Effects of visual uncertainty on grasping movements.

To successfully lift an object, a person's fingers must be moved to locations where forces can be applied that are sufficient for maintaining contact and that allow for easy object manipulation. Obtaining such finger positions becomes more difficult when there is perceptual uncertainty about the location of the hand and object. However, knowledge about the amount of uncertainty could be incorporated into grasp plans to mitigate its effect. For example, during peripheral viewing the fingers could open wider to avoid colliding with or missing the object. The goal of this study is to determine the degree to which people incorporate their understanding of visual uncertainty when making a precision grasp. To investigate, subjects reached to a spatially fixed object whose retinal location was varied by fixating points 0-80 degrees to the left of the object. This manipulation controlled the visual uncertainty of the hand and target without affecting the kinematic demands of the task. We found that people systematically changed their grasping behavior as a function of the amount of visual uncertainty in the task. Specifically, subjects' maximum grip aperture increased linearly with target eccentricity. Moreover, the effect of visual uncertainty on finger trajectories could be captured by a single dimension of change along an axis. Together, these findings suggest that the sensorimotor system estimates visual uncertainty and behaviorally adjusts for it during grasping movements.