GESTALT PERCEPTION IN HEMIANOPIC PATIENTS: PERFORMANCE MEASURES AND VEPS
Perceptual grouping, an essential mechanism of object recognition, is accomplished at early visual processing stages. Its electrophysiological correlate is the N1 component of the visual evoked potential (VEP). In the present VEP study, we investigated neural correlates of perceptual grouping in the perimetrically intact visual field of hemianopic patients. We studied six patients with homonymous hemianopia resulting from posterior brain lesions and compared them to nine healthy subjects. All observers were presented either random arrays of Gabor patches or arrays with an embedded circle. For the hemianopic patients, the circle was presented in their intact hemifield only. The subjects were instructed to detect the circle by pressing a corresponding button (â€œyes-noâ€ paradigm). In addition to VEPs, error rates and response times were registered. Performance data did not show any differences between the patients and controls. In the VEPs for healthy subjects, N1 amplitudes were significantly larger for circular patterns than for random arrays. This N1 effect, however, was not found for the hemianopic group. Furthermore, P1 amplitudes were generally augmented in the patientsâ€™ VEPs. The present findings provide electrophysiological evidence of impaired perceptual grouping in the intact hemifield of hemianopic patients. This confirms previous behavioral observations of impaired perceptual grouping in the intact field.