Visuomotor control of hand movement : beyond planning and perception

Abstract

The roles of two visual streams in visuomotor control have long been a topic of debate. To better understand how the dual visual streams contribute, the studies reported in this thesis explore how visual information is processed for online control of hand movements, and the relationship with volition and consciousness. In all the experiments, subjects reached to grasp a virtual object that was sometimes changed during movement. Responses to these perturbations allowed us to observe the visual processing for online visuomotor correction. In the first study (Chapter 3), it was found that subjects were able to quickly adjust their grasping in response to changes in the orientation or shape of the target object. Grasp points were as optimal as when no perturbation occurred. In the second study (Chapter 4), subjects were asked to try to ignore a perturbation (and continue their reaching‐to‐grasp toward the original object) or stop their movements whenever they noticed it. Despite consciously inhibiting movement, they still made automatic adjustments in response to the perturbations in both two experiments. The third study (Chapter 5) tested whether perturbation responses could be induced by stimuli that are not consciously perceived due to backward masking. Although subjects did not consciously perceive the perturbed stimuli, they automatically adjusted their grip in response to changes in the target. These findings demonstrate that online visual processing can guide visuomotor correction in response to abrupt perturbation of targets without involvement of volition or visual awareness. The overall findings suggest a dominant role of the dorsal stream in the visual processing for online visuomotor control, and generally support the Perception vs. Action model by Goodale and Milner (1992).