Activity in visual area V4 correlates with surface perception.

The neural mechanisms responsible for unifying noncontiguous regions of a visual image into a percept of a single surface remain largely unknown. To investigate these mechanisms, we used a novel stimulus in which local luminance was the only cue for surface segmentation. Subjects viewed an array of small adjoining elements that were randomly assigned as either surface or noise every 100 ms. On each trial, the luminance of surface elements was fixed to a single value and the luminance of noise elements was randomly assigned. As the ratio of surface to noise elements changed, subjects perceived either a surface embedded in noise or noise alone. In three functional magnetic resonance imaging (fMRI) experiments, early visual area V1 responded most strongly during trials with a low surface-to-noise ratio while later areas responded most strongly during trials with a high ratio. Furthermore, even at identical surface-to-noise ratios, responses in area V4 were higher during trials in which the subject perceived a surface than during trials in which the subject did not. Early visual areas did not show this pattern. These results suggest that visual area V4 contains neurons critical for the representation of surfaces.