Time course of the involvement of the ventral and dorsal visual processing streams in a visuospatial task.

A previous transcranial magnetic stimulation (TMS) study [Ellison, A., & Cowey, A. (2006). TMS can reveal contrasting functions of the dorsal and ventral visual processing streams. Experimental Brain Research, 175, 618-625] showed that both the dorsal and ventral cortical visual processing streams are involved in the processing of a task in which judgement of relative spatial position is required. In order to determine whether both streams are active in a parallel or serial manner, a double pulse TMS (20 Hz) experiment was carried out to expose peaks of disruption, indicative of when each of the areas under investigation is most potently involved. Results show that TMS over lateral occipital cortex produces greater disruption of performance than that provoked by TMS over posterior parietal cortex, significantly so when applied at 50 and 100 ms post-visual array onset. Both areas showed peaks of disruption up to 350 ms after visual stimulus onset. The results are discussed with respect to why each of these areas is involved in this task and what the pattern of their involvement reveals.