Motion direction tuning in human visual cortex.

A number of electrophysiological studies have been conducted in recent years in order to clarify the dynamics of visual motion processing in the human brain. Using a variety of event-related potential (ERP) measures, several parameters such as onset, offset, contrast and velocity have been investigated, while a critical aspect of visual motion, that of direction, has received less attention. Here we used multichannel electroencephalography and distributed source localization to study brain activity for different directions of visual motion using random dot stimuli. Our data reveal differential extrastriate activation at 164-226 ms after motion onset that coded for motion direction with different ERP maps and underlying electrical generators for each tested direction. This activation was paralleled initially (164-186 ms) by a distinct extrastriate activation encoding whether the motion stimulus consisted of directed motion stimuli (as above) or contained undirected incoherent motion (control stimulus). Application of a linear inverse solution localized the brain activity for each tested motion direction to distinct brain regions within the same larger network of extrastriate brain regions. These regions included bilateral temporo-occipital and bilateral parieto-occipital cortex. The present data in healthy subjects are compatible with extrastriate activity that is tuned to different directions of visual motion. This extends previous clinical data and suggests the presence of distributed macroscopic motion direction tuning in primate extrastriate cortex that may complement the classical microscopic motion tuning at the columnar level.