Sensory Prediction of Limb Movement Is Critical for Automatic Online Control.

Fast, 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.

Do prism and other adaptation paradigms really measure the same processes?

Sensorimotor plasticity allows the nervous system to set up appropriate motor and sensory compensations when individuals face changing demands in a given motor task. A much-debated question in neuroscience research is the identification of processes that encompass this capacity of plasticity. Prism adaptation is the oldest experimental paradigm that has been used to achieve this goal (Helmholtz, 1867).

Adapting terminology: clarifying prism adaptation vocabulary, concepts, and methods.

When individuals are exposed to a constant change of the interplay with their environment, they are able to develop compensatory alterations of visuo-motor coordination in order to counteract the perturbation. Prism adaptation (PA) is a very simple tool that has been used for several decades to investigate adaptive processes. However, the specific terminology used in PA literature has continuously evolved and is still subjected to broad inconsistency.

Paradoxical adaptation of successful movements: The crucial role of internal error signals.

Visuo-motor adaptation has been classically studied using movements aimed at visual targets with visual feedback. In this type of experimental design, the respective roles of the different error signals cannot be fully disentangled. Here, we show that visuo-motor adaptation occurs despite the terminal success of the action and the compensation of the external error by a jump of the visual target.

Visuomotor adaptation needs a validation of prediction error by feedback error.

The processes underlying short-term plasticity induced by visuomotor adaptation to a shifted visual field are still debated. Two main sources of error can induce motor adaptation: reaching feedback errors, which correspond to visually perceived discrepancies between hand and target positions, and errors between predicted and actual visual reafferences of the moving hand. These two sources of error are closely intertwined and difficult to disentangle, as both the target and the reaching limb are simultaneously visible.

Automatic online control of motor adjustments in reaching and grasping.

Following the princeps investigations of Marc Jeannerod on action-perception, specifically, goal-directed movement, this review article addresses visual and non-visual processes involved in guiding the hand in reaching or grasping tasks. The contributions of different sources of correction of ongoing movements are considered; these include visual feedback of the hand, as well as the often-neglected but important spatial updating and sharpening of goal localization following gaze-saccade orientation. The existence of an automatic online process guiding limb trajectory toward its goal is highlighted by a series of princeps experiments of goal-directed pointing movements.

Kinematic markers dissociate error correction from sensorimotor realignment during prism adaptation.

This study investigated the motor control mechanisms that enable healthy individuals to adapt their pointing movements during prism exposure to a rightward optical shift. In the prism adaptation literature, two processes are typically distinguished. Strategic motor adjustments are thought to drive the pattern of rapid endpoint error correction typically observed during the early stage of prism exposure.

How perceived egocentric distance varies with changes in tonic vergence.

According to the eye muscle potentiation (EMP) hypothesis, sustained vergence leads to changes in egocentric perceived distance. This perceptual effect has been attributed to a change in the resting or tonic state of vergence. The goal of the present study was to test the EMP hypothesis by quantifying the relationship between prism-induced changes in tonic vergence and corresponding changes in perceived distance and by measuring the dynamics of changes in perceived distance.

Generalization properties of a "saccadic-like" hand-reaching adaptation along a single degree of freedom.

Visuomotor-adaptation experiments devoted to the study of plasticity are also used to indirectly test hypotheses about how the brain encodes the spatio-temporal characteristics of arm movement directed at a visual target. A current major theory, the vectorial coding hypothesis, postulates that arm movements are processed differentially for direction and amplitude. This approach, at first developed in an extrinsic Cartesian frame of references, has been also adopted in an intrinsic joint space.

Saccadic-like visuomotor adaptation involves little if any perceptual effects.

Studies on visuomotor adaptation provide crucial clues on the functional properties of the human motor system. The widely studied saccadic adaptation paradigm is a major example of such a fruitful field of investigation. Magescas and Prablanc (J Cogn Neurosci 18(1):75-83, 2006) proposed a transposition of this protocol to arm pointing behavior, by designing an experiment in which the informational context of the upper limb visuomotor system is comparable to that of the saccadic system.

Integration of visual information for saccade production.

To foveate a visual target, subjects usually execute a primary hypometric saccade (S1) bringing the target in perifoveal vision, followed by a corrective saccade (S2) or by more than one S2. It is still debated to what extent these S2 are pre-programmed or dependent only on post-saccadic retinal error. To answer this question, we used a visually-triggered saccade task in which target position and target visibility were manipulated.

Partitioning the components of visuomotor adaptation to prism-altered distance.

While the mechanisms of short-term adaptation to prism-altered apparent visual direction have been widely investigated, the processes underlying adaptation to prism-altered perceived distance are less well known. This study used a hand-pointing paradigm and exposure with base-out prisms to evaluate the relative contributions of sensory (visual and proprioceptive) and motor components of adaptation to perceived-distance alteration. A main experiment was designed to elicit adaptation at the sensory and motor levels, by giving subjects altered visual feedback.

Pointing to double-step visual stimuli from a standing position: very short latency (express) corrections are observed in upper and lower limbs and may not require cortical involvement.

How fast can we correct a planned movement following an unexpected target jump? Subjects, starting in an upright standing position, were required to point to a target that randomly and unexpectedly jumps forward to a constant spatial location. Rapid motor corrections in the upper and lower limbs, with latency responses of less than 100 ms, were revealed by contrasting electromyographic activities in perturbed and unperturbed trials. The earliest responses were observed primarily in the anterior section of the deltoïdus anterior (shoulder) and the tibialis anterior (leg) muscles.

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

Telestereoscopic 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.

Two modes of error processing in reaching.

Several processes are devoted to error reduction for the production of purposeful actions. When motor responses deviate from their goal, online corrections can be performed either under voluntary control with additional sub-movements or under fast automatic control with smooth velocity profiles. When errors cannot be corrected online and are repeated over trials, subsequent responses can be improved iteratively through adaptation, a progressive adjustment of motor commands that acts to reduce the magnitude of error.

Saccade control and eye-hand coordination in optic ataxia.

The aim of this work was to investigate ocular control in patients with optic ataxia (OA). Following a lesion in the posterior parietal cortex (PPC), these patients exhibit a deficit for fast visuo-motor control of reach-to-grasp movements. Here, we assessed the fast visuo-motor control of saccades as well as spontaneous eye-hand coordination in two bilateral OA patients and five neurologically intact controls in an ecological "look and point" paradigm.

Enhancing visuomotor adaptation by reducing error signals: single-step (aware) versus multiple-step (unaware) exposure to wedge prisms.

Neglect patients exhibit both a lack of awareness for the spatial distortions imposed during visuomanual prism adaptation procedures, and exaggerated postadaptation negative after-effects. To better understand this unexpected adaptive capacity in brain-lesioned patients, we investigated the contribution of awareness for the optical shift to the development of prism adaptation. The lack of awareness found in neglect was simulated in a multiple-step group where healthy subjects remained unaware of the optical deviation because of its progressive stepwise increase from 2 degrees to 10 degrees .

A joint-centred model accounts for movement curvature and spatial variability.

Hand reaching toward a visual target needs the central nervous system (CNS) to encode target location and initial limb posture. Once these sensory inputs are encoded within a common frame of reference, the motor system builds a motor command to drive the limb towards the target. In order to face the controversy concerning the variables (in task-space versus joint-space) the motor system may manipulate to build a motor command, we propose to compare recorded hand pointing movements to a task-space vector model and to a simplified joint-centred model.

Automatic drive of limb motor plasticity.

The ability to perform accurate limb movements may require learning mechanisms that continually tune the motor system. In the current study, we isolate a form of pure limb motor plasticity. Participants reached to targets that were turned off just after the onset of an initial eye movement, reappearing at a new location at the end of the reaching movement.

Updating target location at the end of an orienting saccade affects the characteristics of simple point-to-point movements.

Six results are reported. (a) Reaching accuracy increases when visual capture of the target is allowed (e.g.

On-line modification of saccadic eye movements by retinal signals.

A saccade is a rapid shift of the position of the eyes (< 100 ms). Saccades are generally considered too quick to be influenced by retinal signals. To address this idea, we displaced the visual target of a rightward horizontal saccade at eye movement onset (when there is suppression of conscious perception).

Neural control of on-line guidance of hand reaching movements.

Orienting one's gaze towards a peripheral target is usually composed of a hypometric primary saccade followed by a secondary 'corrective saccade' triggered automatically (without conscious perception) by the retinal error at the end of the primary saccade and characterised by a short latency. Due to visual suppression during the saccade, the artificial introduction of a random small target jump during that short period remains undetected and triggers after the end of the primary saccade a normal 'corrective saccade'. As a result this procedure simulates an error in the planning of the primary saccade.

A lesion of the posterior parietal cortex disrupts on-line adjustments during aiming movements.

It is long known that the posterior parietal cortex (PPC) is critically involved in goal-directed movements. Nevertheless, there are still some controversies about its specific functions. Although most published studies have emphasised the role of PPC in sensorimotor planning processes, it has been recently suggested that PPC can also participate to on-line movement control.

Functional anatomy of nonvisual feedback loops during reaching: a positron emission tomography study.

Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed).

Postural invariance in three-dimensional reaching and grasping movements.

The question of whether the final arm posture to be reached is determined in advance during prehension movements remains widely debated. To address this issue, we designed a psychophysical experiment in which human subjects were instructed to reach and grasp, with their right arm, a small sphere presented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement onset, to a new unpredictable position.