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Aarlenne Khan, Laure Pisella, Denise Henriques; Spatial perception deficits in optic ataxia patients. Journal of Vision 2011;11(11):938. doi: 10.1167/11.11.938.
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Damage to the posterior parietal cortex (PPC) is believed to cause deficits in motor actions but not perception. Here we challenge this view by showing that patients with PPC damage also display deficits in perceiving the absolute and relative positions of objects in space. We tested two patients with PPC damage, one with left unilateral optic ataxia (OA) and one with bilateral OA, and 5 neurologically intact controls in 4 paradigms. Subjects were to judge whether a 2nd vertical bar (presented horizontally, randomly −9 deg −9 deg, 1 deg intervals) was left or right of a memorized initial vertical bar (presented 2 deg left, 0 deg or 2 deg right). They did so in 4 paradigms, 1) while fixating throughout the trial on a cross either 12 deg left or right of centre - memory no frame, 2) while switching fixation from one side to the other between the presentation of the two bars - updating no frame, 3–4) same as 1–2 but with the presence of a large frame centered on the screen (20 deg*16 deg) - memory / updating frame. OA subjects showed greater variability in the perceptual judgment than controls (bilateral OA being the most variable). In addition, both patients showed deficits in perceiving/remembering the locations of bars presented in their contralesional visual fields in all paradigms, resulting in a perceptual shift. The presence of the frame shifted the remembered location of the bar in opposite directions for the two patients, not affecting controls. While perceptual updating was unimpaired in unilateral OA, the bilateral OA patient was unable to update at all. Our results show that damage to the PPC results in perceptual deficits in addition to movement impairments. This suggests that some of the reach related deficits in OA may in part be due to difficulty in correctly perceiving object locations.
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