Parallel motion signals to the medial and lateral motion areas V6 and MT+

MT+ and V6 are key motion areas of the dorsal visual stream in both macaque and human brain. In the present study, we combined electrophysiological and neuroimaging methods (including retinotopic brain mapping) to find the electrophysiological correlates of V6 and to define its temporal relationship with the activity observed in MT+. We also determined the spatio-temporal profile of the motion coherency effect on visual evoked potentials (VEPs), and localized its neural generators. We found that area V6 participates in the very early phase of the coherent motion processing and that its electroencephalographic activity is almost simultaneous with that of MT+. We also found a late second activity in V6 that we interpret as a re-entrant feedback from extrastriate visual areas (e.g. area V3A). Three main cortical sources were differently modulated by the motion coherence: while V6 and MT+ showed a preference for the coherent motion, area V3A preferred the random condition. The response timing of these cortical sources indicates that motion signals flow in parallel from the occipital pole to the medial and lateral motion areas V6 and MT+, suggesting the view of a differential functional role. MT+ would be involved in the analysis of motion signals (direction and speed of movement) in the central visual field whereas V6 would be involved in 'subtracting out' self motion signals across the whole visual field.