The useful visual field and ERP topography in persons with intellectual disabilities

Abstract

In order to elucidate physiological aspects underlying the narrower useful visual field in persons with intellectual disabilities, we examined ERP topography evoked by geometric figures presented within the perifovea. Three groups (persons with intellectual disabilities, normal children, and normal adults) have participated. A crosshatch for fixation was constantly presented at the center of CRT screen. Geometric figures were presented simultaneously for 300 ms at all four CRT corners. Four hexagons represented the non-target stimuli (85%), while one circle and three hexagons represented the target stimuli (15%). Eccentricity (distance between the crosshatch and stimulus shapes) was randomized (4°, 6°, 8°, 10° and 12°). EEGs were recorded under the following two conditions: (a) NON-TASK: subjects were asked to fixate the crosshatch. (b) DISCRIMINATE: subjects were asked to fixate and push a button if the stimulus contained a target. EEGs were averaged for non-targets. ERP differences between groups were more remarkable than those between conditions. ERP components at 100 ms (P1 with posterior positivity) and 160 ms (N1 with posterior negativity) after stimulation were observed in all groups. However, the latency of N1 in normal children and in persons with intellectual disabilities was longer than in normal adults. Furthermore, in normal children and adults, a P2 with posterior positivity appeared 200 ms after stimulation. No such P2 was observed in persons with intellectual disabilities. The P2 might reflect cerebral process responsible for particularly precise pattern analysis within the useful visual field for the visual cognition.