Electro-acoustic pitch matching experiments in patients with single-sided deafness and a cochlear implant: Is there a need for adjustment of the default frequency allocation tables?

Abstract

Patients with single-sided deafness (SSD) and a cochlear implant (CI) can compare the pitch of stimulated electrodes and acoustic tones. A pitch mismatch may negatively bear on the fusion of the signals from the two ears, which may limit auditory performance. We aimed to explore this mismatch, correlate it to performance, and finally to discuss its possible clinical consequences. Ten patients with SSD and a CI (Cochlear Ltd.) compared the pitch of electrical and acoustic stimuli. Patients had to choose one of two acoustic stimuli, with the pitch closest to the pitch of the electrical stimulus at electrodes 3, 7, 11, 15, and 19. The difference between the two acoustic stimuli iteratively decreased from 2 octaves to 1/8 octave, resulting in a "pitch match" per electrode. Furthermore, we computed the insertion angle of the CI electrode array based on high-resolution computed-tomography scans. Subsequently, we created frequency-place maps. The difference between our pitch matches and two references (the spiral ganglion map and the default frequency allocation by Cochlear Ltd.) was defined as "mismatch". We observed large intra- and intersubject variability. Following the tonotopic organization of the cochlea, we observed that the pitch matches decreased with increasing insertion angle. The pitch-matched frequencies were on average 2.0 and 1.3 octaves lower than the spiral ganglion map and the default frequency allocation, respectively. There was no significant correlation between performance (consonant-vowel-consonant phoneme recognition score) and mismatch (R(2) = 0.06, P > 0.1). Given the methodological considerations, and the insignificant correlation between mismatch and performance, pitch matching results must not necessarily lead to a change in clinical fitting strategies.