Cochlear nucleus small cells use olivocochlear collaterals to encode sounds in noise
Adam Hockley, Calvin Wu, Susan E Shore
Understanding speech, especially in noisy environments, is crucial to social interactions. Yet, as we age, speech processing can be disrupted by cochlear damage and the subsequent auditory nerve fiber degeneration. The most vulnerable-medium and high-threshold-auditory nerve fibers innervate various cell types in the cochlear nucleus, among which, the small cells are unique in receiving this input exclusively. Here, we characterize small cell firing characteristics, demonstrating superior temporal as well as intensity coding. We show that small-cell unique coding properties are facilitated by direct cholinergic input from the medial olivocochlear system. These results highlight the small cell-olivocochlear circuit as a key player in signal processing in noisy environments, which may be selectively degraded in aging or after noise insult.
Pre-emptive MOC-pathway enhancement protects the cochlea from synaptopathic acoustic
304 trauma (Boero et al., 2018), but whether increased MOC activation following synaptopathic noise
305 exposure could reverse speech-in-noise deficits is still unknown. Conversely, in humans with
306 tinnitus or hyperacusis, an increased contralateral suppression of otoacoustic emissions
307 (Knudson et al., 2014) suggests an over-responsive MOC system in this pathology. MOC neurons
308 are modulated by descending input from the inferior colliculus (Malmierca et al., 1996; Robertson
309 & Mulders, 2000; Schofield & Cant, 1999), auditory cortex (Coomes & Schofield, 2004; Mulders
310 & Robertson, 2000), and local inhibitory interneurons of the medial nucleus of the trapezoid body
311 (Torres Cadenas et al., 2020). Descending modulation may be a route for auditory attention to
312 affect peripheral processing, which may explain attentional facilitation of speech-in-noise
313 understanding (de Boer et al., 2012). Descending modulation of MOC activity would also affect
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314 MOC collateral input to CN SCs, which accurately encode intensity, providing an additional action
315 pathway for auditory attention. The SCC in humans occupies a larger proportion of the CN than
316 in rodents, and is therefore poised to play a major role in central mechanisms of speech
317 perception (Moore & Osen, 1979).