Rapid completion effects in human high-order visual areas.

Object completion is an inherent property of visual recognition in which objects can be accurately perceived in the presence of substantial obstructions. We have previously shown [Cereb. Cortex 12 (2002) 163] that high-order human object areas are driven partially by local object fragments and partially by global completion effects. Here we explored, through a backward masking paradigm, whether the balance of local and global processing is time dependent, that is, to what extent completion effects evolve at a different time compared to local image representations. In two separate experiments, subjects were presented with three types of images: (a) unobstructed line drawings of animal shapes ("whole"), (b) the same shapes obstructed by a set of parallel stripes ("grid"), and (c) a scrambled version of b in which the stripe position was shifted horizontally, disrupting the relative position of image regions but maintaining the local feature distribution ("scrambled"). Images were presented either for 60 or 250 ms followed by a mask. Both behavioral and fMRI findings from high-order occipitotemporal object areas showed consistently that object selectivity emerges at the same time as the local feature representation. Thus, object completion effects were evident at the same relative magnitude (LO: 0.5 +/- 0.3 and 0.58 +/- 0.04; pFs: 0.62 +/- 0.3 and 0.6 +/- 0.04; 60 and 250 ms, respectively) even at the short presentation durations when overall object activation was greatly reduced.