Cortical dynamics underlying face completion in human visual system.

In natural images, visual objects are typically occluded by other objects. A remarkable ability of our visual system is to complete occluded objects effortlessly and see whole, uninterrupted objects. How object completion is implemented in the visual system is still largely unknown. In this study, using a backward masking paradigm, we combined psychophysics and functional magnetic resonance imaging to investigate the temporal evolvement of face completion at different levels of the visual processing hierarchy. Human subjects were presented with two kinds of stimuli that were designed to elicit or not elicit the percept of a completed face, although they were physically very similar. By contrasting subjects' behavioral and blood oxygenation level-dependent (BOLD) responses to completed and noncompleted faces, we measured the psychophysical time course of the face completion and its underlying cortical dynamics. We found that face completion manifested its effect between 50 and 250 ms after stimulus onset. Relative to noncompleted faces, completed faces induced weaker BOLD response at early processing phases in retinotopic visual areas V1 and V2 and stronger BOLD response at late processing phases in occipital face area and fusiform face area. Attending away from the stimuli largely abolished these effects. These findings suggest that face completion consists of two synergetic phases: early suppression in lower visual areas and late enhancement in higher visual areas; moreover, attention is necessary to these neural events.