Experimental assessment of polyphonic tones with cochlear implants

Hypothesis: We hypothesized that cochlear implant (CI) users are able to discriminate tones consisting of 1 and 2 modulation frequencies when the stimuli are applied through direct electrical stimulation. Background: Music perception is a very challenging task for CI users. In music, multiple tones often occur simultaneously, an essential feature of harmony. Proper encoding of simultaneous tones is crucial to musical perception and appreciation. With current implant processing strategies, CI users are severely impaired in the perception of pitch and polyphony. Methods: The ability of CI users to identify the number of simultaneous tones was assessed. Stimuli were applied with direct electrical stimulation. Stimuli with 1 modulation frequency were applied on a basal, a middle, and an apical electrode to determine if there was an effect of cochlear region. Stimuli with 2 modulation frequencies were applied on combinations of an apical electrode together with a basal or a middle electrode. Additionally, 2 modulations frequencies were presented at the same time on an apical electrode only. Results: Results demonstrate that subjects were generally able to identify the number of modulation frequencies in the presented stimuli. Performance for 1 modulation frequency stimuli was significantly above chance level on all 3 electrodes tested. Performance was best on the apical and the middle electrode, followed by the basal electrode. Subjects were also able to identify 2 modulation frequencies significantly above chance level on all 3 combinations tested. Performance was best on combination apical-basal followed by apical-middle. Performance was worst when 2 modulation frequencies were applied on an apical electrode only, but it was still significantly above chance level. Conclusion: If sound processing strategies were to use concurrent modulation frequencies on multiple or single electrodes, then possibly polyphonic tones would be better perceived by CI users yielding better music and language perception.