Saccade accuracy as an indicator of the competition between functional asymmetries in vision.

Hemispheric specialization refers to the fact that cerebral hemispheres are not equivalent and that cognitive processes are lateralized in the brain. Although the potential links between handedness and the left hemisphere specialization for language have been widely studied, little attention has been paid to other motor preferences, such as eye dominance, that also are lateralized in the brain. For example, saccadic accuracy is higher in the hemifield contralateral to the dominant eye compared to the ipsilateral hemifield. Saccade accuracy is, however, also known to be sensitive to other functional asymmetries, such as the lateralization of visuo-spatial attention in the right hemisphere of the brain. Using a global effect paradigm in three different saccade latency ranges, we here propose to use saccade accuracy as an indicator of visual functional asymmetries. We show that for the shortest latencies, saccade accuracy is higher in the left than in the right visual hemifield, which could be due to the lateralization of visuo-spatial attention in the right hemisphere. For the longest latencies, however, saccade accuracy is higher toward the right than the left hemifield, probably due to the lateralization of local and global processing in the left and right hemispheres, respectively. These results could have a major impact on studies designed to measure the degree of lateralization of individuals. We here discuss both the theoretical and clinical contributions of these results.

Time dependency of the SNARC effect for different number formats: evidence from saccadic responses.

In line with the suggestion that the strength of the spatial numerical association of response codes (SNARC) effect was time dependent, the aim of the present study was to assess whether the association strength depends on the processing time of numerical quantity and/or of the time to initiate responses. More specifically, we examined whether and how the SNARC effect could be modulated by number format and effector type. Experiment 1 compared the effect induced by Arabic numbers and number words on the basis of saccadic responses in a parity judgment task. Indeed, previous studies have shown that Arabic numbers lead to faster processing than number words. The results replicated the SNARC effect with Arabic numbers, but not with number words. Experiment 2 was similar to Experiment 1, but this time manual responses (i.e., responses far slower than saccadic ones) were recorded. A strong SNARC effect was observed for both number formats. Further analyses revealed a correlation between mean individual response times and the strength of the SNARC effect. We proposed that the initiation times for saccadic responses may be too short for the SNARC effect to appear, in particular with the written number format for which activation of magnitude takes time. We conclude in terms of time variations resulting from processing specificities related with number format, effector type and also individual reaction and processing speed.

Recentering bias for temporal saccades only: Evidence from binocular recordings of eye movements.

It is well known that the saccadic system presents multiple asymmetries. Notably, temporal (as opposed to nasal) saccades, centripetal (as opposed to centrifugal) saccades (i.e., the recentering bias) and saccades from the abducting eye (as opposed to the concomitant saccades from the adducting eye) exhibit higher peak velocities. However, these naso-temporal and centripetal-centrifugal asymmetries have always been studied separately. It is thus unknown which asymmetry prevails when there is a conflict between both asymmetries, i.e., in case of centripetal nasal saccades or centrifugal temporal saccades. This study involved binocular recordings of eye movements to examine both the naso-temporal and centripetal-centrifugal asymmetries so as to determine how they work together. Twenty-eight participants had to make saccades toward stimuli presented either centrally or in the periphery in binocular conditions. We found that temporal and abducting saccades always exhibit higher peak velocities than nasal and adducting saccades, irrespective of their centripetal or centrifugal nature. However, we showed that the velocity advantage for centripetal saccades is only found for temporal and not for nasal saccades. Such a result is of importance as it could provide new insights about the physiological origins of the asymmetries found in the saccadic system.

Interference between oculomotor and postural tasks in 7-8-year-old children and adults.

Several studies in adults having observed the effect of eye movements on postural control provided contradictory results. In the present study, we explored the effect of various oculomotor tasks on postural control and the effect of different postural tasks on eye movements in eleven children (7.8 ± 0.5 years) and nine adults (30.4 ± 6.3 years). To vary the difficulty of the oculomotor task, three conditions were tested: fixation, prosaccades (reactive saccades made toward the target) and antisaccades (voluntary saccades made in the direction opposite to the visual target). To vary the difficulty of postural control, two postural tasks were tested: Standard Romberg (SR) and Tandem Romberg (TR). Postural difficulty did not affect oculomotor behavior, except by lengthening adults' latencies in the prosaccade task. For both groups, postural control was altered in the antisaccade task as compared to fixation and prosaccade tasks. Moreover, a ceiling effect was found in the more complex postural task. This study highlighted a cortical interference between oculomotor and postural control systems.

Development and learning of saccadic eye movements in 7- to 42-month-old children.

From birth, infants move their eyes to explore their environment, interact with it, and progressively develop a multitude of motor and cognitive abilities. The characteristics and development of oculomotor control in early childhood remain poorly understood today. Here, we examined reaction time and amplitude of saccadic eye movements in 93 7- to 42-month-old children while they oriented toward visual animated cartoon characters appearing at unpredictable locations on a computer screen over 140 trials. Results revealed that saccade performance is immature in children compared to a group of adults: Saccade reaction times were longer, and saccade amplitude relative to target location (10° eccentricity) was shorter. Results also indicated that performance is flexible in children. Although saccade reaction time decreased as age increased, suggesting developmental improvements in saccade control, saccade amplitude gradually improved over trials. Moreover, similar to adults, children were able to modify saccade amplitude based on the visual error made in the previous trial. This second set of results suggests that short visual experience and/or rapid sensorimotor learning are functional in children and can also affect saccade performance.

Characteristics of contralesional and ipsilesional saccades in hemianopic patients.

In order to further our understanding of action-blindsight, four hemianopic patients suffering from visual field loss contralateral to a unilateral occipital lesion were compared to six healthy controls during a double task of verbally reported target detection and saccadic responses toward the target. Three oculomotor tasks were used: a fixation task (i.e., without saccade) and two saccade tasks (eliciting reflexive and voluntary saccades, using step and overlap 600 ms paradigms, respectively), in separate sessions. The visual target was briefly presented at two different eccentricities (5° and 8°), in the right or left visual hemifield. Blank trials were interleaved with target trials, and signal detection theory was applied. Despite their hemifield defect, hemianopic patients retained the ability to direct a saccade toward their contralesional hemifield, whereas verbal detection reports were at chance level. However, saccade parameters (latency and amplitude) were altered by the defect. Saccades to the contralesional hemifield exhibited longer latencies and shorter amplitudes compared to those of the healthy group, whereas only the latencies of reflexive saccades to the ipsilesional hemifield were altered. Furthermore, healthy participants showed the expected latency difference between reflexive and voluntary saccades, with the latter longer than the former. This difference was not found in three out of four patients in either hemifield. Our results show action-blindsight for saccades, but also show that unilateral occipital lesions have effects on saccade generation in both visual hemifields.

The way we look at an image or a webpage can reveal personality traits.

Personality is a central concept and a cross-domain explanatory factor in psychology to characterize and differentiate individuals. Surprisingly, among the many studies on oculomotor behavior, only a few have investigated how personality influences the exploration of a visual stimulus. Due to the limited number of existing studies, it is still uncertain if markers of personality in eye movements are always observable in eye movements across various exploration contexts. Here, introducing a novel concept of gaze-based signatures of personality, we used visual exploration metrics to detect personality signatures across various exploration contexts (visual search and free-viewing on images and webpages) in 91 participants. Personality data were collected as in the reference paper that validated the French version of the Big Five Inventory. Linear regression analyses demonstrated that while Extraversion and Openness to Experience did not correlate with any particular exploration metric, the other three traits-Conscientiousness, Agreeableness, and Neuroticism-correlated robustly with all exploration metrics in different visual exploration contexts. Our study provides evidence for the capture of the gaze-based signature of personality from very brief eye movement recordings.

Perisaccadic compression in two-saccade sequences.

Around the onset of a saccade toward a target, localization judgments are systematically biased toward the saccade endpoint. This perisaccadic compression is thought to be related to transsaccadic reorganization and due to interfering motor signals in visual maps. It has, however, only been investigated for saccades targeting a single target. Here, we examined whether saccade-sequence programming to stationary target(s) would affect the pattern of localization judgments of a briefly flashed stimulus. We presented saccade targets that could induce either a single saccade or two-saccade sequences and we flashed a bar around saccade onsets. For all two-saccade-sequence conditions, we showed that localization judgments of the stimulus flashed around the first saccade onset are biased toward an intermediate position between the two-saccade landing position, indicating the influence of the second saccade's parallel planning. This implies that motor-planning signals are most likely responsible for perisaccadic compression than motor execution feedback.

Distractor-induced saccade trajectory curvature reveals visual contralateral bias with respect to the dominant eye.

The functional consequences of the visual system lateralization referred to as "eye dominance" remain poorly understood. We previously reported shorter hand reaction times for targets appearing in the contralateral visual hemifield with respect to the dominant eye (DE). Here, we further explore this contralateral bias by studying the influence of laterally placed visual distractors on vertical saccade trajectories, a sensitive method to assess visual processing. In binocular conditions, saccade trajectory curvature was larger toward a distractor placed in the contralateral hemifield with respect to the DE (e.g., in the left visual hemifield for a participant with a right dominant eye) than toward one presented in the ipsilateral hemifield (in the right visual hemifield in our example). When two distractors were present at the same time, the vertical saccade showed curvature toward the contralateral side. In monocular conditions, when one distractor was presented, a similar larger influence of the contralateral distractor was observed only when the viewing eye was the DE. When the non dominant eye (NDE) was viewing, curvature was symmetric for both distractor sides. Interestingly, this curvature was as large as the one obtained for the contralateral distractor when the DE was viewing, suggesting that eye dominance consequences rely on inhibition mechanisms present when the DE is viewing. Overall, these results demonstrate that DE influences visual integration occurring around saccade production and support a DE-based contralateral visual bias.

Similarities and Differences Between Eye and Mouse Dynamics During Web Pages Exploration.

The study of eye movements is a common way to non-invasively understand and analyze human behavior. However, eye-tracking techniques are very hard to scale, and require expensive equipment and extensive expertise. In the context of web browsing, these issues could be overcome by studying the link between the eye and the computer mouse. Here, we propose new analysis methods, and a more advanced characterization of this link. To this end, we recorded the eye, mouse, and scroll movements of 151 participants exploring 18 dynamic web pages while performing free viewing and visual search tasks for 20 s. The data revealed significant differences of eye, mouse, and scroll parameters over time which stabilize at the end of exploration. This suggests the existence of a task-independent relationship between eye, mouse, and scroll parameters, which are characterized by two distinct patterns: one common pattern for movement parameters and a second for dwelling/fixation parameters. Within these patterns, mouse and eye movements remained consistent with each other, while the scrolling behaved the opposite way.

Presaccadic attention interferes with feature detection.

Preparing a saccadic eye movement to a particular spatial location enhances the perception of visual targets at this location and decreases perception of nearby targets prior to movement onset. This effect has been termed the orientation of pre-saccadic attention. Here, we investigated whether pre-saccadic attention influenced the detection of a simple visual feature-a process that has been hypothesized to occur without the need for attention. Participants prepared a saccade to a cued location and detected the occurrence of a "pop-out" feature embedded in distracters at the same or different location. The results show that preparing a saccade to a given location decreased detection of features at non-aimed-for locations, suggesting that the selection of a location as the next saccade endpoint influences sensitivity to basic visual features across the visual field.

Adaptation of within-object saccades can be induced by changing stimulus size.

Saccadic adaptation maintains saccade accuracy and has been studied with the intrasaccadic target displacement procedure: displacing a target backwards (or forwards) during saccade execution gradually decreases (or increases) subsequent saccade amplitude. Adaptation has traditionally been studied with targeting saccades which bring the eyes onto a new object. Within-object saccades take the eye from one position in an object to another position in the same object and have been shown to resist the intrasaccadic target displacement procedure. The amplitude of within-object saccades depends on object size rather than position, and we therefore hypothesized that within-object saccades might adapt in response to an intrasaccadic change in object size. In separate sessions, we increased or decreased object size during within-object saccade execution. Results showed amplitude lengthening or shortening, respectively. Furthermore, within-object saccade adaptation seems to share several characteristics with targeting-saccade adaptation.

The spatial pattern of peri-saccadic compression for small saccades.

In the temporal vicinity of a saccade onset, visual stability is transiently disrupted and briefly flashed visual stimuli undergo a systematic perceptual mislocalization. Specifically, when a stimulus is flashed around saccade onset, localization judgments are grossly biased toward the saccade endpoint. This peri-saccadic compression increases with saccade amplitude. Previous studies of peri-saccadic compression have typically used rather large saccade amplitudes. In the present study, we investigate systematically the pattern of errors for small saccade sizes (2°-10°), taking into account both the amplitude of the saccade and the position of the flashed stimulus (11 positions tested from 1 to 12°). Our results show a weaker compression effect for the smallest saccades. Moreover, we found that the strength of the compression depends on both stimulus side and relative distance to the saccade target.

Saccadic adaptation shapes perceived size: Common codes for action and perception.

Recent findings suggest that perceptual and motor systems share common codes; for instance, perceived object location is known to correlate with motor changes in the oculomotor system. Here, we investigate whether modifying saccade amplitude affects object size perception. Participants saw in peripheral vision a test disk that could vary in size across trials. This disk was then replaced by a small target cross, which was the signal to make a saccade. After the saccade, the target cross was extinguished and replaced by a reference disk (thus seen in foveal vision). Participants had to compare the post- to the pre-saccade disk sizes. Psychometric functions were obtained before and after one session of 142 saccades made toward the cross that either stepped toward the fixation point during the saccade (backward adaptation group) or remained stationary (control group). In the experimental group, stepping the target cross toward fixation during saccades decreased movement amplitude, a phenomenon called saccadic adaptation. We observed a concurrent shift in the psychometric functions reflecting a decrease in perceived object size. Such a perceptual modification did not occur in the control group. Our results reveal that motor changes co-occur with changes in perceived object size. Unlike previous studies evaluating the impact of saccadic adaptation on perceived location, we measured here the perception of another spatial feature (the object size) that is not relevant for the sensorimotor transformation. Theoretical implications of the strong links between oculomotor parameters and object perception are discussed.

Saccade dynamics before, during, and after saccadic adaptation in humans.

PURPOSE: When saccade amplitude is systematically inadequate relative to the desired target position, the saccadic system adaptively modifies the amplitude of subsequent saccades so as to recover precise targeting capabilities. The effect of saccadic adaptation on saccade metrics (amplitude, direction) is well documented, but the effect on dynamics (velocity, duration, acceleration, deceleration) remains to be fully elucidated. METHODS: The dynamics of adapted saccades were compared with that of baseline saccades of similar amplitudes. RESULTS: The peak deceleration and skewness (duration of the acceleration period/duration of the deceleration period) were modified by adaptation. CONCLUSIONS: The results point toward involvement of the cerebellum rather than the brain stem saccade generator in human saccadic adaptation.

Quantifying eye dominance strength - New insights into the neurophysiological bases of saccadic asymmetries.

The saccadic system presents asymmetries. Notably, saccadic peak velocity is higher in temporal than in nasal saccades, and in centripetal than in centrifugal saccades. It has already been shown that eye dominance strength relates to naso-temporal asymmetry, but its links with centripetal-centrifugal asymmetry has never been tested. The current study tested both naso-temporal and centripetal-centrifugal asymmetries simultaneously to provide a finer and continuous measure of eye dominance strength. We asked 63 participants to make centripetal and centrifugal saccades from five different locations. Analysis of saccadic peak velocity shows that eye dominance strength modulates every saccadic asymmetry tested. For the first time, we propose a graduated measure of eye dominance strength on a continuum model. The model ranges from weak to very strong eye dominance. Weak eye dominance corresponds to increased saccadic asymmetries whereas strong eye dominance corresponds to no asymmetries. Furthermore, our results provide new insights into the neurophysiological origins of saccadic asymmetries. Modulation of both naso-temporal and centripetal-centrifugal asymmetries by eye dominance strength supports the involvement of V1 in these saccadic asymmetries.

Motor space structures perceptual space: evidence from human saccadic adaptation.

Saccadic adaptation is the progressive correction of systematic saccade targeting errors. When a saccade to a particular target is adapted, saccades within a spatial window around the target, the adaptation field, are affected as a function of their distance from the adapted target. Furthermore, previous studies suggest that saccadic adaptation might modify the perceptual localization of objects in space. We investigated the localization of visual probes before and after saccadic adaptation, and examined whether the spatial layout of the observed mislocalizations was structurally similar to the saccadic adaptation field. We adapted a horizontal saccade directed towards a target 12 degrees to the right. Thirty-eight saccades towards the right visual hemifield were then used to measure the adaptation field. The adaptation field was asymmetric: transfer of adaptation to saccades larger than the adapted saccade was greater than transfer to smaller saccades. Subjects judged the localization of 39 visual probes both within and outside the adaptation field. The perceived localization of a probe at a given position was proportional to the amount of transfer from the adapted saccade to the saccade towards that position. This similar effect of saccadic adaptation on both the action and perception representations of space suggests that the system providing saccade metrics also contributes to the metric used for the perception of space.

Saccadic adaptation depends on object selection: evidence from between- and within-object saccadic eye movements.

The accuracy of saccadic eye movements is maintained by saccadic adaptation. Post-saccadic visual feedback about the error between the target position and the saccade endpoint is crucial to the adaptive process. The present experiments examine the adaptation of saccades that select a new target object (between-object saccades) and that of saccades that would not aim for a selected target but execute a fixed motor vector (within-object saccades). We show that the post-saccadic visual error, induced by the intra-saccadic back step, leads to the adaptation of between-object saccades but not of within-object saccades. Furthermore, between-object saccade adaptation does not transfer to within-object saccades. These results suggest that saccadic adaptation depends on the selection of a precise target object.

Asymmetry in visual information processing depends on the strength of eye dominance.

Unlike handedness, sighting eye dominance, defined as the eye unconsciously chosen when performing monocular tasks, is very rarely considered in studies investigating cerebral asymmetries. We previously showed that sighting eye dominance has an influence on visually triggered manual action with shorter reaction time (RT) when the stimulus appears in the contralateral visual hemifield with respect to the dominant eye (Chaumillon et al. 2014). We also suggested that eye dominance may be more or less pronounced depending on individuals and that this eye dominance strength could be evaluated through saccadic peak velocity analysis in binocular recordings (Vergilino-Perez et al. 2012). Based on these two previous studies, we further examine here whether the strength of the eye dominance can modulate the influence of this lateralization on manual reaction time. Results revealed that participants categorized as having a strong eye dominance, but not those categorized as having a weak eye dominance, exhibited the difference in RT between the two visual hemifields. This present study reinforces that the analysis of saccade peak velocity in binocular recordings provides an effective tool to better categorize the eye dominance. It also shows that the influence of eye dominance in visuo-motor tasks depends on its strength. Our study also highlights the importance of considering the strength of eye dominance in future studies dealing with brain lateralization.

The use of recurrent signals about adaptation for subsequent saccade programming depends on object structure.

Executing sequences of accurate saccadic eye movements supposes the use of signals carrying information about the first saccade for updating the predetermined motor plan of the subsequent saccades. The present study examines the signals used in planning a second saccade when subjects made two successive saccades towards one long or two short peripheral objects displayed before the first saccade execution. Different first eye movement signals could be used: desired eye movement signals, representing the movement necessary for attaining the intended target, or actual eye movement signals, representing the movement actually executed. Experimental dissociation of desired and actual eye movement signals is made possible by adaptive modifications of the first saccade, obtained by transfer of single saccade adaptation, during which the motor vector was progressively modified in response to the systematic intra-saccadic step of a single target. Whether the second saccade used the actual eye movement signal to compensate or not for the adaptive changes in the first saccade depended on which object properties were relevant for saccade planning. Compensation was observed for saccades that aimed for a new object (between-object saccades) because adaptation modifies relative object location. No compensation was observed for saccades that explored an extended object (within-object saccades). Implications for the on-line control of subsequent eye movements are discussed.