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B. T. Barrett, G. K. Panesar, I. E. Pacey, S. Hadwin, R. Kukadia, J. Buckley; Amblyopes Have Difficulty Judging Object Distance but Not Size When Reaching and Grasping in Habitual Viewing. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1843. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Prehension deficits have previously been reported in amblyopes when reaching for a single, isolated object in binocular viewing. Here we examine prehension for objects that are flanked by nearby distracters.
Twenty amblyopes (age 36.4 ±11.7 years) and twenty visual normals (age 27.5 ± 6.3 years) reached for a solid cylindrical object (height 12cm, diameter 3 or 4cm) located directly in front of the participant. The object was flanked by two similarly-sized distracters placed either side of the object, or in front and behind it. The spacing between the object and distracters was scaled according to finger-width of individual participants and was set at spacings of 2- or 4- finger-widths. Shutter-goggles prevented a view of spatial configuration until the movement was to be initiated. Opening the shutters provided the cue for participants to pick up the object in their own time. Movements of the arm, hand and object were recorded using an 8-camera, Vicon-IR recording system.
Our results show differences in both the reaching and grasping phases in amblyopes compared to visual normals. These group differences mainly relate to the speed and timing of prehension. Specifically, the average reach velocity was slower in amblyopes (p=0.02). Peak grip aperture was scaled to object size and distracter spacing equally well in both groups, but it occurred later in the reach in amblyopic subjects (p=0.006) as did peak reach velocity (p=0.004). Amblyopes also took longer to pick up the object once they had contacted it (p=0.002). Both normals and amblyopes opened and closed their grip aperture more quickly when the distracters were more widely separated from the target but the increase in speed was smaller in amblyopes (p<0.03). A group-by-direction effect for reach time indicated a greater time-increase for amblyopes when distracters were separated in depth from the object compared to when they were placed on either side of it (p=0.036). A supplementary analysis indicated there was no difference in prehension performance in amblyopes with and without strabismus.
Motor control for prehension in habitual viewing is executed more slowly in individuals with amblyopia. Our results are consistent with the view that amblyopes have difficulty judging object distance but not size when reaching and grasping.
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