Electrical stimulation of auditory nerve fibers using cochlear implants (CI) shows psychophysical forward masking (pFM) up to several hundreds of milliseconds. By contrast, recovery of electrically evoked compound action potentials (eCAPs) from forward masking (eFM) was shown to be more rapid, with time constants no greater than a few milliseconds. These discrepancies suggested two main contributors to pFM: a rapid-recovery process due to refractory properties of the auditory nerve and a slow-recovery process arising from more central structures. In the present study, we investigate whether the use of different maskers between eCAP and psychophysical measures, specifically single-pulse versus pulse train maskers, may have been a source of confound.
In experiment 1, we measured eFM using the following: a single-pulse masker, a 300-ms low-rate pulse train masker (LTM, 250 pps), and a 300-ms high-rate pulse train masker (HTM, 5000 pps). The maskers were presented either at same physical current (Φ) or at same perceptual (Ψ) level corresponding to comfortable loudness. Responses to a single-pulse probe were measured for masker-probe intervals ranging from 1 to 512 ms. Recovery from masking was much slower for pulse trains than for the single-pulse masker. When presented at Φ level, HTM produced more and longer-lasting masking than LTM. However, results were inconsistent when LTM and HTM were compared at Ψ level. In experiment 2, masked detection thresholds of single-pulse probes were measured using the same pulse train masker conditions. In line with our eFM findings, masked thresholds for HTM were higher than those for LTM at Φ level. However, the opposite result was found when the pulse trains were presented at Ψ level.
Our results confirm the presence of slow-recovery phenomena at the level of the auditory nerve in CI users, as previously shown in animal studies. Inconsistencies between eFM and pFM results, despite using the same masking conditions, further underline the importance of comparing electrophysiological and psychophysical measures with identical stimulation paradigms.
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We would like to thank all subjects for their participation. We would also like to thank the editors and two anonymous reviewers for their insightful comments and helpful suggestions. This study was funded by the Agence Nationale de Recherche (ANR-11-PDOC-0022) and by the Mission pour l’interdisciplinarité du CNRS (Project DEFI-SENS). Parts of this work were presented at the 8th International Symposium on Objective Measures in Auditory Implants October 15–18, 2014 in Toronto, Ontario, Canada and at the Conference on Implantable Auditory Prostheses July 12–17, 2015 in Granlibakken, Lake Tahoe, CA, USA.
Compliance with Ethical Standards
A local ethics committee approved this study (Eudract 2012-A00438-35).
Conflict of Interest
The authors declare that they have no conflict of interest.
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