Cross-Modal Brain Plasticity in Congenital Blindness: Lessons from the Tongue Display Unit

The tongue display unit (TDU) is a sensory substitution device that translates visual information into electrotactile stimulation that is applied to the tongue. Blind subjects can learn to use the TDU in various visual tasks, including orientation, motion and shape identification, and spatial navigation. We used the TDU in conjunction with brain imaging techniques in order to study the cerebral correlates of cross-modal brain plasticity. The results show that when blind subjects use the TDU to perform visual tasks that are known to activate the dorsal and ventral visual streams in the sighted, they activate the same brain areas. This suggests that motion and shape processing are organized in a supramodal manner in the human brain and that vision is not necessary for the development of the functional architecture in motion and shape processing areas. We also used the TDU in spatial navigation tasks. The results showed that blind but not blindfolded sighted subjects activated their visual cortex and right parahippocampus during navigation, suggesting that in the absence of vision, cross-modal plasticity permits the recruitment of the same cortical network used for spatial navigation tasks in sighted subjects