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.