Primary Saccade Research Articles

No evidence for differential saccadic adaptation in children and adults with an autism spectrum diagnosis.

Altered patterns of eye-movements during scene exploration, and atypical gaze preferences in social settings, have long been noted as features of the Autism phenotype. While these are typically attributed to differences in social engagement and interests (e.g., preferences for inanimate objects over face stimuli), there are also reports of differential saccade measures to non-social stimuli, raising the possibility that fundamental differences in visuo-sensorimotor processing may be at play. Here, we tested the plasticity of the eye-movement system using a classic saccade-adaptation paradigm to assess whether individuals with ASD make typical adjustments to their eye-movements in response to experimentally introduced errors. Saccade adaptation can be measured in infants as young as 10 months, raising the possibility that such measures could be useful as early neuro-markers of ASD risk. Saccade amplitudes were measured while children and adults with ASD (N = 41) and age-matched typically developing (TD) individuals (N = 68) made rapid eye-movements to peripherally presented targets. During adaptation trials, the target was relocated from 20-degrees to 15-degrees from fixation once a saccade to the original target location was initiated, a manipulation that leads to systematic reduction in saccade amplitudes in typical observers. Neither children nor adults with ASD showed any differences relative to TD peers in their abilities to appropriately adapt saccades in the face of persistently introduced errors. Of the three studies to date of saccade adaptation in ASD, none have shown deficits in saccade adaptation that are sufficient to generalize to the whole or a subgroup of the ASD population. Unlike prior studies, we found no evidence for a slower adaptation rate during the early adaptation phase, and no of evidence greater variance of saccade amplitudes in ASD. In post hoc analysis, there was evidence for larger primary saccades to non-adapted targets, a finding requiring replication in future work.

More (corrective) consecutive saccades after a lesion to the posterior parietal cortex.

To reach a target, primary saccades (S1s) are often followed by (corrective) consecutive saccades (S2, and potentially S3, S4, S5), which are based on retinal and extraretinal feedback. Processing these extraretinal signals was found to be significantly impaired by lesions to the posterior parietal cortex (PPC). Recent studies, however, added a more nuanced view to the role of the PPC, where patients with PPC lesions still used extraretinal signals for S2s and perceptual judgements (Fabius et al., 2020; Rath-Wilson & Guitton, 2015). Hence, it seems that a PPC lesion is not disrupting extraretinal processing per se. Yet, a lesion might still result in less reliable processing of extraretinal signals. Here, we investigated whether this lower reliability manifests as decreased or delayed S2 initiation. Patients with PPC lesions (n = 7) and controls (n = 26) performed a prosaccade task where the target either remained visible or was removed after S1 onset. When S1 is removed, accurate S2s (corrections of S1 error) rely solely on extraretinal signals. We analysed S2 quantity and timing using linear mixed-effects modelling and additive hazards analyses. Patients demonstrated slower S1 execution and lower S1 amplitudes than controls, but their S2s still compensated the S1 undershoot, also when they only relied on extraretinal information. Surprisingly, patients showed an increased amount of S2s. This deviation from control behaviour can be seen as suboptimal, but given the decreased accuracy of the primary saccade, it could be optimal for patients to employ more (corrective) consecutive saccades to overcome this inaccuracy.

Temporal Eye-Hand Coordination During Visually Guided Reaching in 7- to 12-Year-Old Children With Strabismus.

We recently found slow visually guided reaching in strabismic children, especially in the final approach. Here, we expand on those data by reporting saccade kinematics and temporal eye-hand coordination during visually guided reaching in children treated for strabismus compared with controls. Thirty children diagnosed with esotropia, a form of strabismus, 7 to 12 years of age and 32 age-similar control children were enrolled. Eye movements and index finger movements were recorded. While viewing binocularly, children reached out and touched a small dot that appeared randomly in one of four locations along the horizontal meridian (±5° or ±10°). Saccade kinematic measures (latency, accuracy and precision, peak velocity, and frequency of corrective and reach-related saccades) and temporal eye-hand coordination measures (saccade-to-reach planning interval, saccade-to-reach peak velocity interval) were compared. Factors associated with impaired performance were also evaluated. During visually guided reaching, strabismic children had longer primary saccade latency (strabismic, 195 ± 29 ms vs. control; 175 ± 23 ms; P = 0.004), a 25% decrease in primary saccade precision (0.15 ± 0.06 vs. 0.12 ± 0.03; P = 0.007), a 45% decrease in the final saccade precision (0.16 ± 0.06 vs. 0.11 ± 0.03; P < 0.001), and more reach-related saccades (16 ± 13% of trials vs. 8 ± 6% of trials; P = 0.001) compared with a control group. No measurable stereoacuity was related to poor saccade kinematics. Strabismus impacts saccade kinematics during visually guided reaching in children, with poor binocularity playing a role in performance. Coupled with previous data showing slow reaching in the final approach, the current saccade data suggest that children treated for strabismus have not yet adapted or formed an efficient compensatory strategy during visually guided reaching.

Cerebellar complex spikes multiplex complementary behavioral information.

Purkinje cell (PC) discharge, the only output of cerebellar cortex, involves 2 types of action potentials, high-frequency simple spikes (SSs) and low-frequency complex spikes (CSs). While there is consensus that SSs convey information needed to optimize movement kinematics, the function of CSs, determined by the PC’s climbing fiber input, remains controversial. While initially thought to be specialized in reporting information on motor error for the subsequent amendment of behavior, CSs seem to contribute to other aspects of motor behavior as well. When faced with the bewildering diversity of findings and views unraveled by highly specific tasks, one may wonder if there is just one true function with all the other attributions wrong? Or is the diversity of findings a reflection of distinct pools of PCs, each processing specific streams of information conveyed by climbing fibers? With these questions in mind, we recorded CSs from the monkey oculomotor vermis deploying a repetitive saccade task that entailed sizable motor errors as well as small amplitude saccades, correcting them. We demonstrate that, in addition to carrying error-related information, CSs carry information on the metrics of both primary and small corrective saccades in a time-specific manner, with changes in CS firing probability coupled with changes in CS duration. Furthermore, we also found CS activity that seemed to predict the upcoming events. Hence PCs receive a multiplexed climbing fiber input that merges complementary streams of information on the behavior, separable by the recipient PC because they are staggered in time.

Differential Change in Oculomotor Performance among Female Collegiate Soccer Players versus Non-Contact Athletes from Pre- to Post-Season.

Sensitive and reliable tools are needed to evaluate potential behavioral and cognitive changes following head impact exposure in contact and collision sport participation. We evaluated change in oculomotor testing performance among female, varsity, collegiate athletes following variable exposure to head impacts across a season. Female, collegiate, contact sport (soccer, CONT) and non-contact sport (NON-CONT) athletes were assessed pre-season and post-season. Soccer athletes were grouped according to total season game headers into low dose (≤40 headers; CONT-Low Dose) or high dose (>40 headers; CONT-High Dose) groups. Performance on pro-saccade (reflexive visual response), anti-saccade (executive inhibition), and memory-guided saccade (MGS, spatial working memory) computer-based laboratory tasks were assessed. Primary saccade measures included latency/reaction time, inhibition error rate (anti-saccade only), and spatial accuracy (MGS only). NON-CONT (n = 20), CONT-Low Dose (n = 17), and CONT-High Dose (n = 7) groups significantly differed on pre-season versus post-season latency on tasks with executive functioning demands (anti-saccade and MGS, p ≤ 0.001). Specifically, NON-CONT and CONT-Low Dose demonstrated shorter (i.e., faster) anti-saccade (1.84% and 2.68%, respectively) and MGS (5.74% and 2.76%, respectively) latencies from pre-season to post-season, whereas CONT-High Dose showed 1.40% average longer anti-saccade, and 0.74% shorter MGS, latencies. NON-CONT and CONT-Low Dose demonstrated reduced (i.e., improved) inhibition error rate on the anti-saccade task at post-season versus pre-season, whereas CONT-High Dose demonstrated relative stability (p = 0.021). The results of this study suggest differential exposure to subconcussive head impacts in collegiate female athletes is associated with differential change in reaction time and inhibitory control performances on executive saccadic oculomotor testing.

The temporal and spatial constraints of saccade planning to double-step target displacements

The double-step paradigm investigates the characteristics of planning and execution when the motor system must rapidly adjust for a new goal location. Studies have provided detailed temporal information based on the duration available for the motor system to prepare a new movement trajectory (here referred to as re-preparation time). However, previous work has largely examined single displacement sizes, limiting the spatiotemporal understanding of movement planning and execution. The lack of a description of this behavioral timecourse across increasing displacement sizes is true for saccades, rapid eye movements that redirect the fovea. Furthermore, during the double-step paradigm, the primary saccade often fails to accurately foveate the final target location and a secondary saccade brings the target onto the fovea. However, it is also unknown how this compensation is concurrently modified with the exposure duration and displacement of the movement goal. Here, we examined the amount of time required to change the initial saccade direction to a new target location for relatively small (20°, 30°, and 40°) and large (60° and 90°) target spatial separations. Interestingly, we found a clear relationship between the saccade direction and the amount of time allowed to redirect the movement; across separations, intermediate saccades occurred when approximately 60–140 ms was available to readjust the movement plan. Additionally, there was a consistent relationship between the timing of the secondary saccade and the re-preparation time across jump sizes, suggesting that concurrent movement correction planning was dependent on the amount of exposure to the final movement goal.

Reward Prediction Error Modulates Saccade Vigor.

Movement vigor, defined as the reciprocal of the latency from availability of reward to its acquisition, changes with reward magnitude: movements exhibit shorter reaction time and increased velocity when they are directed toward more rewarding stimuli. This invigoration may be due to release of dopamine before movement onset, which has been shown to be modulated by events that signal reward prediction error (RPE). Here, we generated an RPE event in the milliseconds before movement onset and tested whether there was a relationship between RPE and vigor. Human subjects (both sexes) made saccades toward an image. During execution of the primary saccade, we probabilistically changed the position and content of that image, encouraging a secondary saccade. On some trials, the content of the secondary image was more valuable than the first image, resulting in a positive RPE (+RPE) event that preceded the secondary saccade. On other trials, this content was less valuable (-RPE event). We found that reaction time of the secondary saccade was affected in an orderly fashion by the magnitude and direction of the preceding RPE event: the most vigorous saccades followed the largest +RPE, whereas the least vigorous saccades followed the largest -RPE. Presence of the secondary saccade indicated that the primary saccade had experienced a movement error, inducing trial-to-trial adaptation. However, this learning from movement error was not modulated by the RPE event. The data suggest that RPE events, which are thought to transiently alter the release of dopamine, modulate the vigor of the ensuing movement.SIGNIFICANCE STATEMENT Does dopamine release in response to a stimulus serve to invigorate the ensuing movement? To test this hypothesis, we relied on the fact that reward prediction error (RPE) is a strong modulator of dopamine. Our innovation was a task in which an RPE event occurred precisely before onset of a stimulus-driven movement. We probabilistically produced a combination of large or small, negative or positive RPE events and observed that saccade vigor carried a robust signature of the preceding RPE event: high vigor saccades followed +RPE events, whereas low vigor saccades followed -RPE events. This suggests that in humans, vigor is partly controlled through release of dopamine in the moments before onset of a movement.

Feature-based guidance of attention during post-saccadic selection

Current models of trans-saccadic perception propose that, after a saccade, the saccade target object must be localized among objects near the landing position. However, the nature of the attentional mechanisms supporting this process is currently under debate. In the present study, we tested whether surface properties of the saccade target object automatically bias post-saccadic selection using a variant of the visual search task. Participants executed a saccade to a shape-singleton target in a circular array. During this primary saccade, the array sometimes rotated so that the eyes landed between the target and an adjacent distractor, requiring gaze correction. In addition, each object in the array had an incidental color value. On Switch trials, the target and adjacent distractor switched colors. The accuracy and latency of gaze correction to the target (measures that provide a direct index of target localization) were compared with a control condition in which no color switch occurred (No-switch trials). Gaze correction to the target was substantially impaired in the Switch condition. This result was obtained even when participants had substantial incentive to avoid encoding the color of the saccade target. In addition, similar effects were observed when the roles of the two feature dimensions (color and shape) were reversed. The results indicate that saccade target features are automatically encoded before a saccade, are retained in visual working memory across the saccade, and instantiate a feature-based selection operation when the eyes land, biasing attention toward objects that match target features.

Processing speed in perceptual visual crowding.

In this study, a perceptual visual crowding paradigm was designed to quantitatively assess the detection speed of (un)crowded meaningful visual targets using eye-movement responses. This paradigm was tested in individuals with dyslexia and age-matched controls. Trials were shown on a monitor with an integrated eye tracker to 25 control and 11 dyslexic subjects without any known ocular problems. Each trial started with fixation of a central target. Next, four peripheral targets were shown (left, right, top, bottom), one being a duplicate of the central target. The duplicate was either surrounded by flankers (crowding trials) or shown in isolation (reference trials). The timing of the primary saccades were obtained as a measure for detection speed. The performance of the reference trials was significantly higher compared to the crowding trials (p < 0.05) and a 54% increase in saccadic reaction time (SRT) was found for the crowding trials. The linear mixed model revealed a significant effect of critical spacing and chart type. For the reference trials, no significant differences in SRT were found between dyslexic and control subjects. However, for the crowding trials, a significant increase of ∼13% in SRT was found in the dyslexic subjects. A first application of this paradigm showed that dyslexic subjects perform equally well in identifying visual targets in crowded as well as uncrowded scenes compared to controls. However, they seem to need more time to identify targets in crowded scenes, which might be related to the reading difficulties that they experience in general.

Saccade latencies during a preferential looking task and objective scoring of grating acuity in children with and without visual impairments.

PurposeWe determined the latencies of orienting responses during a preferential looking task in children with normal vision and in children with visual impairments between 6 and 12 years old, and assessed the feasibility of scoring grating detection in these populations with video‐based eye tracking.MethodsChildren performed a computerized preferential looking test, while a remote eye tracker measured the children's eye movements. The stimuli consisted of a 2 × 2 grid, with three uniform grey fields and one target field consisting of a black‐and‐white square wave grating. The grating was presented randomly at one of the four locations. The spatial frequencies (1.05, 2.11 and 7.02 cyc/deg) were randomly interleaved, with 10 trials per spatial frequency. Three different methods were used to score the accuracy of the responses: (1) primary saccade ends on target, (1) gaze 50% of the presentation time on target, and (3) a combination of method 1 and 2 (i.e. primary saccade ends on target, and/or gaze 50% of the presentation time on target).ResultsThe combined scoring method was most reliable to determine whether children could resolve the gratings. Children with visual impairments had significantly lower accuracies than children with normal vision with all three scoring methods. In addition, saccade latencies decreased with age and were significantly longer (62 ± 15 ms) in children with visual impairments.ConclusionThe use of eye tracking to assess grating detection with a preferential looking task in clinical populations provides valuable additional information, including objective detection measures and developmental delays in saccade latencies.

Effects of combat training on visuomotor performance in children aged 9 to 12 years - an eye-tracking study

BackgroundData on visuomotor performance in combat training and the effects of combat training on visuomotor performance are limited. This study aimed to investigate the effects of a specially designed combat sports (CS) training program on the visuomotor performance levels of children.MethodsA pre–post comparative design was implemented. A total of 26 students aged 9–12 years underwent 40-min CS training sessions twice a week for 8 weeks during their physical education classes. The CS training program was designed by a karate coach and a motor control specialist. The other 30 students continued their regular activities and were considered as a control group. Each student’s eye movement was monitored using an eye tracker, whereas the motor performance was measured using a target hitting system with a program-controlled microprocessor. The measurements were taken 8 weeks before (baseline), 1 day before (pretest), and 1 week after (posttest) the designated training program. The task used for evaluating these students was hitting or tracking random illuminated targets as rapidly as possible. A two-way analysis of variance [group(2) × time(3)] with repeated measures of time was performed for statistical analysis.ResultsFor the children who received combat training, although the eye response improvement was not significant, both the primary and secondary saccade onset latencies were significantly earlier compared to the children without combat training. Both groups of students exhibited improvement in their hit response times during the target hitting tasks.ConclusionThe current finding supported the notion that sports training efforts essentially enhance visuomotor function in children aged 9–12 years, and combat training facilitates an earlier secondary saccade onset.

Effects of wrist tendon vibration and eye movements on manual aiming.

In the present study, we investigated whether visual information mediates a proprioceptive illusion effect induced by muscle tendon vibration in manual aiming. Visual information was gradually degraded from a situation in which the targets were present and participants (n = 20; 22.3 ± 2.7years) were permitted to make saccadic eye movements to designated target positions, to a condition in which the targets were not visible and participants were required to perform cyclical aiming while fixating a point between the two target positions. Local tendon vibration applied to the right wrist extensor muscles induced an illusory reduction of 15% in hand movement amplitude. This effect was greater in the fixation than in the saccade condition. Both anticipatory control and proprioceptive feedback are proposed to contribute to the observed effects. The primary saccade amplitude was also reduced by almost 4% when muscle tendon vibration was locally applied to the wrist. These results confirm a tight link between eye movements and manual perception and action. Moreover, the impact of the proprioceptive illusion on the ocular system indicates that the interaction between systems is bidirectional.

Saccade adaptation deficits in developmental dyslexia suggest disruption of cerebellar-dependent learning

BackgroundEstimates of the prevalence of developmental dyslexia in the general population range from 5% to as many as 10%. Symptoms include reading, writing, and language deficits, but the severity and mix of symptoms can vary widely across individuals. In at least some people with dyslexia, the structure and function of the cerebellum may be disordered. Saccadic adaptation requires proper function of the cerebellum and brainstem circuitry and might provide a simple, noninvasive assay for early identification and sub-phenotyping in populations of children who may have dyslexia.MethodsChildren between the ages of 7 and 15 served as participants in this experiment. Fifteen had been diagnosed with developmental dyslexia and an additional 15 were typically developing children. Five of the participants diagnosed with dyslexia were also diagnosed with an attention deficit hyperactivity disroder and were excluded from further analyses. Participants performed in a saccadic adaptation task in which visual errors were introduced at the end of saccadic eye movements. The amplitudes of primary saccades were measured and plotted as a function of the order in which they occurred. Lines of best fit were calculated. Significant changes in the amplitude of primary saccades were identified.Results12/15 typically developing children had significant adaptation of saccade amplitude in this experiment. 1/10 participants with dyslexia appropriately altered saccade amplitudes to reduce the visual error introduced in the saccade adaptation paradigm.ConclusionsProper cerebellar function is required for saccadic adaptation, but in at least some children with dyslexia, cerebellar structure and function may be disordered. Consistent with this hypothesis, the data presented in this report clearly illustrate a difference in the ability of children with dyslexia to adapt saccade amplitudes in response to imposed visual errors. Saccadic adaptation might provide a noninvasive assay for early identification of dyslexia. Future work will determine whether reduced saccadic adaptation is pervasive in dyslexia or whether this identifies a sub-phenotype within the larger population of people identified with reading and language deficits.

Change in sensitivity to visual error in superior colliculus during saccade adaptation

Saccadic eye movements provide a valuable model to study the brain mechanisms underlying motor learning. If a target is displaced surreptitiously while a saccade is underway, the saccade appears to be in error. If the error persists gradual neuronal adjustments cause the eye movement again to land near the target. This saccade adaptation typically follows an exponential time course, i.e., adaptation speed slows as adaptation progresses, indicating that the sensitivity to error decreases during adaptation. Previous studies suggested that the superior colliculus (SC) sends error signals to drive saccade adaptation. The objective of this study is to test whether the SC error signal is related to the decrease in the error sensitivity during adaptation. We show here that the visual activity of SC neurons, which is induced by a constant visual error that drives adaptation, decreases during saccade adaptation. This decrease of sensitivity to visual error was not correlated with the changes of primary saccade amplitude. Therefore, a possible interpretation of this result is that the reduction of visual sensitivity of SC neurons contributes an error sensitivity signal that could help control the saccade adaptation process.

Eye Control Deficits Coupled to Hand Control Deficits: Eye-Hand Incoordination in Chronic Cerebral Injury.

It is widely accepted that cerebral pathology can impair ocular motor and manual motor control. This is true in indolent and chronic processes, such as neurodegeneration and in acute processes such as stroke or those secondary to neurotrauma. More recently, it has been suggested that disruptions in these control systems are useful markers for prognostication and longitudinal monitoring. The utility of examining the relationship or the coupling between these systems has yet to be determined. We measured eye and hand-movement control in chronic, middle cerebral artery stroke, relative to healthy controls, in saccade-to-reach paradigms to assess eye–hand coordination. Primary saccades were initiated significantly earlier by stroke participants relative to control participants. However, despite these extremely early initial saccades to the target, reaches were nevertheless initiated at approximately the same time as those of control participants. Control participants minimized the time period between primary saccade onset and reach initiation, demonstrating temporal coupling between eye and hand. In about 90% of all trials, control participants produced no secondary, or corrective, saccades, instead maintaining fixation in the terminal position of the primary saccade until the end of the reach. In contrast, participants with stroke increased the time period between primary saccade onset and reach initiation. During this temporal decoupling, multiple saccades were produced in about 50% of the trials with stroke participants making between one and five additional saccades. Reaches made by participants with stroke were both longer in duration and less accurate. In addition to these increases in spatial reach errors, there were significant increases in saccade endpoint errors. Overall, the magnitude of the endpoint errors for reaches and saccades were correlated across participants. These findings suggest that in individuals with otherwise intact visual function, the spatial and temporal relationships between the eye and hand are disrupted poststroke, and may need to be specifically targeted during neurorehabilitation. Eye–hand coupling may be a useful biomarker in individuals with cerebral pathology in the setting of neurovascular, neurotraumatic, and neurodegenerative pathology.

Amplitude and Width Manipulations in Fitts' Paradigm: Comment on Heath et al. (2011) and Heath et al. (2016)

ABSTRACTHeath, Weiler, Marriott, Elliott, and Binstead (2011) and Heath, Samani, Tremblay, and Elliott (2016) have claimed that there are dissociable movement times for amplitude and width manipulations in the Fitts' paradigm. This may to some extent be true, but the Heath et al. (2011) and Heath et al. (2016) data do not support that contention, as shown in this comment. It is shown that data from the research of Heath et al. is not in the range of index of difficulty where ongoing visual control would be used and hence their fits of data to Fitts' law are spurious. The data of Heath et al. (2011) for arm movements and for the primary eye saccade submovements of Heath et al. (2016) are well correlated by the ballistic movement time equation of Hoffmann (1981) and Gan and Hoffmann (1988). When the ballistic equation is used to model data for constant amplitude and constant target width conditions, the effects of the amplitude and width manipulations disappear. Other research indicates that the independent effects of movement amplitude occur due to changes in the time for the distance-covering phase of the movement.

Revealing the time course of signals influencing the generation of secondary saccades using Aalen’s additive hazards model

Saccadic eye movements are frequently followed by smaller secondary saccades which are generally assumed to correct for the error in primary saccade landing position. However, secondary saccades can also occur after accurate primary saccades and they are often as small as microsaccades, therefore raising the need to further scrutinize the processes involved in secondary saccade generation. Following up a previous study, we analyzed secondary saccades using rate analysis which allows us to quantify experimental effects as shifts in distributions, therefore going beyond comparisons of mean differences. We use Aalen’s additive hazards model to delineate the time course of key influences on the secondary saccade rate. In addition to the established effect of primary saccade error, we observed a time-varying influence of under- vs. overshooting – with a higher risk of generating secondary saccades following undershoots. Moreover, increasing target eccentricity influenced the programming of secondary saccades, therefore demonstrating that error-unrelated variables co-determine secondary saccade programs. Our results provide new insights into the generative mechanisms of small saccades during postsaccadic fixation that need to be accounted for by secondary saccade models.

Corrections in saccade endpoints scale to the amplitude of target displacements in a double-step paradigm

It is widely held that discrete goal-directed eye movements (saccades) are ballistic in nature because their durations are too short to allow for sensory-based online correction. Recent studies, however, have provided evidence that saccadic endpoints can be mediated via online corrections. Specifically, it has been reported that saccade trajectories adapt to the eccentricity of an unexpectedly perturbed target location (i.e., target ‘jump’ paradigm). If saccades are subject to online correction mechanisms, then the magnitude of such changes should scale to the amplitude of the target jump. To test this hypothesis, saccadic endpoints for trials on which the target jumped one of three amplitudes (Small: 2.5°, Medium: 5.0°, and Large: 7.5°; i.e., Jump trials) immediately after saccade onset were compared with the endpoints of trials in which the target location did not change (i.e., Reference trials). Results showed that primary saccade endpoints for Jump trials were longer than for Reference trials. Importantly, the magnitude of this increase in endpoint scaled with the amplitude of the target jump. Thus, these results support emerging and coalescent evidence that saccade trajectories are subject to online corrections.

Context cue-dependent saccadic adaptation in rhesus macaques cannot be elicited using color.

When the head does not move, rapid movements of the eyes called saccades are used to redirect the line of sight. Saccades are defined by a series of metrical and kinematic (evolution of a movement as a function of time) relationships. For example, the amplitude of a saccade made from one visual target to another is roughly 90% of the distance between the initial fixation point (T0) and the peripheral target (T1). However, this stereotypical relationship between saccade amplitude and initial retinal error (T1-T0) may be altered, either increased or decreased, by surreptitiously displacing a visual target during an ongoing saccade. This form of motor learning (called saccadic adaptation) has been described in both humans and monkeys. Recent experiments in humans and monkeys have suggested that internal (proprioceptive) and external (target shape, color, and/or motion) cues may be used to produce context-dependent adaptation. We tested the hypothesis that an external contextual cue (target color) could be used to evoke differential gain (actual saccade/initial retinal error) states in rhesus monkeys. We did not observe differential gain states correlated with target color regardless of whether targets were displaced along the same vector as the primary saccade or perpendicular to it. Furthermore, this observation held true regardless of whether adaptation trials using various colors and intrasaccade target displacements were randomly intermixed or presented in short or long blocks of trials. These results are consistent with hypotheses that state that color cannot be used as a contextual cue and are interpreted in light of previous studies of saccadic adaptation in both humans and monkeys.

Disrupting saccadic updating: visual interference prior to the first saccade elicits spatial errors in the secondary saccade in a double-step task.

When we explore the visual environment around us, we produce sequences of very precise eye movements aligning the objects of interest with the most sensitive part of the retina for detailed visual processing. A copy of the impending motor command, the corollary discharge, is sent as soon as the first saccade in a sequence is ready to monitor the next fixation location and correctly plan the subsequent eye movement. Neurophysiological investigations have shown that chemical interference with the corollary discharge generates a distinct pattern of spatial errors on sequential eye movements, with similar results also from clinical and TMS studies. Here, we used saccadic inhibition to interfere with the temporal domain of the first of two subsequent saccades during a standard double-step paradigm. In two experiments, we report that the temporal interference on the primary saccade led to a specific error in the final landing position of the second saccade that was consistent with previous lesion and neurophysiological studies, but without affecting the spatial characteristics of the first eye movement. On the other hand, single-step saccades were differently influence by the flash, with a general undershoot, more pronounced for larger saccadic amplitude. These findings show that a flashed visual transient can disrupt saccadic updating in a double-step task, possibly due to the mismatch between the planned and the executed saccadic eye movement.