Contour-integration deficits in intact visual field of hemianopia patients
We investigated Gestalt perception in the intact visual field (VF) of hemianopia patients. Three patients and matched controls performed a Yes-No figure detection task. Gabor patches of one orientation, making up the outline of a square, were embedded in randomly oriented “background” Gabor patches. Continuity of the square outline was modified by changing the orientation of 4 to 12 out of 16 Gabor patches. In addition, background density (BD) varied from low to high. Figure detection in the intact visual field was impaired in a patient with a temporal-parietal lesion but not in a patient with an occipital lesion. Both patients had frequent false positives when only the background elements or fragmented squares were presented. ‘Pathological completion’ occurred more frequently (i) for higher BDs, (ii) when the fragment ends faced the blind VF, and (iii) for central compared to peripheral presentation. The patient with an optical tract lesion had almost normal figure detection at low BD but impaired performance at higher BDs; no ‘pathological completion’ was observed. Our findings indicate that crowding engenders contour integration deficits in the “intact” VF of hemianopia patients. We attribute the deficits to malfunction of the cortical stage of processing in the visual system.