Tinnitus, or phantom sound perception, likely results from aberrant central auditory neural activity. Gaps in identifying brain changes in human tinnitus are mainly due to limited compatible technology to map auditory regions in real-time. The ability to identify changes is critical to mapping aberrant brain regions that could objectify the disease. Functional near infrared-spectroscopy (fNIRS) is an ideal brain imaging tool to investigate central auditory changes in human tinnitus. Using fNIRS, we demonstrated increased hemodynamic activity (Issa et al., 2016), and brain connectivity (San Juan et al., 2017) between auditory and non-auditory cortices that may serve as potential objective markers of human tinnitus. A limitation of existing fNIRS technology lies in restricted (to outer cerebral cortex) depth of IR penetration through skin and skull using traditional ?cap? recording configurations. Since brain changes in tinnitus likely extend to deeper cortical/sub-cortical regions, it is necessary to improve IR brain penetration to measure putative tinnitus markers in these regions. The primary goal of this project is to expand brain surveillance using fNIRS by adapting the IR-source and detector pair of conventional probes deeper within the skull through natural openings (ear canal). Physically placing fNIRS probes deeper into the skull using this highly innovative approach will broaden brain surveillance to regions never measured before with fNIRS. The purpose of this application is not to identify/uncover the underlying mechanisms of tinnitus, but rather to use published changes (hemodynamic responses and connectivity) in human tinnitus as a platform to validate the adapted probes. To test our central hypothesis that fNIRS technology adapted to the ear canal will expand brain surveillance beyond traditional probes and provide quantifiable correlates (increased hemodynamic rates and brain connectivity) of tinnitus and potentially, other brain diseases two specific aims will be utilized.
Aim 1 will measure changes in human tinnitus and normal controls using simultaneous ?cap? probes and adapted fNIRS probes that contain an IR source or detector placed on the ventral/medial temporal lobe via the ear canal. Here, we expect to validate the adapted probes to determine efficacy/optimal parameters as compared to ?cap? probes to improve anatomic localization and expand brain surveillance.
Aim 2 will employ an advanced probe that retains both IR-source and detector on a single-fiber engineered for ear canal placement. The goal here is to optimize this single-fiber to measure hemodynamic changes and brain connectivity as a stand-alone technology not reliant on ?cap? probes. Once validated, single-fiber probes could be adapted to other clinical questions (i.e., frontal lobe studies via the anterior cranial base/nasal cavity) or used to investigate brain regions not measurable with existing fNIRS technology. The proposed adaptation of fNIRS probes could expand brain surveillance beyond cortical studies and provide critical information about brain diseases including tinnitus in a non-invasive, portable manner to be applied to future R01 applications.
An unclear understanding of the causes of tinnitus and restricted therapies are due, in part, to limited imaging technology capable of characterizing human brain changes in tinnitus. Functional near-infrared spectroscopy (fNIRS) is a non-invasive method for measuring cerebral cortical activity that shows increased neural activity and brain connectivity in human tinnitus. These brain changes in tinnitus may be objective correlates of the disease that could also occur in deeper brain regions beyond the reach of current fNIRS technology. Therefore, it is critical to modify existing fNIRS technology to probe deeper and wider within the brain. The primary goal of this project is to use objective brain changes as a platform to validate new probes to expand brain surveillance.