Feeling with the mind's eye: contribution of visual cortex to tactile perception

Visual imagery is implicated in the normal tactile perception of certain object properties. This is an example of cross-modal interactions that characterize normal perception. Here we review recent studies from our laboratory on cross-modal interactions between vision and touch in normally sighted humans. Positron emission tomography was used to demonstrate activation of a region of extrastriate visual cortex, near the parieto-occipital fissure, during tactile discrimination of grating orientation. Transcranial magnetic stimulation (TMS) over this region interfered with performance of this tactile task. In both studies, visual cortical involvement was found for tactile discrimination of orientation but not spatial frequency. Thus, this cortical region is not only active during but also necessary for optimal tactile sensing of orientation. Recent findings implicating visual cortex in Braille-reading in the blind should be evaluated from this perspective.