Visual feedback reduces bimanual coupling of movement amplitudes, but not of directions.

To what extent does visual feedback shape the coordination between our arms? As a first step towards answering this question, this study compares bimanual coupling in simultaneous bimanual reversal movements that control cursor movements on a vertical screen. While both cursors were visible in the control condition, visual feedback was prevented in the experimental condition by deleting one or both cursors from the screen. Absence of visual feedback for one or both arms significantly increased the reaction times of both arms and the movement amplitude of the occluded arm. Temporal coupling between the arms remained unchanged in all feedback conditions. The same was true for spatial coupling of movement directions. Amplitude coupling, however, was significantly affected by visual feedback. When no feedback for either arm was available, amplitude correlations were significantly higher than when feedback for one or both arms was present. This finding suggests that online visual feedback decreases bimanual amplitude coupling, presumably through independent movement corrections for the two arms. The difference between movement amplitudes and movement directions in their susceptibility to visual feedback supports the idea that they are subserved by different control mechanisms. Analysis of eye movements during task performance revealed no major differences between the different feedback conditions. The eye movements of all subjects followed a stereotypical pattern, with generally only one saccade after target onset, directed towards the average position of all possible targets, irrespective of feedback condition and target direction.