Measurement of phase-locked neural responses to cochlear-implant stimulation from multiple stages of the auditory system.

Studies investigating objective measures of auditory perception when using cochlear implants (CIs) differ in the stimuli used and in the neural response measured. The present study aims to disentangle the effects of response type and of stimulus type on neural response properties. We report three measures corresponding with separate neural latencies, all collected from the same 19 CI recipients and with the same stimulus, consisting of a continuous low-rate electrical pulse train of about 40 pulses per second (pps). These measures comprise the electrically evoked compound action potential (eCAP), auditory brainstem response (eABR), and auditory steady-state response (eASSR), having putative neural generators in peripheral (auditory nerve), subcortical (brainstem), and thalamo-cortical auditory regions, respectively. When assessing correlations across CI recipients, we found that the growth of eASSR amplitude with stimulus level was correlated with that of eCAP amplitude and predictive of the just-noticeable-to-comfortable electrical dynamic range, as measured through behavioral loudness scaling. Additionally, we compared eASSRs measured with the ∼40-pps stimulus and with a high-rate (500-pps) pulse train modulated in amplitude at rates around 40 Hz, and found that both showed similar recipient-specific effects of stimulation rate on response phase and amplitude. Our results highlight that neural responses to different electrical stimuli and at different central processing stages share common idiosyncratic properties, and support the validity of eASSRs for loudness-growth estimation in CI recipients.