How does unilateral visual loss affect motor responses of reaching and grasping?

Abstract

Skilled and accurate hand movements are required in order to reach and grasp a cup. The movements are analysed under the ‘transport’ and ‘grasp’ mechanisms. We investigated whether a decrease in vision of one eye to create unequal vision during binocular viewing affects reaching and grasping hand movements compared to monocular viewing. Unequal vision was simulated using a +6.00 DS blurring lens. Subjects picked up a cup placed at two different distances. Two different cup sizes were used. Measurements were carried out under binocular, monocular and unequal binocular conditions. Hand movements were analysed using a 6 infra-red camera system. Prehensile indices measured were maximum velocity, time to maximum velocity, time to maximum grasp, total time and maximum grip aperture. Binocular performance with bin (equal) and bin (unequal) were compared to the ‘best’ monocular (mon) performance. Under bin (equal) the grasp component was significantly smaller than under monocular viewing, showing uncertainty under monocular conditions. In the bin (unequal) condition, the grasp component was significantly larger than monocular, showing worse performance compared to monocular. Maximum velocity showed a similar effect. Bin (equal) and mon conditions were similar. Maximum velocity under bin (unequal) was significantly lower than the monocular condition. These data demonstrate that a monocular visual impairment affects binocular performance for reaching and grasping to an extent that it becomes worse than the performance of the good eye alone. These data have implications for patients with a visual impairment in one eye, or in whom one eye deteriorates faster than the other.