The exact role of posterior parietal cortex (PPC) in visually directed reaching is unknown. We propose that, by building an internal representation of instantaneous hand location, PPC computes a dynamic motor error used by motor centers to correct the ongoing trajectory. With unseen right hands, five subjects pointed to visual targets that either remained stationary or moved during saccadic eye movements. Transcranial magnetic stimulation (TMS) was applied over the left PPC during target presentation. Stimulation disrupted path corrections that normally occur in response to target jumps, but had no effect on those directed at stationary targets. Furthermore, left-hand movement corrections were not blocked, ruling out visual or oculomotor effects of stimulation.

When we reach towards an object that suddenly appears in our peripheral visual field, not only does our arm extend towards the object, but our eyes, head and body also move in such a way that the image of the object falls on the fovea. Popular models of how reaching movements are programmed have argued that while the first part of the limb movement is ballistic, subsequent corrections to the trajectory are made on the basis of dynamic feedback about the relative positions of the hand and the target provided by central vision. These models have assumed that the adjustments are dependent on seeing the hand moving with respect to the target. Here we present evidence that a change in the position of a visual target during a reaching movement can modify the trajectory even when vision of the hand is prevented. Moreover, these dynamic corrections to the trajectory of the moving limb occur without the subject perceiving the change in target location. These findings demonstrate that visual feedback about the relative position of the hand and target is not necessary for visually driven corrections in reaching to occur, and the mechanisms that maintain the apparent stability of a target in space are dissociable from those that mediate the visuomotor output directed at that target.

No abstract

1. The aim of this study was to demonstrate that goal-directed pointing movements, executed at normal speed to a small visual target, but without vision of the movement, do not rely on preprogrammed commands (open-loop process); by contrast these responses are under the control of a feedback loop, which compares the ongoing response and the goal (or its internal representation). When the location of this goal is changed at the onset of the movement, an automatic correction of the path occurs. Modification of the goal was obtained by presenting a target in the peripheral visual field that the subject had to look at and point at as quickly and accurately as possible. When the orienting ocular saccade reached its peak velocity, statistically corresponding to the hand movement onset, the target was suddenly shifted 10 degrees in a random direction. This perturbation was undetected by the subject because of the absence of perception during the saccade. For the compensation to occur, the initial orientation of the movement and also its extent had to be modified. The results revealed 1) a nearly complete compensation of the movement path and a 66- to 80-ms duration lengthening; 2) relatively short reaction times to the perturbations (from 145 to 174 ms, with effective reaction times even 40 ms shorter); 3) nearly identical spatiotemporal movement characteristics to the perturbations, regardless of whether vision of the hand was allowed, suggesting that corrections were subserved by the same mechanisms. 2. The spatiotemporal characteristics of these unconscious corrections were similar to those observed in the classical double-step experiments investigating the intentional modifications of ongoing movements and suggest that they might share some common low-level mechanisms. That is, they could rely on visuokinesthetic feedback loops, which compare the updated information provided by the eye at the end of the saccade and the proprioceptive information of the end point effector (the fingertip here); they could also rely on feed-forward processes detecting the discrepancy between an efference copy of the movement and the new goal; or they could rely on a combination of those two main processes.

1. Subjects were asked to point toward visual targets without visual reafference from the moving hand in two conditions. In both conditions the pointing fingertip was viewed only before movement onset. 2. In one condition, the pointing fingertip was viewed through prisms that created a visual displacement without altering the view of the target. In another experimental condition, vision of the fingertip was not displaced. Comparison of these two conditions showed that virtually shifting finger position before movement through prisms induced a pointing bias in the direction opposite to the shift. The extent of this pointing bias was about one third of the prismatic shift applied to the fingertip. 3. Analysis of movement initial direction demonstrated that it was also less deviated than predicted from the prismatic shift. In addition, the reaction time and movement time of the reaching movement were increased. 4. This result is interpreted in the framework of the vectorial coding of reaching movement. Proprioception and vision provide two possible sources of information about initial hand position, i.e., the origin of the movement vector. The question remains as to how these two sources of information interact in specifying initial hand position when they are simultaneously available. 5. Our results are thus discussed with respect to a visual-to-visual movement vector hypothesis and a proprioceptive-to-visual vector hypothesis. It is argued that the origin of the putative movement vector is encoded by weighted fusion of the visual and the proprioceptive information about hand initial position.

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 . We contrasted this method with the classical single-step group in which subjects were aware of the visual shift because they were directly exposed to the full 10 degrees shift. Because the number of pointing trials was identical in the two groups, the total amount of deviation exposure was 50% larger in the single-step group. Negative after-effects were examined with an open-loop pointing task performed with the adapted hand, and generalization was tested with open-loop pointing with the nonexposed hand to visual and auditory targets. The robustness of adaptation was assessed by an open-loop pointing task after a simple de-adaptation procedure. The progressive, unaware condition was associated with larger negative after-effects, transfer to the non-exposed hand for the visual and auditory pointing tasks, and greater robustness. The amount of adaptation obtained remained, nevertheless, lower than the exaggerated adaptive capacity seen in patients with neglect. Implications for the functional mechanisms and the anatomical substrates of prism adaptation are discussed.

No abstract

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). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-armjumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.