Adaptation to noise in amplitude modulation detection without the medial olivocochlear reflex

Abstract

The detection of amplitude modulation (AM) in quiet or in noise improves when the AM carrier is preceded by noise, an effect that has been attributed to the medial olivocochlear reflex (MOCR). We investigate whether this improvement can occur without the MOCR by measuring AM sensitivity for cochlear implant (CI) users, whose MOCR effects are circumvented as a result of the electrical stimulation provided by the CI. AM detection thresholds were measured monaurally for short (50 ms) AM probes presented at the onset (early condition) or delayed by 300 ms (late condition) from the onset of a broadband noise. The noise was presented ipsilaterally, contralaterally and bilaterally to the test ear. Stimuli were processed through an experimental, time-invariant sound processing strategy. On average, thresholds were 4 dB better in the late than in the early condition and the size of the improvement was similar for the three noise lateralities. The pattern and magnitude of the improvement was broadly consistent with that for normal hearing listeners [Marrufo-P erez et al., 2018, J Assoc Res Otolaryngol 19:147e161]. Because the electrical stimulation provided by CIs is independent from the middle-ear muscle reflex (MEMR) or the MOCR, this shows that mechanisms other than the MEMR or the MOCR can facilitate AM detection in noisy backgrounds.

Direct Cochlear Recordings in Humans Show a Theta Rhythmic Modulation of Auditory Nerve Activity by Selective Attention

“…To our knowledge, attentional modulation via the LOC remains completely unknown, as direct recordings of auditory nerve activity are normally not feasible in humans. Given the absence of the efferent MOC reflex in CI recipients ( Wilson et al, 2003 ; Lopez-Poveda et al, 2016 ; Marrufo-Pérez et al, 2019 ) in addition to substantial OHC degeneration, as well as the overall scarcity of efferent MOC projections ( Liberman and Liberman, 2019 ), potential alterations of respective auditory nerve activity in a selective attention paradigm should largely reflect top-down signals from the LOC ( Lopez-Poveda, 2018 ). Crucially, LOC innervation density peaks at the apical end of the cochlea, whereas MOC projections peak in mid-cochlear regions ( Liberman and Liberman, 2019 ).…”

“…This technique allows for discrete sampling of that period within a single trial that can later be processed like standard EEG recordings. Interestingly, CI recipients lack the efferent MOC reflex that leads to cochlear dynamic compression in normal hearing ( Wilson et al, 2003 ; Lopez-Poveda et al, 2016 ; Marrufo-Pérez et al, 2019 ). As they additionally show substantial degeneration of OHCs which are further damaged during surgical implant insertion, proposed CI recordings during selective attention should mostly reflect modulations of spiral ganglion cell activity via LOC connections ( Lopez-Poveda, 2018 ) rather than modulations of scattered OHC populations by residual MOC efferents.…”

Direct cochlear recordings in humans show a theta rhythmic modulation of the auditory nerve by selective attention

“…To our knowledge, attentional modulation via the LOC remains completely unknown as direct recordings of auditory nerve activity are normally not feasible in humans. Given the absence of the efferent MOC reflex in CI recipients (Wilson et al, 2003;Lopez-Poveda et al, 2016;Marrufo-Pérez et al, 2019) in addition to substantial OHC degeneration, potential alterations of respective auditory nerve activity in a selective attention paradigm should largely reflect top-down signals from the LOC (Lopez-Poveda, 2018). Our results show that ongoing auditory nerve activity is top-down modulated, putatively suggesting a role of the LOC pathway in selective attention.…”

“…This technique allows for discrete sampling of that period within a single trial that can later be processed like standard electroencephalographic (EEG) recordings. Interestingly, CI recipients lack the efferent MOC reflex that leads to cochlear dynamic compression in normal hearing (Wilson et al, 2003;Lopez-Poveda et al, 2016;Marrufo-Pérez et al, 2019). As CI recipients additionally show substantial degeneration of OHCs which are further damaged during surgical insertion of the implant, proposed CI recordings during selective attention should mostly reflect modulations of spiral ganglion cell activity via LOC synaptic connections (Lopez-Poveda, 2018) rather than modulations of scattered OHC populations by residual MOC efferents.…”