FUNCTIONAL RELEVANCE OF CROSS-MODAL PLASTICITY IN BLIND HUMANS

Functional imaging studies of people who were blind from an early age have revealed that their primary visual cortex can be activated by Bra ille reading and other tactile discrimination task(1). Other studies h ave also shown that visual cortical areas can be activated by somatose nsory input in blind subjects but not those with sight(2-7). The signi ficance of this cross-modal plasticity is unclear, however, as it is n ot known whether the visual cortex can process somatosensory informati on in a functionally relevant way. To address this issue, we used tran scranial magnetic stimulation to disrupt the function of different cor tical areas in people who were blind from an early age as they identif ied Braille or embossed Roman letters, Transient stimulation of the oc cipital (visual) cortex induced errors in both tasks and distorted the tactile perceptions of blind subjects, In contrast, occipital stimu l ation had no effect on tactile performance in normal-sighted subjects, whereas similar stimulation is known to disrupt their visual performa nce. We conclude that blindness from an early age can cause the visual cortex to be recruited to a role in somatosensory processing. We prop ose that this cross-modal plasticity may account in part for the super ior tactile perceptual abilities of blind subjects.