Opposite dependencies on visual motion coherence in human area MT+ and early visual cortex.

In order to understand the relationship between brain activity and visual motion perception, knowledge of the cortical areas participating in signal processing alone is insufficient. Rather knowledge on how responses vary with the characteristics of visual motion is necessary. In this study, we measured whole brain activity using magnetoencephalography in humans discriminating the global motion direction of a random dot kinematogram whose strength was systematically varied by the percentage of coherently moving dot elements. Spectral analysis revealed 2 components correlating with motion coherence. A first component in the low-frequency domain ( approximately 3 Hz), linearly increasing with motion coherence, could be attributed to visual cortex including human area middle temporal (MT) +. A second component oscillating in the alpha frequency range and emerging after stimulus offset showed the inverse dependence on motion coherence and arose from early visual cortex. Based on these results, we first of all conclude that motion coherence is reflected in the population response of human extrastriate cortex. Second, we suggest that the occipital alpha activity represents a gating mechanism protecting visual motion integration in later cortical areas from disturbing upcoming signals.