Abstract
The neuronal mechanisms underlying tinnitus are yet to be revealed. Tinnitus, an auditory phantom sensation, used to be approached as a purely auditory domain symptom. More recently, the modulatory impact of non-auditory brain regions on the percept and burden of tinnitus are explored. The thalamus is uniquely situated to facilitate the communication between auditory and non-auditory subcortical and cortical structures. Traditionally, animal models of tinnitus have focussed on subcortical auditory structures, and research with human participants has been concerned with cortical activity in auditory and non-auditory areas. Recently, both research fields have investigated the connectivity between subcortical and cortical regions and between auditory and non-auditory areas. We show that even though the different fields employ different methods to investigate the activity and connectivity of brain areas, there is consistency in the results on tinnitus between these different approaches. This consistency between human and animals research is observed for tinnitus with peripherally instigated hearing damage, and for results obtained with salicylate and noise-induced tinnitus. The thalamus integrates input from limbic and prefrontal areas and modulates auditory activity via its connections to both subcortical and cortical auditory areas. Reported altered activity and connectivity of the auditory, prefrontal, and limbic regions suggest a more systemic approach is necessary to understand the origins and impact of tinnitus.
Highlights
Tinnitus, commonly known as ‘ringing in the ears’, is a sensation of sound attributed to a location inside the head, to both ears, or to one ear
Both the Prefrontal cortex (PFC) and hippocampal formation send excitatory projections to the NAc and are interconnected with the amygdala. This route is in line with the proposal of Leaver et al (2011) that decreasing activity in the VMPFC could disinhibit the action of the thalamic reticular nucleus (TRN) on the ventral part of the medial geniculate body (MGBv), thereby allowing spontaneous subcortical activity more access to the auditory cortex (ACx) (Eggermont, 2021)
In addition to changes in functional connectivity, correlations or coherence based on the lowfrequency fluctuations in the BOLD response are affected (Bijsterbosch et al, 2020; Duff et al, 2018)
Summary
Commonly known as ‘ringing in the ears’, is a sensation of sound attributed to a location inside the head, to both ears, or to one ear. Current animal research related to tinnitus is primarily concerned with subcortical structures, notably the cochlear nucleus, the inferior colliculus, and the thalamus. Most of these studies perform behavioral tests for tinnitus, employing innate auditory reflexes or paradigms that involve training an animal to respond to the absence or presence of sound stimuli. Even though human and animal research on tinnitus used to work at different ends of the auditory pathway, this gap is bridged in more recent work In light of this development, the thalamus is of specific interest due to its unique position to facilitate communication between subcortical and cortical structures. Note that the mean spontaneous firing rate of the MGBv in anesthetized mice is much higher (8.8 spikes/second) than both the MGBd (2.2 s/s) and MGBm (2.6 s/s) divisions (Anderson and Linden, 2011)
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