the composite eye movements did not depend on the tracking condition. The delay of the eye movements with respect to the (sinusoidal) target movements also showed no dependence on the tracking condition. Visual feedback from the tracking hand was found not to play a role in the difference in eye movements for the two tracking conditions.     

Small effects of neck torsion on healthy human voluntary eye movements

Purpose

Although several lines of research suggest that the head and eye movement systems interact, previous studies have reported that applying static neck torsion does not affect smooth pursuit eye movements in healthy controls. This might be due to several methodological issues. Here we systematically investigated the effect of static neck torsion on smooth pursuit and saccadic eye movement behavior in healthy subjects.

Methods

In twenty healthy controls, we recorded eye movements with video-oculography while their trunk was in static rotation relative to the head (7 positions from 45° to the left to 45° to right). The subject looked at a moving dot on the screen. In two separate paradigms, we evoked saccadic and smooth pursuit eye movements, using both predictable and unpredictable target motions.

Results

Smooth pursuit gain and saccade peak velocity decreased slightly with increasing neck torsion. Smooth pursuit gains were higher for predictable target movements than for unpredictable target movements. Saccades to predictable targets had lower latencies, but reduced gains compared to saccades to unpredictable targets. No interactions between neck torsion and target predictability were observed.

Conclusion

Applying static neck torsion has small effects on voluntary eye movements in healthy subjects. These effects are not modulated by target predictability.