The organization of global motion and transparency

The visual system has the task of computing global motions associated with objects and surfaces. This task strongly involves extrastriate brain areas, particularly V5/MT. Motion transparency provides a particular challenge for understanding how global motions are computed and represented in the brain. Psycho-physical experiments show that, for a single region, multiple motions can be quantitatively represented. However, at the most local scale, motion signals have a suppressive interaction so that only a single motion can be represented. Neurophysiological experiments show that this suppression is a property of MT, not of V1, reflecting a subunit structure within MT receptive fields and showing that transparency perception is related to MT rather than V1 activity. A full understanding of transparency perception and other global motion phenomena will require us to understand how perceived motions are related to the distribution of activity across a population of directionally selective neurones, and how the brain implements the representation of motions assigned to extended objects rather than to specific retinotopic locations.