Tactual perception of speech-like stimuli with an electrocutaneous vocoder.

A deficiency in current tactual vocoder designs is the relatively poor transmission of rapid spectral changes (formant transitions). To understand better the application of electrocutaneous tactual stimulation as a basis for an artificial hearing system, the tactual discrimination of rapidly changing patterns was studied. Stimuli were presented on a belt containing 32 electrodes that was worn in a linear array 5 cm above the navel. Bipolar pulse trains (height 10 mA; width 13 microseconds) of specified frequencies (200, 400, or 1,000 Hz) were presented sequentially to adjacent electrodes to simulate a tactual vocoder equivalent of simple frequency transitions. Discrimination of the direction of stimulation movement (i.e., stimulation on the belt to either a subject's left or right) was assessed with one experienced subject as a function of both electrode number (four or eight electrodes) and stimulation frequency. Identification of the direction of stimulation movement was influenced both by the number of stimulus channels and by the frequency of electrocutaneous stimulation. These preliminary results indicate that the discrimination of formant trajectory transmitted in an electrocutaneous vocoder scheme is within the temporal limits typically imposed by natural speech.