Processing of retinal and extraretinal signals for memory-guided saccades during smooth pursuit

It is an essential feature for the visual system to keep track of self-motion to maintain space constancy. Therefore the saccadic system uses extraretinal information about previous saccades to update the internal representation of memorized targets, an ability that has been identified in behavioral and electrophysiological studies. However, a smooth eye movement induced in the latency period of a memory-guided saccade yielded contradictory results. Indeed some studies described spatially accurate saccades, whereas others reported retinal coding of saccades. Today, it is still unclear how the saccadic system keeps track of smooth eye movements in the absence of vision. Here, we developed an original two-dimensional behavioral paradigm to further investigate how smooth eye displacements could be compensated to ensure space constancy. Human subjects were required to pursue a moving target and to orient their eyes toward the memorized position of a briefly presented second target (flash) once it appeared. The analysis of the first orientation saccade revealed a bimodal latency distribution related to two different saccade programming strategies. Short-latency (<175 ms) saccades were coded using the only available retinal information, i.e., position error. In addition to position error, longer-latency (>175 ms) saccades used extraretinal information about the smooth eye displacement during the latency period to program spatially more accurate saccades. Sensory parameters at the moment of the flash (retinal position error and eye velocity) influenced the choice between both strategies. We hypothesize that this tradeoff between speed and accuracy of the saccadic response reveals the presence of two coupled neural pathways for saccadic programming. A fast striatal-collicular pathway might only use retinal information about the flash location to program the first saccade. The slower pathway could involve the posterior parietal cortex to update the internal representation of the flash once extraretinal smooth eye displacement information becomes available to the system.     

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.     

Memory-guided saccades: what is memorized?

Summary In order to find out whether extraretinal (oculomotor, internal) input suffices to provide the oculomotor system with the information necessary for saccadic control, two subjects were asked to make memoryguided saccades in complete darkness, after three different location acquisition conditions. These conditions were visually-guided saccades (SA), providing retinal (external) and extraretinal input, visual peripheral target presentation during central target fixation (FI) (external input only), and smooth pursuit (PU) (internal input only). Either 2 or 12 s (delay) after locating the target, the subjects had to make a memory-guided saccade toward it in complete darkness. The results show that whereas these memory-guided saccades were quite accurate for trials with preceding external input, this was not the case with acquisition through internal input alone. Moreover, the accuracy of memory-guided saccades decreased when the delay increased from 2 to 12s for both conditions with retinal input, whereas the accuracy increased for the one condition without retinal input, i.e., the smooth pursuit location acquisition. Furthermore, when both retinal and oculomotor inputs were provided, better accuracy of the memory-guided saccades was observed than with single input.     

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.     

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.     

Role of retinal slip in the prediction of target motion during smooth and saccadic pursuit

Visual tracking of moving targets requires the combination of smooth pursuit eye movements with catch-up saccades. In primates, catch-up saccades usually take place only during pursuit initiation because pursuit gain is close to unity. This contrasts with the lower and more variable gain of smooth pursuit in cats, where smooth eye movements are intermingled with catch-up saccades during steady-state pursuit. In this paper, we studied in detail the role of retinal slip in the prediction of target motion during smooth and saccadic pursuit in the cat. We found that the typical pattern of pursuit in the cat was a combination of smooth eye movements with saccades. During smooth pursuit initiation, there was a correlation between peak eye acceleration and target velocity. During pursuit maintenance, eye velocity oscillated at approximately 3 Hz around a steady-state value. The average gain of smooth pursuit was approximately 0.5. Trained cats were able to continue pursuing in the absence of a visible target, suggesting a role of the prediction of future target motion in this species. The analysis of catch-up saccades showed that the smooth-pursuit motor command is added to the saccadic command during catch-up saccades and that both position error and retinal slip are taken into account in their programming. The influence of retinal slip on catch-up saccades showed that prediction about future target motion is used in the programming of catch-up saccades. Altogether, these results suggest that pursuit systems in primates and cats are qualitatively similar, with a lower average gain in the cat and that prediction affects both saccades and smooth eye movements during pursuit.     

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.     

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.     

Eye movement disorders after frontal eye field lesions in humans

Eye movements were recorded electro-oculographically in three patients with a small ischemic lesion affecting the left frontal eye field (FEF) and in 12 control subjects. Reflexive visually guided saccades (gap and overlap tasks), antisaccades, predictive saccades, memory-guided saccades, smooth pursuit and optokinetic nystagmus (OKN) were studied in the three patients. Staircase saccades and double step saccades were also studied in one of the three patients. For both leftward and rightward saccades, latency in the overlap task (but not in the gap task) and that of correct antisaccades and of memory-guided saccades was significantly increased, compared with the results of controls. There was a significant decrease in the amplitude gain of all rightward saccades programmed using retinotopic coordinates (gap and overlap tasks, predictive and memory-guided saccades), whereas the amplitude gain of corresponding leftward saccades was preserved. Such an asymmetry between leftward and rightward saccades was significant. In the staircase paradigm as well as for the first saccade in the double step paradigm (with the use of retinotopic coordinates in both cases), the amplitude gain of rightward saccades was also significantly lower than that of leftward saccades. Moreover, in the double step paradigm, the amplitude gain of the first rightward saccade was significantly lower than that of the second rightward saccade (programmed using extraretinal signals), which was preserved. The percentage of errors in the antisaccade task did not differ significantly from that of normal subjects. In the predictive saccade paradigm, the percentage of predictive rightward saccades was significantly decreased. The left smooth pursuit gain for all tested velocities, the right smooth pursuit gain for higher velocities, and the left OKN gain were significantly decreased. The results show, for the first time in humans, that the FEF plays an important role in (1) the disengagement from central fixation, (2) the control of contralateral saccades programmed using retinotopic coordinates, (3) saccade prediction and (4) the control of smooth pursuit and OKN, mainly ipsilaterally. In contrast, the left FEF did not appear to be crucial for the control of the only type of saccades programmed using extraretinal signals studied here.     

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.     

Eye position signals in human saccadic processing

Summary 1. We studied saccades to briefly flashed targets in 8 human subjects. The target flash occurred (i) during smooth pursuit (ramp-flash), (ii) just before a saccade to another target (step-flash), or (iii) during steady fixation (flash only). All lights were extinguished after the target flash so that smooth pursuit or saccadic eye movements occurred during the interval of complete darkness between the target flash and the saccade to it. We compared these saccades to those made without intervening eye movement (flash only), and quantified the extent to which the saccadic system compensated for the change in eye position that occurred during the dark interval. 2. Saccades to control flashes were reasonably accurate (mean gain 0.87) and consistent. Compensation for the intervening eye movement in the ramp-flash and step-flash paradigms was highly variable from trial to trial. On average, subjects compensated for 27% of the intervening pursuit eye movement on ramp-flash trials and for 58% of intervening saccadic movement on step-flash trials. 3. Multiple regression analysis showed that the variability did not depend on factors such as variations in underlying saccadic gain, response latency, timing of stimuli or size of the required response. We conclude that this variability is intrinsic to saccadic responses that require the use of an eye position signal. 4. These results show that an eye position signal is available to the saccadic system but that this signal has low fidelity. The high variability and low fidelity of the eye position signal suggest that the saccadic system does not normally operate in spatial coordinates, which require the use of an accurate eye position signal, but rather in retinal coordinates.     

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.     

The initiation of smooth pursuit eye movements and saccades in normal subjects and in “express-saccade makers”

A vast knowledge exists about saccadic reaction times (RT) and their bi- or multimodal distributions with very fast (express) and regular RT. Recently, there has been some evidence that the smooth pursuit system may show a similar RT behavior. Since moving targets usually evoke a combined pursuit/saccade response, we asked which processes influence the initiation of pursuit and saccadic eye movements. Furthermore, we investigated whether and how the pursuit and saccadic system interact during the initiation of eye movements to moving targets. We measured the RT of the initial smooth pursuit (iSP) response and of the first corrective saccade and compared the RT behavior of both. Furthermore we compared the behavior of the corrective saccades to moving targets to that of saccades to stationary targets, known from the literature. The stimulus consisted of a target that moved suddenly at constant velocity (ramp). In addition, prior to the movement, a temporal gap, a position step or a combination of both could occur (gap-ramp, step-ramp, gap-step-ramp, respectively). Differently from most previous studies, we chose step and ramp with the same direction to provoke competition between the pursuit and saccade system. For the first time we investigated pursuit initiation in "express-saccade makers" (ES makers), a subject group known to produce an abnormally high percentage of short-latency saccades in saccade tasks. We compared their results with subject groups who were either naive or trained with respect to saccade tasks. The iSP started at approximately 100 ms, which corresponds to express saccade latencies. These short iSP-RT occurred reflex-like and almost independent of the experimental task. A bimodal frequency distribution of RT with a second peak of longer iSP-RT occurred exclusively in the ramp paradigm. The RT of the first corrective saccades in a pursuit task were comparable with that in a saccade task and depended on the stimulus. The ability of ES makers to produce a high number of express saccades was transferred to corrective saccades in the pursuit task, but not to pursuit initiation. In summary, short-latency pursuit responses differ from express saccades with respect to their independence of experiment and subject group. Therefore, a simple analogy to express saccades cannot be drawn, although some mechanisms seem to act similarly on both the pursuit and the saccade system (such as disengagement of attention with the gap effect). Furthermore, we found evidence that the initial pursuit response and the first corrective saccade are processed independently of each other. The first corrective saccades to moving targets behave like saccades to stationary targets. Normal pursuit but abnormal saccade RT of ES makers can be explained by recent theories of superior colliculus (SC) function in terms of retinal error handling.     

Coordination of smooth pursuit and saccade target selection in monkeys

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

Interactions between visually and electrically elicited saccades before and after superior colliculus and frontal eye field ablations in the rhesus monkey

Recent work has shown that humans and monkeys utilize both retinal error and eye position signals to compute the direction and amplitude of saccadic eye movements (Hallett and Lightstone 1976a, b; Mays and Sparks 1980b). The aim of this study was to examine the role the frontal eye fields (FEF) and the superior colliculi (SC) play in this computation. Rhesus monkeys were trained to acquire small, briefly flashed spots of light with saccadic eye movements. During the latency period between target extinction and saccade initiation, their eyes were displaced, in total darkness, by electrical stimulation of either the FEF, the SC or the abducens nucleus area. Under such conditions animals compensated for the electrically induced ocular displacement and correctly reached the visual target area, suggesting that both a retinal error and eye position error signal were computed. The amplitude and direction of the electrically induced saccades depended not only on the site stimulated but also on the amplitude and direction of the eye movement initiated by the animal to acquire the target. When the eye movements initiated by the animal coincided with the saccades initiated by electrical stimulation, the resultant saccade was the weighted average of the two, where one weighing factor was the intensity of the electrical stimulus. Animals did not acquire targets correctly when their eyes were displaced, prior to their intended eye movements, by stimulating in the abducens nucleus area. After bilateral ablation of either the FEF or the SC monkeys were still able to acquire visual targets when their eyes were displaced, prior to saccade initiation, by electrical stimulation of the remaining intact structure. These results suggest that neither the FEF nor the SC is uniquely responsible for the combined computation of the retinal error and the eye position error signals.     

The assessment of position of stationary targets perceived during saccadic eye movement

Summary The experiment was performed to establish the accuracy with which visual targets perceived during saccadic eye movement are localised. Subjects were presented with the task of executing saccades of 30° plus amplitude, passing through primary gaze, about the time of peak velocity a 5 ms red flash was presented at some random position (up to 30° left or right of centre) on a horizontal visual display. Subjects were required to indicate the direction in which they thought the flash was localised by fixating in that direction. Observations were made under conditions of prolonged total darkness and in the presence of a contrasting background. Measurement was made of saccade velocity and eye displacement as an index of target positions. Eye displacement was linearly scaled with respect to true target direction. Targets were localised with an average error of 5°–6° although the variance was high. No systematic differences were found between conditions or subjects. Error was unrelated to saccade velocity. It is concluded that during saccadic eye movements the appreciation of target position is maintained with an acceptable degree of accuracy.     

The influence of briefly presented randomized target motion on the extraretinal component of ocular pursuit

We assessed the ability to extract velocity information from brief exposure of a moving target and sought evidence that this information could be used to modulate the extraretinal component of ocular pursuit. A step-ramp target motion was initially visible for a brief randomized period of 50, 100, 150, or 200 ms, but then extinguished for a randomized period of 400 or 600 ms before reappearing and continuing along its trajectory. Target speed (5-20 degrees /s), direction (left/right), and intertrial interval (2.7-3.7 s) were also randomized. Smooth eye movements were initiated after about 130 ms and comprised an initial visually dependent component, which reached a peak velocity that increased with target velocity and initial exposure duration, followed by a sustained secondary component that actually increased throughout extinction for 50- and 100-ms initial exposures. End-extinction eye velocity, reflecting extraretinal drive, increased with initial exposure from 50 to 100 ms but remained similar for longer exposures; it was significantly scaled to target velocity for 150- and 200-ms exposures. The results suggest that extraretinal drive is based on a sample of target velocity, mostly acquired during the first 150 ms, that is stored and forms a goal for generating appropriately scaled eye movements during absence of visual input. End-extinction eye velocity was significantly higher when target reappearance was expected than when it was not, confirming the importance of expectation in generating sustained smooth movement. However, end-extinction eye displacement remained similar irrespective of expectation, suggesting that the ability to use sampled velocity information to predict future target displacement operates independently of the control of smooth eye movement.     

Perisaccadic mislocalization without saccadic eye movements

Despite frequent saccadic gaze shifts we perceive the surrounding visual world as stable. It has been proposed that the brain uses extraretinal eye position signals to cancel out saccade-induced retinal image motion. Nevertheless, stimuli flashed briefly around the onset of a saccade are grossly mislocalized, resulting in a shift and, under certain conditions, an additional compression of visual space. Perisaccadic mislocalization has been related to a spatio-temporal misalignment of an extraretinal eye position signal with the corresponding saccade. Here, we investigated perceptual mislocalization of human observers both in saccade and fixation conditions. In the latter conditions, the retinal stimulation during saccadic eye movements was simulated by a fast saccade-like shift of the stimulus display. We show that the spatio-temporal pattern of both the shift and compression components of perceptual mislocalization can be surprisingly similar before real and simulated saccades. Our findings suggest that the full pattern of perisaccadic mislocalization can also occur in conditions which are unlikely to involve changes of an extraretinal eye position signal. Instead, we suggest that, under the conditions of our experiments, the arising difficulty to establish a stable percept of a briefly flashed stimulus within a given visual reference frame yields mislocalizations before fast retinal image motion. The availability of visual references appears to exert a major influence on the relative contributions of shift and compression components to mislocalization across the visual field.     

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.     

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.