The selectivity and timing of motion processing in human temporo-parieto-occipital and occipital cortex: a transcranial magnetic stimulation study.

An extrastriate visual area near the human temporo parieto occipital junction (TPO) may selectively mediate motion processing, while contributing little to the perception of color or form. This TPO area may be the human analogue of the monkey middle temporal (MT or V5) and medial superior temporal (MST) extrastriate visual areas. The selectivity of the effect of transcranial magnetic stimulation (TMS) on motion processing was unknown, as was the timecourse of occipital to TPO motion processing. In the first experiment, unilateral TMS was delivered over TPO 50-250 ms after the onset of a random dot motion discrimination display that was presented in the right or left hemifield. TMS reduced the correct discrimination of motion direction only when it was delivered in a discrete time window 100-175 ms following the onset of the display. TMS did not significantly affect hemifield spatial acuity in the same time window. In the second experiment, bilateral TMS delivered over occipital cortex also degraded the discrimination of motion-defined form (MDF) in a discrete time window following the onset of a display presented foveally. Bilateral focal TMS delivered over TPO disrupted the discrimination of MDF in a time window beginning 20-40 ms later than the effect of TMS delivered over occipital cortex. Bilateral focal TMS delivered over TPO also degraded the discrimination of color-defined form, motion direction and color. TMS can trace the timing of visual processing from occipital to extrastriate visual areas.