Increasing interhemispheric connectivity between human visual motion areas uncovers asymmetric sensitivity to horizontal motion

Our conscious perceptual experience relies on a hierarchical process involving integration of low-level sen-sory encoding and higher-order sensory selection.1 This hierarchical process may scale at different levels of brain functioning, including integration of information between the hemispheres.2-5 Here, we test this hypoth-esis for the perception of visual motion stimuli. Across 3 experiments, we manipulated the connectivity be-tween the left and right visual motion complexes (V5/MT+) responsible for horizontal motion perception2,3 by means of transcranial magnetic stimulation (TMS).4,5 We found that enhancing the strength of connections from the left to the right V5/MT+, by inducing spike-timing-dependent plasticity6 in this pathway, increased sensitivity to horizontal motion. These changes were present immediately and lasted at least 90 min after intervention. Notably, little perceptual changes were observed when strengthening connections from the right to the left V5/MT+. Furthermore, we found that this asymmetric modulation was mirrored by an asym-metric perceptual bias in the direction of the horizontal motion. Overall, observers were biased toward left-ward relative to rightward motion direction. Crucially, following the strengthening of the connections from right to left V5/MT+, this bias could be momentarily reversed. These results suggest that the projections con-necting left and right V5/MT+ in the human visual cortex are asymmetrical, subtending a hierarchical role of hemispheric specialization7-10 favoring left-to-right hemisphere processing for integrating local sensory input into coherent global motion perception.