How well you reach depends on where you look

To make a successful reach, the visual system must take into account the accuracy of its knowledge of the location and size of an object. The spatial certainty of a target's location with respect to the hand is limited by the eccentricity of viewing (Hess & Hayes, 1994), and by the ability to convert a target in retinal coordinates to arm-centered coordinates in the presence of noise in this sensorimotor transformation. If the observer takes into account the spatial uncertainty of a target when conducting a reach, then we would expect grasp parameters to vary as a function of eccentricity and eye direction. Observers reached and grasped a target (1.1cm radius cylinder spatially fixed at 40deg while fixating 1 of 9 marks. This allowed identical reaches to be performed with different retinal eccentricities for the target. In each trial, an auditory cue indicated the appropriate fixation mark 1 second before the reach. Observers performed this task in two conditions: with vision, and with vision of fixation points but with the target and hand occluded from view. Observer's peak grip-width (forefinger to thumb) was used as a measure of spatial uncertainty (Paulignon, 1991). We found that even with highly practiced and repeated reaches to the same location, grip-width increased with eccentricity (14% increase from 0deg to 80deg). Further, we found that the width of observer's grip increased with eccentricity even when he/she could not see their hand or the target to be grasped (4% increase). Although the maximum grip-widths were at greater eccentricities, the minimum grip-widths occurred near the target location (40deg) when the target was in view, and near 0deg when the target was occluded. These findings suggest that there is a greater degree of uncertainty involved when reaching with low visual resolution, and greater uncertainty in the sensorimotor transformation when the eyes are eccentric with respect to the head.