rTMS over the human medial parietal cortex impairs online reaching corrections

Abstract

Indirect correlational evidence suggests that the posteromedial sector of the human parietal cortex (area hV6A) is involved in reaching corrections. We interfered with hV6A functions using repetitive transcranial magnetic stimulation (rTMS) while healthy participants performed reaching movements and in-flight adjustments of the hand trajectory in presence of unexpected target shifts. rTMS over hV6A specifically altered action reprogramming, causing deviations of the shifted trajectories, particularly along the vertical dimension (i.e., distance). This study provides evidence of the functional relevance of hV6A in action reprogramming while a sudden event requires a change in performance and shows that hV6A also plays a role in state estimation during reaching. These findings are in line with neurological data showing impairments in actions performed along the distance dimension when lesions occur in the dorsal posterior parietal cortex.

Keywords: Area V6A; Correction of reach direction and position; Human brain; Limb state estimation; Posterior parietal cortex.

Fig. 1

Experimental setup. A Lateral (left) and top (right) view of the target arrangements in the experimental task. The participants performed reaching movements with their right hand toward one of the targets (black dots represent the targets of both Experiments, whereas the gray dots correspond with the targets used only in Experiment 1) located at different distances and directions in two target arrangements: FAR arrangement (top) and NEAR arrangement (right) while fixating a fixation point (FP, cross). Reaching movements were performed from the initial hand position (Home button, HB). In the stable target trials, reaching was directed to one of the ten stationary targets (left). In the shifted target trials (right), reaching was directed to the central target in each arrangement but was suddenly reprogrammed and redirected toward a new location of the target within the same arrangement. The new target location could be in a different horizontal direction (shift in horizontal direction, only in Experiment 1) or at a different vertical direction (shift in vertical direction, in both Experiments) within each arrangement of targets. B Time sequence of the task in both Experiments (only one target position is shown for conciseness). The eye represents the fixation point; the filled black circle shows the reaching target. The fixation point stayed visible for 1.3 or 1.5 s and then the reaching target was turned on in one of the locations. Immediately, the participant reached for the target with her/his right hand while maintaining his/her gaze on the fixation point. Movement onset triggered the target switching off. The target appeared again at the previous location (stable target trials) or in another location (shifted target trials), requiring the participants to correct the movement online. During the movement time, rTMS was delivered with a time-course sketched below the timeline

Fig. 2

Impairments in the amount of trajectory deviations in shifted target trials of Experiment 1. Mean population euclidean distance (ED) between the shifted trajectory and the corresponding central route of the FAR arrangement of targets (A) and of the NEAR arrangement of targets (B). Within each arrangement, data are separated for direction of shift, stimulation sites, and time bins. The black line below the plot in each graph represents the stimulation time. Top left inset: tPAcc = time of maximum acceleration and tPVel = time of maximum velocity (shown only once, because the time bins of their occurrence were the same in all positions). Asterisks indicate significant post hoc comparisons (green: comparison hV6A vs SHAM; red: comparison V1/V2 vs SHAM; black: comparison hV6A vs V1/V2). In each plot, a larger area under the curve should be interpreted as a larger deviation from the central trajectory. To the right of each inset a schematic representation of the task is shown. Other conventions as in Fig. 1

Fig. 3

Impairments in the amount of trajectory deviations in shifted target trials of Experiment 2. Conventions as in Fig. 2