What is the role of the hippocampus and parahippocampal gyrus in the persistence of tinnitus?

Abstract

The hippocampus and parahippocampal gyrus have been implicated as part of a tinnitus network by a number of studies. These structures are usually considered in the context of a "limbic system," a concept typically invoked to explain the emotional response to tinnitus. Despite this common framing, it is not apparent from current literature that this is necessarily the main functional role of these structures in persistent tinnitus. Here, we highlight a different role that encompasses their most commonly implicated functional position within the brain-that is, as a memory system. We consider tinnitus as an auditory object that is held in memory, which may be made persistent by associated activity from the hippocampus and parahippocampal gyrus. Evidence from animal and human studies implicating these structures in tinnitus is reviewed and used as an anchor for this hypothesis. We highlight the potential for the hippocampus/parahippocampal gyrus to facilitate maintenance of the memory of the tinnitus percept via communication with auditory cortex, rather than (or in addition to) mediating emotional responses to this percept.

Keywords: auditory; hippocampal; memory; neuroscience.

FIGURE 1

Key structures in the medial temporal lobe.

FIGURE 2

The proposed model by which hippocampal activity may be preserved when hearing loss is present with tinnitus. In one of the models (Model 2) put forward in Griffiths et al. (2020), some hippocampal dysfunction may occur as a result of diminished auditory input, which results in degraded communication between auditory cortex and MTL regions (top panel). In the scenario where tinnitus is maintained by an ongoing prediction of the memory of a phantom signal (bottom panel), communication between MTL regions is at least somewhat preserved, due to a perpetual updating of this signal in both directions; predictions held in memory update to correspond with ongoing auditory cortex activity, and sensory representations in auditory cortex are adjusted in light of predictions in memory.