Speed-dependent responses in V5: A replication study

In a previous paper, we used fMRI to examine motion-sensitive responses in human area V5 as a function of stimulus speed. As predicted by electrophysiological findings, we observed optimal responses at intermediate speeds of around 7 to 30 degrees /s. These results revealed a nonlinear (inverted "U") dependency on speed that was also evident in V3a. In this paper we repeated the experiment using an improved stimulus and a larger range of speeds. We replicated our previous findings and extended our characterization of speed-dependent responses: Optimal responses were seen in V5 at speeds of 4 and 8 degrees /s and in V3a at speeds of 4 to 16 degrees /s. We were also able to show an interaction between speed (fast vs slow) and contrast (color > luminance) in V5. This interaction was anticipated on the basis of the different properties of the geniculate and extrageniculate inputs to V5. Finally, we were also able to demonstrate an interaction between motion (moving vs stationary) and contrast (color > luminance) in V4. This suggests that for V4, color-specific responses are augmented in the context of motion; or equivalently, that color contrast enhances any motion-sensitive responses in V4.