How well you reach depends on where you look

Research output: Contribution to journalReview articlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)320a
JournalJournal of vision
Issue number3
StatePublished - 2001


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