Optimising frequency-to-electrode allocation for individual cochlear implant users

Abstract

Pitch perception for cochlear implant (CI) users is known to vary between individuals due to differences of insertion depth, of the function of neural tissue in the cochlea, of acclimatisation and CI stimulation parameters. In this study, frequency-to-electrode allocation was adjusted in a group of 12 adult cochlear implant users, to ascertain if the use of a default setting results in optimum perception of speech and music for individual recipients. Participants in the experiment trialled a map in which the frequency allocation was adjusted to the frequency-position function of the normal cochlea and a map which allocated sounds to a limited area of the cochlea, in addition to the default. Performance with the two alternative maps did not exceed that of the default allocation and was poorer for the majority of participants: [F(2,14) = 51.3, p<0.001] for a sentence test in noise. Performance was negatively correlated with the magnitude of the adjustment from the default [r=0.838, p=0.002 and r=-0.700, p=0.024] for the two maps, suggesting that participants had acclimatised to their clinical maps. Electrode discrimination was found to be at chance levels for some participants at the apical end of the array but above chance in the middle of the array. Another alternative map, with logarithmic frequency spacing and some basal shift was trialled and gave improved performance on a sentence test in noise for three participants with poor electrode discrimination at the apical end of the array.

A second experiment was conducted, with 13 adult CI users, in which perception of speech and music was assessed with ten frequency allocations, including the default. The ability to follow a pitch contour was measured for centre frequencies of neighbouring filters. Performance with the different allocations varied between individuals; some individuals performed better with alternative allocations from the default. A strategy was developed for the selection of frequency allocation for individuals, based on pitch contour scores for different electrodes, which offered improved performance on the sentence test for the group [t(12)=-3.31, p=0.006, r=0.69]. The overall results show that optimisation of frequency allocation for individuals can be achieved by adjustment of the frequency-to-electrode allocation based on pitch perception ability in different areas of the cochlea.

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