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G. T. Timberlake, B. M. Quaney, E. Omoscharka, J. B. Nolan, J. H. Maino; The Effect of Bilateral Macular Scotomas on Reach-to-Grasp Hand Movements. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2531.
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When reaching to grasp an object, the trajectory of the hand and the grip aperture (distance between the index finger and thumb) are influenced by visual information about the object and the hand. Thus, vision loss from bilateral macular scotomas may affect reach-to-grasp hand movements. To investigate this possibility, we measured reach-to-grasp movements of individuals with bilateral macular scotomas in comparison to normally sighted controls.
Three-dimensional hand trajectories and grip apertures were tracked using digital infrared video. Subjects reached and grasped 3 sizes of wooden blocks placed on the subject’s midline, 20 or 40 cm from the starting position of the hand. Subjects were instructed to reach and grasp as "quickly and accurately as possible." Ten subjects with dense, bilateral central scotomas due to Age-related Macular Degeneration and 10 age-matched, visually-normal control subjects were tested. Each subject completed 30 trials (2 distances X 3 blocks X 5 trials). A between-group repeated measures ANOVA was performed
Scotoma subjects had 39% longer reaction times (341 vs. 246 msec, p < 0.0001) and took 46% longer to reach for and grasp the block than controls (1,374 vs. 941 msec, p < 0.0001). Hand trajectory lengths, measured by the directness ratio (actual trajectory length/shortest possible length) were 17% longer for scotoma subjects than for controls (1.54 vs. 1.32, p < 0.0001). Peak hand velocity was 14% lower for scotoma subjects than controls (1,104 vs. 1,255 mm/sec, p = 0.01). Maximum grip aperture was not significantly different between control and scotoma subjects.
These results indicate that vision loss from bilateral macular scotomas affects reach-to-grasp hand movements that require accuracy. Subjects with bilateral macular scotomas had slower reach-to-grasp trajectories than did normally-sighted controls. Thus, individuals with bilateral macular scotomas sacrifice speed in order to accurately reach to grasp an object.
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