Critical periods for the transmission of tactual information.

There is good evidence for critical periods in the development of sensory processes (particularly vision and hearing) and language functions. If appropriate experience is absent during these critical periods, later performance is impaired. This principle is of importance to designers of devices which use the skin as an alternative channel of communication for the blind of the deaf. The implications are that: (a) Some promising devices may have been prematurely abandoned because they were tested on adults rather than children. (b) Tactile devices should be evaluated in studies with children who have not yet passed through the critical period for touch and language development. It is argued that early exposure to a tactile transformation of speech (for example) might provide the profoundly deaf with two significant advantages. Firstly, early exposure increases the chance that the cutaneous system will develop neural connections appropriate to the handling of the transformed speech signal. In other words, tactile sensitivity can be optimized. Secondly, early exposure to language as a process associated with the cutaneous system (rather than the auditory system) might favour the development of cortical connections consistent with the sensory substitution system. Such plasticity might no longer exist when the critical period has passed.