Speech perception by congenitally deaf subjects using an electrocutaneous vocoder

Two congenitally profoundly deaf adults were trained to perceive words through the Tacticon 1600 electrocutaneous vocoder, an artificial hearing prosthesis. The subjects learned to identify 50 words during 47 hours (Subject One) and 41 hours (Subject Two) of training, with a 41.6 percent rate of success across all sessions. Both subjects showed consistent error patterns during the training phase. Analysis of these error patterns suggested that they were employing word identification strategies based on some general aspects of tactual patterns. Specific characteristics of the tactual patterns that they appeared to be using included: syllable number, tactual locus of word ending, direction of pattern movement, and position of bursts (/t/, /k/, /d/, for example). Following training, the subjects were tested for their abilities to integrate tactual and aided-auditory cues in word identification. Three conditions of aided-audition alone (A), tactual vocoder alone (TV), and aided-audition with tactual vocoder (TV + A) were used. The stimulus-word list for this phase consisted of the 50 words acquired in tactual vocoder training, and 50 "tactually-new" words, i.e., words that had not been presented to them in tactual vocoder training sessions. They correctly identified 93 percent (Subject One) and 56 percent (Subject Two) more trials in the TV + A condition than in the A condition. Tactually-new vocabulary was correctly identified 78 percent (Subject One) and 50 percent (Subject Two) more often when sensory modalities were combined, than when only aided-audition was used. Subjects identified tactually-new vocabulary better than chance in the TV condition.