Automatic shape processing and visuomotor corrections during grasping

Abstract

In a previous study, we demonstrated that grasp points for smooth irregular shapes can be determined during an ongoing movement (Chen & Saunders, 2015). When the shape of a target object was unexpectedly changed during a virtual grasping movement, subjects made smooth corrective adjustments toward appropriate contact points for the new target object, detectable within about 300 ms. In this study, we presented task-irrelevant shape perturbations during movements to test whether corrective responses are automatic, or can be suppressed volitionally. Subjects reached to touch projected 2D objects with smooth irregular shapes at locations they would use to grasp the objects. In Experiment 1, half of the trials had transient perturbations in which the target object was replaced by a different object for 200 ms and then reappeared. Subjects were told that the original object would always reappear and were asked to ignore the perturbations. Despite the instructions, we observed automatic corrective adjustments to the grip axis, with similar latency as in the previous study. Experiment 2 tested similar conditions, but subjects were instructed to stop their movement and withdraw their hand as soon as they detected a perturbation. Perturbations were only present on 10% of trials to discourage compensatory strategies. We again observed automatic adjustments of the grip in response to shape perturbations, with similar latency. The grip adjustments continued even after the forward motion of the hand had been stopped, indicating that automatic adjustments co-occurred with volitional control. These results indicate that grip adjustments in response to updated shape information occur automatically and beyond the control of volition.