Discrimination of synthetic vowels by using tactile vocoder and a comparison to that of an eight-channel cochlear implant

Both the tactile vocoder and the cochlear implant divide speech signals into 16 frequency components using bandpass filters and lateral inhibition circuits. In the tactile vocoder, these 16 components are converted into a vibration with 200-Hz frequency and applied to a 3*16 element vibrator array using bimorph piezoelectric elements. The vibratory patterns are sensed on the fingertip. In the cochlear implant, the 16 components are reduced to eight current stimulation signals, consisting of biphasic pulses with 200-Hz frequency, which are applied to an eight-channel electrode array implanted in the scala tympani. The electrode array passes through the round window into the scala tympani to a depth of 23 mm. These psychophysical experiments investigate the ability of human subjects to discriminate synthetic vowels as a function of the number of channels used. The results suggest that an eight-channel and a 16-channel tactile vocoder provide essentially the same discrimination scores. However, the ability to discriminate synthetic vowels decreases rapidly when fewer than eight channels are used.