Relationship Between Intra-Cochlear Current Spread and Speech Intelligibility in Adult Cochlear Implant Users

Background: Inter-channel interactions are thought to affect speech intelligibility in cochlear implant (CI) users. While current psychoacoustic and electrophysiological tests measuring these interactions require cochlear neurons to be activated, none has studied so far the spread of excitation across electrodes, upstream of the neural tissue.

Objectives: As broader intra-cochlear current spread implies higher interactions, we aimed at evaluating the relationship between intra-cochlear current spread and speech intelligibility.

Material and methods: Using the voltage matrices recorded during impedance measurement in 36 adults with more than 11 months of CI experience, exponential spread coefficient (ESC) was computed for each subject.

Speech audiometry was realized during the same session, with intelligibility in quiet at 40 dB SPL (I40) and 50% speech reception threshold (SRT50) being collected.

Correlation between ESC and either I40 or SRT50 was assessed using the Bravais-Pearson test.

Results: SRT50 was obtained in 33 subjects. When analyses were done for these 33 subjects, no significant correlation was found between ESC and either I40 (r = -0.251, p = 0.158) or SRT50 (r = 0.200, p = 0.263). However, when analyses were done in the 27 subjects for whom all electrodes were activated, ESC was found to be correlated not only with I40 (r = -0.516, p = 0.017) but also with SRT50 (r = 0.471, p = 0.031).

Conclusion: Results indicate that speech intelligibility in adult CI users is influenced by the spread of intra-cochlear current spread upstream of the neural tissue.