Toward a common circle: interhemispheric contextual modulation in human early visual areas.

Humans can readily and effortlessly perceive a rich, stable, and unified visual world from a complex visual scene. Yet our internal representation of a visual object appears to be sparse and fragmented. How and where in the brain are such fragmented representations organized into a whole percept? Recent studies have accumulated evidence that some global feature integration is mediated at the early stage of visual processing. However, the spatial operating range of the integration still remains unclear. The present human functional magnetic resonance imaging study provides support that the global integration process in early visual areas, including even the primary visual area V1, is mediated beyond the separated projection of visual hemifields from right and left sides of the fixation to the visual cortex of the contralateral cerebral hemisphere. Retinotopic neural responses corresponding to a visual target were significantly enhanced when another target was simultaneously presented at the point-symmetrical position in the nonassociated visual field quadrant. The result makes a convincing case that the contextual effects involve feedback from higher areas, because there are no direct callosal connections that allow such interhemispheric contextual modulation. This enhancement from the ipsilateral hemifield may help rapid position-and-size-invariant detection of a circular pattern, which may be special among visual structures because of its ubiquity in natural scenes. Early visual areas as well as higher ones may play a more essential role in perceiving the unity of the real world than previously thought.