Temporal interactions between pulses in CI users: a look through the modelling of neural distortion responses

Authors: Francois Guerit1, Charlotte Garcia2, Robert P. Carlyon1

1MRC Cognition and Brain Sciences Unit, University of Cambridge
2University of Cambridge

Background: In Cochlear Implant (CI) users, temporally interleaving 2 pulse trains modulated at 80 and at 120 Hz produces a neural distortion response (NDR) at 40 Hz in the cortical EEG response (Carlyon et al. 2021). The NDR arises because the auditory system “undoes” the interleaving and is non-linear. We used modelling to investigate whether phenomena such as refractoriness and facilitation as early as the auditory nerve are sufficient to create an NDR that matches the observed pattern of results.

Methods: We used two phenomenological models of the auditory nerve (AN), one that includes only refractoriness and spike-rate adaptation (Brochier et al. 2022), and a second that additionally includes facilitation and accommodation (Joshi et al. 2017). The stimuli were two 480-pps pulse trains modulated at 80 and 120 Hz, separated by a variable inter-pulse interval (IPI) ranging from 0 to 1000 µs. We then measured the summed neural activity at 40 Hz.

Results: Both models created an NDR. In the Joshi et al. model and at IPIs below 400 µs, the NDR stemmed from every third cycle of the 120-Hz pulse train being facilitated by the 80-Hz pulse train. At longer IPIs, and at all IPIs for the Brochier et al. model, every third cycle was instead masked due to refractoriness, yet also resulted in an NDR. None of the models could accurately predict the reduction in NDR at longer IPIs seen in humans, but a modified version (with a faster release from refractoriness) of the Brochier et al. model could. Preliminary data showed that the phase of the NDR was better predicted by refractoriness mechanisms than facilitation.

Conclusion: Neural distortions caused by temporal interactions between pulses may already be present at the level of the AN. Modelling studies may reveal the nature of these interactions.