Differential and co-involvement of areas of the temporal and parietal streams in visual tasks

Transcranial magnetic stimulation (TMS) is particularly useful in teasing apart the contrasting contributions of different anatomical and functional systems in particular aspects of behaviour, for example the involvement of the dorsal and ventral visual streams in tasks involving the perception of distance, shape and colour. In order to investigate the dual involvement of two areas, namely right posterior parietal cortex (PPC) and lateral occipital cortex (LO), in a distance discrimination task, neural processing in both areas was concurrently disrupted using dual site TMS. Although there was no change in error rates, reaction time was significantly lengthened over that seen with TMS over either site alone. This additive effect indicates that both PPC and LO are concurrently active and essential for efficient processing of this task. The second experiment investigated the specificity of function within the ventral stream. Performance was assessed for distance and shape discrimination when TMS was applied to our original LO site and an area rostral to V5 but still part of the lateral occipital complex (rostral LOC) that is activated in form and colour discrimination. Performance deficits were restricted to TMS over LO; no significant impairment for either task followed TMS at the rostral LOC site.