Subcortical pathways: Towards a better understanding of auditory disorders

Abstract

Hearing loss is a significant problem that affects at least 15% of the population. This percentage, however, is likely significantly higher because of a variety of auditory disorders that are not identifiable through traditional tests of peripheral hearing ability. In these disorders, individuals have difficulty understanding speech, particularly in noisy environments, even though the sounds are loud enough to hear. The underlying mechanisms leading to such deficits are not well understood. To enable the development of suitable treatments to alleviate or prevent such disorders, the affected processing pathways must be identified. Historically, mechanisms underlying speech processing have been thought to be a property of the auditory cortex and thus the study of auditory disorders has largely focused on cortical impairments and/or cognitive processes. As we review here, however, there is strong evidence to suggest that, in fact, deficits in subcortical pathways play a significant role in auditory disorders. In this review, we highlight the role of the auditory brainstem and midbrain in processing complex sounds and discuss how deficits in these regions may contribute to auditory dysfunction. We discuss current research with animal models of human hearing and then consider human studies that implicate impairments in subcortical processing that may contribute to auditory disorders.

When Sound Stops: Offset Responses in the Auditory System

“…Abnormalities in gap detection and brain responses to stimulus transients, including sound offsets, have also been reported in subjects with developmental or neurological disorders [128,129] and in animal models of aging and disease [1,11].…”

“…A likely cause of gap-detection deficits and other auditory temporal processing difficulties is alteration in excitatory and inhibitory synaptic transmission, particularly weakening of inhibition [1,11,130]. Notably, in mouse models of auditory neuropathy and tinnitus, emergence of auditory temporal processing deficits has been linked to abnormalities in excitation-inhibition balance in the auditory midbrain and cortex [131,132].…”

“…These neurons appear to be especially concentrated in particular nuclei of the auditory brainstem [9] and specific subregions of the thalamus [10], suggesting the existence of a dedicated "offset pathway" in the auditory brain. Moreover, recent studies indicate that neural representation of sound transients, including offsets, may be key to perception of communication sounds such as speech [1,11]. It is time to re-assess the importance of sound-offset responses in the auditory system.…”

A Novel Class of Inferior Colliculus Principal Neurons Labeled in Vasoactive Intestinal Peptide-Cre Mice

“…Neurons in the IC exhibit selective responses to the spectral and temporal content of sounds and 27 perform computations important for sound localization and the identification of speech and other 28 communication sounds (Felix et al, 2018;Winer and Schreiner, 2005). Despite these critical functions, 29…”

Auditory cortical generators of the Frequency Following Response are modulated by intermodal attention

“…In principle, auditory cortical processes could affect cochlear activity via only two synapses (Winer 2006; Suga 2008; Dragicevic et al 2015). These corticofugal modulations are essential in adapting responses of subcortical neurons, for example, by modulating their spectral tuning curves (Suga 2008; Felix et al 2018). However, the extent to which subcortical auditory brain regions are implicated in selective attentional modulation is not well established.…”