Limits of the Classical Functionalist Perspective on Sensory Substitution

The tongue display unit (TDU) is a sensory substitution device that translates visual images into electrotactile stimulation that is transmitted to the tongue and leads to new perceptual skills following training. Trained users, including blind individuals, become capable of orientation discrimination, motion detection, shape recognition and they can also successfully use the TDU to navigate in an environment, locate objects and avoid obstacles. Many studies and discussions have focused on the effects of training at the behavioural level, and assumed that the effects shown in training blindfolded sighted individuals are similar to those observed in blind people. In doing so, we argue that behavioural research on sensory substitution shows a functionalist bias. Functionalism claims that mental processes can be individuated by their characteristic inputs and outputs, and that the physical realization of a given function introduces no relevant difference, as long as the function is the same. We emphasize here why this assumption biases the interpretation of sensory substitution devices.

Crossmodal Recruitment of the Ventral Visual Stream in Congenital Blindness

We used functional MRI (fMRI) to test the hypothesis that blind subjects recruit the ventral visual stream during nonhaptic tactile-form recognition. Congenitally blind and blindfolded sighted control subjects were scanned after they had been trained during four consecutive days to perform a tactile-form recognition task with the tongue display unit (TDU). Both groups learned the task at the same rate. In line with our hypothesis, the fMRI data showed that during nonhaptic shape recognition, blind subjects activated large portions of the ventral visual stream, including the cuneus, precuneus, inferotemporal (IT), cortex, lateral occipital tactile vision area (LOtv), and fusiform gyrus. Control subjects activated area LOtv and precuneus but not cuneus, IT and fusiform gyrus. These results indicate that congenitally blind subjects recruit key regions in the ventral visual pathway during nonhaptic tactile shape discrimination. The activation of LOtv by nonhaptic tactile shape processing in blind and sighted subjects adds further support to the notion that this area subserves an abstract or supramodal representation of shape. Together with our previous findings, our data suggest that the segregation of the efferent projections of the primary visual cortex into a dorsal and ventral visual stream is preserved in individuals blind from birth.