A novel anxiety process biomarker via electrovestibulography
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Date
2022-04-28
Authors
Bosecke, Cory
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Abstract
Anxiety disorders have no known biomarkers, and are thus diagnosed based on symptoms. Identifying biomarkers for these disorders could help improve the accuracy of anxiety disorder diagnoses, but may be difficult because some of the deepest brain regions are implicated in anxiety. Directly measuring activity from these regions non-invasively is highly complex; however, it may be possible to indirectly measure activity in these regions with the electrophysiological technique ‘Electrovestibulography’ (EVestG). This technique records electrical activity from the ear canals with cotton tipped wire electrodes, and detects field potentials (FPs) caused by vestibular neurons. These FPs may reveal an anxiety biomarker because they are mediated by the brainstem vestibular nucleus (VN), which receives dense projections from three anxiety-implicated brainstem regions, that in turn receive projections from anxiety-implicated regions whose homologues in rodents are known to exhibit the rhythmicity of the anxiety-implicated ‘hippocampal theta rhythm’ (hTheta). Vitally, depth recording has revealed a human analogue of hTheta in the hippocampus (HPC); and Shadli et al. [1] found an anxiety process biomarker ‘goal conflict specific rhythmicity’ (GCSR) that seems to be driven by a human analogue of hTheta, in frontal electroencephalography (EEG) data, via an anxiety-related ‘Stop Signal Task’ (SST). It may therefore be possible to test whether a human analogue of hTheta entrains vestibular FPs by eliciting this analogue with the SST while recording FPs with EVestG, where altered FP firing patterns would indicate a GCSR ‘analogue.’
The current study was comprised of two experiments: an ‘EEG Experiment’ and ‘EVestG Experiment,’ where healthy volunteers (five and four, respectively) performed the SST by pressing keyboard keys in response to images on a screen while frontal EEG or EVestG data were recorded. The ‘EEG Experiment’ was aimed at partially replicating an experiment in Shadli et al. [1] by reproducing GCSR in frontal EEG power spectra; while the ‘EVestG Experiment’ — a novel experiment — was aimed at eliciting an analogue of GCSR in vestibular FP firing patterns. The EEG Experiment showed no GCSR, indicating that frontal EEG power was not significantly altered during the anxiogenic conditions of the SST. The EVestG Experiment, however, revealed a GCSR ‘analogue’ in the right ear EVestG data of volunteers and indicated that vestibular FPs occurred at a higher than average (9 Hz) firing rate during the anxiogenic conditions of the SST. The theory behind GCSR implies that a human analogue of hTheta may have entrained these FPs. This could have caused vestibular neurons to burst fire, grouping FPs and making them easier to detect with EVestG (which there was evidence of, in the results). Critically, the GCSR ‘analogue’ validates a novel anxiety biomarker (caused by a human analogue of hTheta) that could possibly provide objective data to clinicians that could inform anxiety disorder diagnoses, in the future. However, due to the low recruitment (related to COVID-19 lockdowns) in the current study, results should be interpreted with caution; and both experiments should be repeated with more volunteers.
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Keywords
EVestG, anxiety, hippocampal theta