Global Motion Processing in Human Visual Cortical Areas V2 and V3

Global motion perception entails the ability to extract the central direction tendency from an extended area of visual space containing widely disparate local directions. A substantial body of evidence suggests that local motion signals generated in primary visual cortex (V1) are spatially integrated to provide perception of global motion, beginning in the middle temporal area (MT) in macaques and its counterpart in humans, hMT. However, V2 and V3 also contain motion-sensitive neurons that have larger receptive fields than those found in V1, giving the potential for spatial integration of motion signals. Despite this, V2 and V3 have been overlooked as sites of global motion processing. To test, free of local-global confounds, whether human V2 and V3 are important for encoding global motion, we developed a visual stimulus that yields a global direction yet includes all possible local directions and is perfectly balanced at the local motion level. We then attempted to decode global motion direction in such stimuli with multivariate pattern classification of fMRI data. We found strong sensitivity to global motion in hMT, as expected, and also in several higher visual areas known to encode optic flow. Crucially, we found that global motion direction could be decoded in human V2 and, particularly, in V3. The results suggest the surprising conclusion that global motion processing is a key function of cortical visual areas V2 and V3. A possible purpose is to provide global motion signals to V6.

Humans can readily detect the overall direction of movement in a flock of birds despite large differences in the directions of individual birds at a given moment. This ability to combine disparate motion signals across space underlies many aspects of visual motion perception and has therefore received considerable research attention. The received wisdom is that spatial integration of motion signals occurs in the cortical motion complex MT+ in both human and nonhuman primates. We show here that areas V2 and V3 in humans are also able to perform this function. We suggest that different cortical areas integrate motion signals in different ways for different purposes.