Music and Pitch Perception of Cochlear Implant Recipients

Users of cochlear implants (CIs) often achieve good performance in speech recognition, but report that music sounds unnatural and is often unrecognisable. This thesis reports experiments to determine the relationship between the perception by CI users of parameters related to music (and to tonal languages) and the parameters of the signals that give rise to them. For electrical stimulation by pulse trains, the perceived pitch decreases approximately linearly with the insertion depth in the cochlear of the place of stimulation. At low rates, perceived pitch also increases approximately logarithmically with the rate of pulse delivery, but it saturates at several hundred pulses per second. The perceived quality of sound is greater when low rates are applied towards the apex of the cochlear, and conversely. The thresholds for temporal jitter discrimination are lower at lower rates ranging from on average 30% at 100 pulses per second (pps) to 50% at 500 pps. Simulations of the processing of tonal languages were conducted using speech coding strategies currently used for Western languages. These suggest that the resolution provided in the frequency domain is insufficient to allow reliable distinction of words differentiated only by their pitch or pitch contour. Pitch discrimination was measured in a melodic context using complex and pure stimuli. The difference limens for recognition of change and for recognition of direction of pitch change were similar for pure tones, around 1 semitone at 400Hz, but recognition of direction was much poorer for complex stimuli, around 6 semitones at this frequency. This is explained in terms of the ambiguity of frequency-place mapping of complex signals in current strategies. In order to improve pitch and music perception, a new coding strategy was implemented that presents rate and place pitch in combination and with the jitter constraints suggested by these studies. Improved pitch discrimination and perceived quality of sound by users are reported.