Effect of Saccadic Adaptation on Sequences of Saccades

Accuracy of saccadic eye movements is maintained thanks to adaptation mechanisms. The adaptive lengthening and shortening of reactive and voluntary saccades rely on partially separate neural substrates. Although in daily-life we mostly perform sequences of saccades, the effect of saccadic adaptation has been mainly evaluated on single saccades. Here, sequences of two saccades were recorded before and after adaptation of rightward saccades. In 4 separate sessions, reactive and voluntary saccades were adaptively shortened or lengthened. We found that the second saccade of the sequence always remained accurate and compensated for the adaptive changes of the first rightward saccade size. This finding suggests that adaptation loci are upstream of the site where the efference copy involved in sequence planning originates

Virtual reality set-up for studying vestibular function during head impulse test

ObjectivesVirtual reality (VR) offers an ecological setting and the possibility of altered visual feedback during head movements useful for vestibular research and treatment of vestibular disorders. There is however no data quantifying vestibulo-ocular reflex (VOR) during head impulse test (HIT) in VR. The main objective of this study is to assess the feasibility and performance of eye and head movement measurements of healthy subjects in a VR environment during high velocity horizontal head rotation (VR-HIT) under a normal visual feedback condition. The secondary objective is to establish the feasibility of VR-HIT recordings in the same group of normal subjects but under altered visual feedback conditions.DesignTwelve healthy subjects underwent video HIT using both a standard setup (vHIT) and VR-HIT. In VR, eye and head positions were recorded by using, respectively, an imbedded eye tracker and an infrared motion tracker. Subjects were tested under four conditions, one reproducing normal visual feedback and three simulating an altered gain or direction of visual feedback. During these three altered conditions the movement of the visual scene relative to the head movement was decreased in amplitude by 50% (half), was nullified (freeze) or was inverted in direction (inverse).ResultsEye and head motion recording during normal visual feedback as well as during all 3 altered conditions was successful. There was no significant difference in VOR gain in VR-HIT between normal, half, freeze and inverse conditions. In the normal condition, VOR gain was significantly but slightly (by 3%) different for VR-HIT and vHIT. Duration and amplitude of head impulses were significantly greater in VR-HIT than in vHIT. In all three altered VR-HIT conditions, covert saccades were present in approximatively one out of four trials.ConclusionOur VR setup allowed high quality recording of eye and head data during head impulse test under normal and altered visual feedback conditions. This setup could be used to investigate compensation mechanisms in vestibular hypofunction, to elicit adaptation of VOR in ecological settings or to allow objective evaluation of VR-based vestibular rehabilitation

Sensory Processing of Motor Inaccuracy Depends on Previously Performed Movement and on Subsequent Motor Corrections: A Study of the Saccadic System

When goal-directed movements are inaccurate, two responses are generated by the brain: a fast motor correction toward the target and an adaptive motor recalibration developing progressively across subsequent trials. For the saccadic system, there is a clear dissociation between the fast motor correction (corrective saccade production) and the adaptive motor recalibration (primary saccade modification). Error signals used to trigger corrective saccades and to induce adaptation are based on post-saccadic visual feedback. The goal of this study was to determine if similar or different error signals are involved in saccadic adaptation and in corrective saccade generation. Saccadic accuracy was experimentally altered by systematically displacing the visual target during motor execution. Post-saccadic error signals were studied by manipulating visual information in two ways. First, the duration of the displaced target after primary saccade termination was set at 15, 50, 100 or 800 ms in different adaptation sessions. Second, in some sessions, the displaced target was followed by a visual mask that interfered with visual processing. Because they rely on different mechanisms, the adaptation of reactive saccades and the adaptation of voluntary saccades were both evaluated. We found that saccadic adaptation and corrective saccade production were both affected by the manipulations of post-saccadic visual information, but in different ways. This first finding suggests that different types of error signal processing are involved in the induction of these two motor corrections. Interestingly, voluntary saccades required a longer duration of post-saccadic target presentation to reach the same amount of adaptation as reactive saccades. Finally, the visual mask interfered with the production of corrective saccades only during the voluntary saccades adaptation task. These last observations suggest that post-saccadic perception depends on the previously performed action and that the differences between saccade categories of motor correction and adaptation occur at an early level of visual processing

Long-lasting modifications of saccadic eye movements following adaptation induced in the double-step target paradigm

The adaptation of saccadic eye movements to environmental changes occurring throughout life is a good model of motor learning and motor memory. Numerous studies have analyzed the behavioral properties and neural substrate of oculomotor learning in short-term saccadic adaptation protocols, but to our knowledge, none have tested the persistence of the oculomotor memory. In the present study, the double-step target protocol was used in five human subjects to adaptively decrease the amplitude of reactive saccades triggered by a horizontally-stepping visual target. We tested the amplitude of visually guided saccades just before and at different times (up to 19 days) after the adaptation session. The results revealed that immediately after the adaptation session, saccade amplitude was significantly reduced by 22% on average. Although progressively recovering over days, this change in saccade gain was still statistically significant on days 1 and 5, with an average retention rate of 36% and 19%, respectively. On day 11, saccade amplitude no longer differed from the pre-adaptation value. Adaptation was more effective and more resistant to recovery for leftward saccades than for rightward ones. Lastly, modifications of saccade gain related to adaptation were accompanied by a decrease of both saccade duration and peak velocity. A control experiment indicated that all these findings were specifically related to the adaptation protocol, and further revealed that no change in the main sequence relationships could be specifically related to adaptation. We conclude that in humans, the modifications of saccade amplitude that quickly develop during a double-step target adaptation protocol can remain in memory for a much longer period of time, reflecting enduring plastic changes in the brain

Compensation for gaze perturbation during inactivation of the caudal fastigial nucleus in the head-unrestrained cat.

International audienceMuscimol injection in the caudal part of the fastigial nucleus (cFN) leads, in the head-unrestrained cat, to a characteristic dysmetria of saccadic gaze shifts toward visual targets. The goal of the current study was to test whether this pharmacological cFN inactivation impaired the ability to compensate for unexpected perturbations in gaze position during the latency period of the saccadic response. Such perturbations consisted of moving gaze away from the target by a transient electrical microstimulation in the deep layers of the superior colliculus simultaneously with extinction of the visual target. After injection of muscimol in the cFN, targets located in the contralesional hemifield elicited gaze shifts that fell short of the target in both "perturbed" and "unperturbed" trials. The amplitude of the compensatory contraversive gaze shifts in perturbed trials coincided with the predicted amplitude of unperturbed responses starting from the same position. Targets located in the opposite hemifield elicited hypermetric gaze shifts in both trial types, and the error of compensatory responses was not statistically different from that of unperturbed gaze shifts. These results indicate that inactivation of the cFN does not interfere with the ability of the head-unrestrained cat to compensate for ipsiversive or contraversive perturbations in gaze position. Thus the gaze-related feedback signals that are used to compute a reference signal of desired gaze displacement are not impaired by cFN inactivation

On-line compensation of gaze shifts perturbed by micro-stimulation of the superior colliculus in the cat with unrestrained head

International audiencePrior studies have led to the gaze feedback hypothesis, which states that quick orienting movements of the visual axis (gaze shifts) are controlled by a feedback system. We have previously provided evidence for this hypothesis by extending the original study of Mays and Sparks (1980) to the cat with unrestrained head (Pélisson et al. 1989). We showed that cats compensated for a stimulation-induced perturbation of initial gaze position by generating, in the dark, an accurate gaze shift towards the remembered location of a flashed target. In the present study, we investigate goal-directed gaze shifts perturbed “in flight” by a brief stimulation of the superior colliculus. The microstimulation parameters were tuned such that significant perturbations were induced without halting the movement. The ambient light was turned off at the onset of the gaze shift, suppressing any visual feedback. We observed that, following stimulation offset, the gaze shift showed temporal and spatial changes in its trajectory to compensate for the transient perturbation. Such compensations, which occurred “on-line” before gaze shift termination, involved both eye and head movements and had dynamic characteristics resembling those of unperturbed saccadic gaze shifts. These on-line compensations maintained gaze accuracy when the stimulation was applied during the early phase of large and medium (about 60 and 40°) movements. These results are compatible with the notion of a gaze feedback loop providing a dynamic gaze error signal

New insights on eye blindness and hand sight: Temporal constraints of visuo-motor networks

International audiencePioneer experiments on saccadic suppression have shown that this effect is not followed by motor disorientation: Conscious perception of a target displacement can be dissociated from correct manual target reaching. It has subsequently been demonstrated that movement corrections with the same latency and spatial characteristics can be produced in conditions allowing perceptual awareness of perturbation of a target as in condition inducing saccadic suppression. In addition to the qualitative dissociation between motor performance and conscious awareness , quantitative temporal dissociations in action can be observed by manipulating different features of the visual target. When the target of an ongoing simple action is perturbed, a temporal advantage is found for responses to perturbations of location relative to colour and shape. Furthermore, there seems to be a temporal advantage for automatic motor corrections made in response to a target displacement as compared to other responses (other ongoing movement adjustments , movement interruption, conditional motor response such as pressing a key, verbal response, delayed matching-to-sample tasks). Thus, this paper reviews evidence for the fact that the temporal characteristics of any given response to a stimulus are dependent both on the sensory processes and on the type of response generated. Accordingly, identification responses (such as verbal report) to a visual stimulus are much slower than motor corrections of an ongoing movement in response to a target location change because of different processing times of the stimulus features ("What" compared to "Where") and of the response itself ("What" compared to "How"). The existence of two continua (What/Where and What/How) is proposed between these two extreme stimulus-response combinations. This model may be a useful framework to better understand visuo-motor transformations and the network of connections between visual and motor areas

Control of saccadic eye movements and combined eye/head gaze shifts by the medio-posterior cerebellum

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The posterior parietal cortex processes visuo-spatial and extra-retinal information for saccadic remapping: A case study

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