Preattentive filling-in of visual surfaces in parietal extinction.

Unilateral brain damage frequently produces "extinction," in which patients can detect brief single visual stimuli on either side but are unaware of a contralesional stimulus if presented concurrently with an ipsilesional stimulus. Explanations for extinction have invoked deficits in initial processes that operate before the focusing of visual attention or in later attentive stages of vision. Preattentive vision was preserved in a parietally damaged patient, whose extinction was less severe when bilateral stimuli formed a common surface, even if this required visual filling-in to yield illusory Kanizsa figures or completion of partially occluded figures. These results show that parietal extinction arises only after substantial processing has generated visual surfaces, supporting recent claims that visual attention is surface-based.