Evaluation of the pulse-spreading harmonic complex vocoder for simulating cochlear implant hearing

International audienceNoise- and sine-carrier vocoders are often used to acoustically simulate the information transmitted by a cochlear implant (CI). However, sinusoids fail to mimic the broad spread of excitation produced by a CI, and noise bands contain intrinsic modulations that are absent in CIs. Pulse spreading harmonic complexes (PSHCs) are complexes with a high-density spectrum that can be shaped spectrally and whose rate can be adjusted to minimize the amount of intrinsic modulations at the output of auditory filters, possibly providing a stimulus qualitatively similar to the electrical pulse trains of a CI. Here, we aimed to evaluate the PSHC vocoder in two experiments with single-sided deaf (SSD) CI listeners. In Experiment 1, electric-acoustic pitch matches were collected from 13 subjects. Electric stimulation was presented to an apical or a middle electrode. Acoustic stimulus types were either sinusoids (SINE), 1/3-octave wide narrow bands of Gaussian noises (NOISE), or 1/3-octave wide PSHCs. Assuming that electric-acoustic pitch matches should be less variable if the acoustic and electric stimuli are qualitatively similar, it was hypothesized that pitch matches of PSHC would yield a lower variance than those for SINE or NOISE. Although this hypothesis was not verified, the mean pitch matches showed significant effects of electrode position and stimulus type. In Experiment 2, 9 subjects were asked to compare the similarity of speech sentences presented through the audio input of their CI with different vocoder implementations that used PSHC, NOISE or SINE carriers. The vocoder using PSHC carriers was judged more similar to the sound of their CI compared to the other vocoders. However, it did not provide an exact match to the sound of their CI, prompting further research to better acoustically simulate CI hearing