Involvement of visual cortex in tactile discrimination of orientation

The primary sense modalities (vision, touch and so on) are generally though t of as distinct. However, visual imagery is implicated in the normal tacti le perception of some object properties, such as orientation(1), shape and size(2). Furthermore, certain tactile tasks, such as discrimination of grat ing orientation(1) and object recognition: are associated with activity in areas of visual cortex. Here we show that disrupting function of the occipi tal cortex using focal transcranial magnetic stimulation (TMS) interferes w ith the tactile discrimination of grating orientation. The specificity of t his effect is illustrated by its time course and spatial restriction over t he scalp, and by the failure of occipital TMS to affect either detection of an electrical stimulus applied to the fingerpad or tactile discrimination of grating texture. In contrast, TMS over the somatosensory cortex blocked discrimination of grating texture as well as orientation. We also report th at, during tactile discrimination of grating orientation, an evoked potenti al is recorded over posterior scalp regions with a latency corresponding to the peak of the TMS interference effect (about 180 ms). The findings indic ate that visual cortex is closely involved in tactile discrimination of ori entation. To our knowledge, this is the first demonstration that visual cor tical processing is necessary for normal tactile perception.