The Perception of Multiple Simultaneous Pitches as a Function of Number of Spectral Channels and Spectral Spread in a Noise-Excited Envelope Vocoder

Abstract

Cochlear implant (CI) listeners typically perform poorly on tasks involving the pitch of complex tones. This limitation in performance is thought to be mainly due to the restricted number of active channels and the broad current spread that leads to channel interactions and subsequent loss of precise spectral information, with temporal information limited primarily to temporal-envelope cues. Little is known about the degree of spectral resolution required to perceive combinations of multiple pitches, or a single pitch in the presence of other interfering tones in the same spectral region. This study used noise-excited envelope vocoders that simulate the limited resolution of CIs to explore the perception of multiple pitches presented simultaneously. The results show that the resolution required for perceiving multiple complex pitches is comparable to that found in a previous study using single complex tones. Although relatively high performance can be achieved with 48 channels, performance remained near chance when even limited spectral spread (with filter slopes as steep as 144 dB/octave) was introduced to the simulations. Overall, these tight constraints suggest that current CI technology will not be able to convey the pitches of combinations of spectrally overlapping complex tones.

Keywords: cochlear implants; complex pitch; vocoder.

Fig. 1

Schematic representation of the stimulus paradigm (left-side panels) and the spectrum of the mixtures (right-side panel s). The top row shows the HIGH condition where the maskers are both lower in frequency than the target. The middle and bottom rows illustrate the MID and LOW conditions, respectively. The vertical grey lines (leftside panels) indicate the start and stop of the background noise.

Fig. 2

Percent correct scores for experiment 1 are plotted for each channel condition (32, 48, and 64 channels) for each position of the target (high, mid, and low). Each dot within the bar represents an individual subject. The black line in the middle of each bar denotes the mean, the lighter portions represent standard error of the mean, and the darker outer portions represent standard deviations.

Fig. 3

Percent correct scores for experiment 2 are plotted. Each subplot shows data for different amounts of spectral overlap conditions (72, 96, 120, 144 dB/octave), for each channel condition (32, 48, and 64 channels) and target positions (high, mid, and low) shown within each panel. Each dot within the bar represents an individual subject. The black line in the middle of each bar denotes the mean, the lighter portions represent standard error of the mean, and the darker outer portions represent standard deviations.

Fig. 4

Spectrum of the three-tone mixture stimulus after vocoding with the No Overlap and 144 dB/oct conditions (upper and lower rows, respectively). The F0s of the tones were 353.55, 471.93, and 561.23 Hz