Using interleaved stimulation and EEG to measure temporal smoothing and growth of the sustained neural response to cochlear-implant stimulation

Two EEG experiments measured the sustained neural response to amplitude-modulated (AM) high-rate pulse trains presented to a single cochlear-implant (CI) electrode. Stimuli consisted of two interleaved pulse trains with AM rates F1 and F2 close to 80 and 120 Hz respectively, and where F2=1.5F1. Following Carlyon et al. (2021) we assume that such stimuli can produce a Neural Distortion Response (NDR) at F0 = F2-F1 Hz if temporal dependencies (“smoothing”) in the auditory system are followed by one or more neural nonlinearities. In experiment 1 the rate of each pulse train was 480 pps and the gap between pulses in the F1 and F2 pulse trains ranged from 0 to 984 μs. The NDR had a roughly constant amplitude for gaps between 0 and about 200-400 μs, and decreased for longer gaps. We argue that this result is consistent with a temporal dependency, such as facilitation, operating at the level of the auditory nerve and/or with co-incidence detection by cochlear-nucleus neurons. Experiment 2 first measured the NDR for stimuli at each listener’s most comfortable level (“MCL”) and for F0 = 37, 40, and 43 Hz. This revealed a group delay of about 42 ms, consistent with a thalamic/cortical source. We then showed that the NDR grew steeply with stimulus amplitude and, for most listeners, decreased by more than 12 dB between MCL and 75% of the listener’s dynamic range. We argue that the NDR is a potentially useful objective estimate of MCL.