Adaptation of egocentric distance perception under telestereoscopic viewing within reaching space.

International audienceTelestereoscopic viewing provides a method to distort egocentric distance perception by artificially increasing the interpupillary distance. Adaptation to such a visual rearrangement is little understood. Two experiments were performed in order to dissociate the effects of a sustained increased vergence demand, from those of an active calibration of the vergence/distance mapping. Egocentric distances were assessed within reaching space through open-loop pointing to small targets in the dark. During the exposure condition of the first experiment, subjects were instructed to point to the targets without feedback, whereas in the second experiment, hand visual feedback was available, resulting in a modified relationship between vergence-specified distance and reach distance. The visual component of adaptation in the second experiment was assessed on the unexposed hand. In the post-tests of both experiments, subjects exhibited a constant distance overestimation across all targets, with a more than twice larger aftereffect in the second one. These findings suggest two different processes: (1) an alteration in the vergence effort following sustained increased vergence; (2) a calibration of the vergence/distance mapping uncovering the visual component of adaptation

Sensory Prediction of Limb Movement Is Critical for Automatic Online Control

International audienceFast, online control of movement is an essential component of human motor skills, as it allows automatic correction of inaccurate planning. The present study explores the role of two types of concurrent signals in error correction: predicted visual reafferences coming from an internal representation of the hand, and actual visual feedback from the hand. While the role of sensory feedback in these corrections is well-established, much less is known about sensory prediction. The relative contributions of these two types of signals remain a subject of debate, as they are naturally interconnected. We address the issue in a study that compares online correction of an artificially induced, undetected planning error. Two conditions are tested, which only differ with respect to the accuracy of predicted visual reafferences. In the first, "Prism" experiment, a planning error is introduced by prisms that laterally displace the seen hand prior to hand movement onset. The prism-induced conflict between visual and proprioceptive inputs of the hand also generates an erroneous prediction of visual reafferences of the moving hand. In the second, "Jump" experiment, a planning error is introduced by a jump in the target position, during the orienting saccade, prior to hand movement onset. In the latter condition, predicted reafferences of the hand remained intact. In both experiments, after hand movement onset, the hand was either visible or hidden, which enabled us to manipulate the presence (or absence) of visual feedback during movement execution. The Prism experiment highlighted late and reduced correction of the planning error, even when natural visual feedback of the moving hand was available. In the Jump experiment, early and automatic corrections of the planning error were observed, even in the absence of visual feedback from the moving hand. Therefore, when predicted reafferences were accurate (the Jump experiment), visual feedback was processed rapidly and automatically. When they were erroneous (the Prism experiment), the same visual feedback was less efficient, and required voluntary, and late, control. Our study clearly demonstrates that in natural environments, reliable prediction is critical in the preprocessing of visual feedback, for fast and accurate movement

Breaking the boundaries of somatosensory plasticity: improved touch at the fingers transfers to the lips

Selected as Hot TopicInternational audienceIt is now well established that cortical plasticity occurs in the adult brain and that plastic phenomena are involved in many brain functions. Understanding the rules governing plasticity and its functional consequences is extremely challenging. A cutaneous coactivation (CA) protocol has been successfully used to study somatosensory plasticity (see Godde et al, 2003). CA relies on the Hebbian postulate and consists of synchronous and passive coactivation of several non-overlapping receptive fields which produces transient somatosensory changes. The aim of this study was to evaluate whether pure somatosensory plasticity, experimentally induced at a specific cutaneous location (i.e., right index fingertip) can cross somatotopically defined boundaries and functionally affect physically distant, but cortically adjacent body-part regions (i.e., the perioral lip region). A two-point discrimination (2PD) task was used to determine the spatial discrimination thresholds at the right and left index finger (RD2 & LD2), and at the right and left sides of the upper lip area. This test was performed during 4 sessions on 2 consecutive days. Between the third and fourth session, a vibrotactile CA was applied for three hours at the RD2 fingertip. Thirty participants were randomly attributed to either of two groups: a test group (n=15, mean age = 20.53 ± 2.26 years), in which subjects received a true CA at the RD2 fingertip; and a control group (n=15, mean age = 23.5 ± 3.25 years), in which the CA was replaced by a sham CA (stimulator fixed at the fingertip but turned off). Results showed that in the test group three hours of CA led to a significant decrease of the 2PD threshold at RD2 fingertip, from 1.77 ± 0.28 mm to 1.52 ± 0.43 mm (mean decrease of 15.26%), thus replicating previous results. Critically, a significant 2PD threshold decrease was also found at both sides of the upper lip area (from 6.02 ± 0.88 mm to 5.44 ± 1.24 mm, mean decrease of 10.29%). No effects were found at either the LD2 fingertip or at any body-part region of the control group. These results demonstrate that experimentally-induced plasticity following RD2 coactivation improves participants’ tactile discrimination performance not only at the coactivated body site, but also at cortically adjacent body sites. Thus, CA-induced somatosensory plasticity appears to cross somatotopically defined boundaries between cortical body-parts representations, spreading its functional consequences to cortically adjacent, but peripherally unstimulated body parts

Saccades and Eye–Head Coordination in Ataxia with Oculomotor Apraxia Type 2

International audienceAtaxia with oculomotor apraxia type 2 (AOA2) is one of the most frequent autosomal recessive cerebellar ataxias. Oculomotor apraxia refers to horizontal gaze failure due to deficits in voluntary/reactive eye movements. These deficits can manifest as increased latency and/or hypometria of saccades with a staircase pattern and are frequently associated with compensatory head thrust movements. Oculomotor disturbances associated with AOA2 have been poorly studied mainly because the diagnosis of oculomotor apraxia was based on the presence of compensatory head thrusts. The aim of this study was to characterise the nature of horizontal gaze failure in patients with AOA2 and to demonstrate oculomotor apraxia even in the absence of head thrusts. Five patients with AOA2, without head thrusts, were tested in saccadic tasks with the head restrained or free to move and their performance was compared to a group of six healthy participants. The most salient deficit of the patients was saccadic hypometria with a typical staircase pattern. Saccade latency in the patients was longer than controls only for memory-guided saccades. In the head-free condition, head movements were delayed relative to the eye and their amplitude and velocity were strongly reduced compared to controls. Our study emphasises that in AOA2, hypometric saccades with a staircase pattern are a more reliable sign of oculomotor apraxia than head thrust movements. In addition, the variety of eye and head movements’ deficits suggests that, although the main neural degeneration in AOA2 affects the cerebellum, this disease affects other structures

Saccadic Adaptation Boosts Ongoing Gamma Activity in a Subsequent Visuoattentional Task

Attention and saccadic adaptation (SA) are critical components of visual perception, the former enhancing sensory processing of selected objects, the latter maintaining the eye movements accuracy toward them. Recent studies propelled the hypothesis of a tight functional coupling between these mechanisms, possibly due to shared neural substrates. Here, we used magnetoencephalography to investigate for the first time the neurophysiological bases of this coupling and of SA per se. We compared visual discrimination performance of 12 healthy subjects before and after SA. Eye movements and magnetic signals were recorded continuously. Analyses focused on gamma band activity (GBA) during the pretarget period of the discrimination and the saccadic tasks. We found that GBA increases after SA. This increase was found in the right hemisphere for both postadaptation saccadic and discrimination tasks. For the latter, GBA also increased in the left hemisphere. We conclude that oculomotor plasticity involves GBA modulation within an extended neural network which persists after SA, suggesting a possible role of gamma oscillations in the coupling between SA and attention.status: publishe

Saccadic adaptation boosts ongoing gamma activity in a subsequent visuo-attentional task

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