Functional imaging of visually expanding motion stimuli: Toward a functional anatomy of visual motion processing

An expanding motion stimulus produces two kinds of image changes: a radial displacement (optic flow) and an increase in the scale of image features (scale change). It is well known that observers can extract the rate of expansion from optic flow alone using random dot stimuli that have no scale-change information. Using novel stochastic stimuli in which the scale of image elements changes over time, but the optic flow pattern is random, Schrater et. al. (2001) has shown that expansion rate can be extracted from scale change information alone. However, it remains unknown whether these two rate-extracting processes are supported by common or separate neural substrates. Using fMRI, we measured activity in visual cortex while subjects viewed expansionary motion of different types of stimuli. Five observers were scanned in a 1.5T magnet while viewing the alternation between random motion and expanding motion of one of three types of stimuli, random dot, rigid texture, and stochastic. A novel discriminant analysis was used to determine cortical areas involved in processing expansionary motion. Our results indicate that a number of visual areas contribute to the processing of expansionary motion including MT+, V3 and V1. In addition, the three types of stimuli produce different activations within each area, and the time series in these areas can be used to reliably predict the current stimulus. However, despite these differences, our analysis shows that there is a set of common brain areas that process expansionary motion independent of stimulus type. These results suggests that the neural substrates for processing expansionary motion are largely common to all three stimulus types, but the differential pattern of activation within these areas uniquely represents the particular type of stimulus.