Perception of Global Flow and Local Motion under Natural Conditions

Our natural visual world contains a variety of different types of motion. Two of the most prominent are global ow, the movement of the entire visual scene that occurs whenever we make an eye or head movement, and local motion, the real movement of people and objects in our environment. We constantly have a mixture of these two kinds of motion, but generally have no problems distinguishing between the two, even though they can produce similar movements on the retina. This ability of the visual system was explored in the present study. Subjects watched a feature movie, used as an approximation to the natural visual world whilst functional magnetic resonance images (fMRI) were made of their brains. Relative amounts of global ow and local motion in the movie were determined using a motion algorithm, and compared to bloodoxygenation level dependent (BOLD) activations in specific visual regions of interest, which were determined from standard retinotopic mapping and localizer techniques. A significant preference to local motion was identified in areas MST, V5/MT, V3A, V2 and V3. Furthermore whole brain analyses showed additional areas with a preference to local motion, and responses to global flow in activity in areas commonly involved in perception of our surrounding spatial environment. These findings further support the idea that there are different brain areas involved in the processing of global flow and local motion