Grouping and extinction: Evidence for low-level modulation of visual selection

The effects of collinear edge- and brightness-based grouping on extinction were investigated in a bilateral parietally damaged patient. If either edge or brightness information supported the grouping of two simultaneously presented items, extinction was reduced. Grouping reduced extinction when the items fell across the midline (Experiment 2) and when they were presented in either visual field (Experiment 3). However, even with edge- and brightness-based grouping cues present, a small increase in item-separation dramatically increased the extent of extinction, though single item detection remained constant. These results support a model of visual processing in which locally acting grouping processes (e.g edge- and brightness-based grouping) provide input for a selection mechanism that processes the resulting structural units (cf. Duncan & Humphreys, 1989). Extinction reflects spatially biased competition for selection, which is reduced when elements form a single structural unit.