Tinnitus and auditory cortex: using adapted functional near-infrared spectroscopy to measure resting-state functional connectivity

Abstract

Objective Tinnitus, phantom sound perception, arises from aberrant brain activity within auditory cortex. In tinnitus animal models, auditory cortex neurons show increased spontaneous firing and neural synchrony. In humans, similar hyperactivation in auditory cortex has been displayed with functional near-infrared spectroscopy (fNIRS). Resting-state functional connectivity (RSFC) or increased connectivity between brain regions has also been shown in tinnitus using fNIRS. However, current fNIRS technology utilizes infrared (IR)-sources and IR-detectors placed on the scalp that restricts (~3 cm depth IR penetration) signal capture to outer cerebral cortex due to skin and skull bone. To overcome this limitation, in this proof of concept study, we adapted fNIRS probes to fit in the external auditory canal (EAC) to physically place IR-probes deeper within the skull thereby extracting neural signals from deeper auditory cortex. Methods Twenty adults with tinnitus and 20 nontinnitus controls listened to periods of silence and broadband noise before and after 5 min of silence to calculate RSFC. Concurrent scalp probes over auditory cortex and an adapted probe placed in the right EAC were utilized. Results For standard probes, left and right auditory cortex in tinnitus showed increased RSFC to each other and to other nonauditory cortices. Interestingly, adapted fNIRS probes showed trends toward increased RSFC. Conclusion While many areas for the adapted probes did not reach significance, these data using a highly innovative and newly created probe adapting fNIRS technology to the EAC substantiates our previously published data in human tinnitus and concurrently validates this technology as a useful and expanded brain imaging modality.

Functional Brain Regions Linked to Tinnitus Pathology and Compensation During Task Performance: A Systematic Review

“…Tinnitus is proposed to occur when the auditory cortex neurons show increased spontaneous firing and neural synchrony 42 . Supporting this theory, Zenke et al observed an increased auditory cortex activation 37 .…”

“…Tinnitus is proposed to occur when the auditory cortex neurons show increased spontaneous firing and neural synchrony. 42 Supporting this theory, Zenke et al observed an increased auditory cortex activation. 37 The authors suggest that a limbic-auditory interaction may be to blame for the presence of tinnitus.…”

Tinnitus und multimodale kortikale Interaktion

“…Currently, the number of sensors used is relatively small, so that a priori decisions must be made as to which regions’ activity will be recorded and evaluated. Initial studies indicate that cortical changes in tinnitus are not limited to auditory regions 55 , but also affect emotion-relevant regions 56 .…”

“…Aktuell ist die Anzahl der verwendeten Sensoren relativ klein, so dass a priori entschieden werden muss, die Aktivität welcher Regionen aufgezeichnet und ausgewertet wird. Erste Untersuchungen weisen auch hier daraufhin, dass sich kortikale Veränderungen bei Tinnitus nicht auf auditive Regionen beschränken 55 , sondern auch hier z. B. emotionsrelevante Regionen betreffen 56 .…”

The Pathological Mechanisms and Treatments of Tinnitus

“…One study showed increasing intensity of perceived tinnitus with increased gamma band activity of the contralateral auditory cortex 43 whilst another study demonstrated increased gamma band activity in the left and right primary and secondary auditory cortex of tinnitus patients 44 . An innovative functional near-infrared spectroscopy (fNIRS) approach demonstrated increased hemodynamic activity in the AC indicating plasticity changes 45 whereas another fNIRS illustrated increased resting state functional activity in the AC of tinnitus patients vs. non tinnitus individuals 46 . A rat model illustrated augmented cortical excitability post- salicylate induced tinnitus 47 , again reiterating the cellular mechanism of hyperactivity 47 discussed previously but now in the context of the AC.…”