Global versus local: double dissociation between MT+ and V3A in motion processing revealed using continuous theta...

The functional properties of motion selective areas in human visual cortex, including V3A, MT+, and intraparietal sulcus (IPS) are not fully understood. To examine the functional specialization of these areas for global and local motion processing, we used off-line, neuronavigated, continuous theta burst (cTBS) transcranial magnetic stimulation to temporarily alter neural activity within unilateral V3A, MT+, and IPS. A within-subjects design was employed and stimulation sessions were separated by at least 24 h. In each session, subjects were asked to discriminate the global motion directions of successively presented random dot kinematograms (RDKs) before and after cTBS. RDKs were presented at either 100 or 40 % coherence in either the left or right visual field. We found that V3A stimulation selectively impaired discrimination of 100 % coherent motion, while MT+ stimulation selectively impaired discrimination of 40 % coherent motion. IPS stimulation impaired discrimination of both motion stimuli. All cTBS effects were specific to stimuli presented contralaterally to the stimulation site and vertex stimulation had no effect. The double dissociation between the cTBS effects on MT+ and V3A indicates distinct roles for these two regions in motion processing. Judging the direction of 100 % coherent motion can rely on local motion processing because every dot moves in the same direction. However, judging the global direction of 40 % coherent motion requires global processing. Thus, our results suggest separate, parallel processing of local and global motion in V3A and MT+, respectively, with the outputs of these two areas being combined within the IPS.