Automatic adjustments to grasping movements from unconscious visual information

Abstract

We investigated whether control of hand movements can be driven by visual information that is not consciously perceived. Previous studies have shown that subjects can make corrective responses to perturbations during hand movements even when they do not notice the perturbations. We tested whether movements can be affected by visual information that is not perceived at all, using backward masking to prevent conscious perception. Subjects performed reach-to-grasp movements toward 2D virtual objects that were projected onto a rigid surface. They were instructed to touch the projection surface at the locations that they would use to pick up the object. On perturbed trials, the target object was briefly shown (33 ms) at an orientation that was ±20° from the original orientation, followed by a 200 ms mask. Perturbations were triggered when the index finger was 20 cm away from the target. After the mask, the original target reappeared and remained visible until completion of movement. Thus, the task did not require any response to the perturbations. Unperturbed trials were identical except that the orientation of the target remained constant. None of the subjects reported noticing the masked perturbations, and a follow-up test found that half of the subjects could not reliably discriminate the perturbations even when trying. Despite the lack of awareness, the brief views of the rotated targets caused detectable changes in the grip axis during movement. Approximately 200 ms after perturbation onset, the grip axes in perturbed trials began to rotate in the direction of the target rotation, reaching a maximum deviation of 1.2° after another 200 ms. These biases were corrected during the final movement, so that the final grasp axes were not significantly different across perturbation conditions. The results demonstrate that visual information can affect control of hand movement even when it is not consciously perceived. (Meeting abstract presented at VSS 2015)