Perceptual Evidence for Interhemispheric Visual Integration

Visual scenes in the left and right halves of visual fields are processed in separate cortical hemispheres of the brain. Corpus callosum is thought to play an important role in stitching together the split at the vertical meridian. Yet we do not notice any consequence of such interhemispheric integration in our daily lives. Can we find any perceptual evidence for such integration? Specifically, do visual tasks involving interhemispheric integration require extra processing time due to transmission delays of signals that must be exchanged via corpus callosum? To address these questions, we used a visual illusion known as the flash lag effect as a ruler for space and time. Two configurations of flash lag stimuli were presented to normal human subjects. In one configuration, the stimuli were presented to only one visual hemifield so that the processing could be completed within a single cortical hemisphere. In the other configuration, moving and flashed parts of the stimuli were presented to separate hemifields straddling the vertical meridian. Positions of flashed stimuli were manipulated to find a spatial offset necessary to cancel the lag. Our results show that the stimulus configuration that involves interhemispheric integration requires 40∼50ms of extra processing time.