The Inferior Colliculus: A Target for Deep Brain Stimulation for Tinnitus Suppression

Abstract

Tinnitus is a neurological condition that manifests as a phantom auditory perception in the absence of an external sound source. Tinnitus is often caused by hearing loss associated with noise exposure or aging and as such, the prevalence is only expected to rise in the coming years. Currently there is no cure for tinnitus and available treatment options have only shown limited success, thus there is an ever present need for continued research into new treatments. In this thesis we propose a new approach to treating tinnitus that uses deep brain stimulation to target the inferior colliculus (IC) with the goal of altering tinnitus-related neural activity, such as hyperactivity and increased neural synchrony, to suppress the tinnitus percept. We hypothesize that stimulation of the outer cortices of the inferior colliculus will modulate the tinnitus-affected neurons in the central region of the inferior colliculus (ICC) and in turn, these neural changes will be carried throughout the central auditory system by the extensive projection network originating in the IC, and will induce modulation in other tinnitus-affected auditory nuclei. The research of this thesis is aimed at determining the feasibility of this tinnitus treatment by assessing the IC as a potential neuromodulation target and identifying optimal stimulation locations and stimulation strategies for achieving maximal suppression. The first study was completed to better understand the auditory coding properties of the IC and to create a three dimensional reconstruction of these functional properties across the entire IC. These results narrowed down the stimulation target to the dorsal cortex of the inferior colliculus (ICD) and produced a tool that could be used to consistently place stimulating and recording electrodes in correct regions in the IC. The second and third studies focused on assessing the best stimulation locations and stimulation paradigms within the ICD, respectively, by stimulating throughout and measuring changes in neural activity in the ICC. These results show that maximal suppression is achieved by stimulation of the rostral-medial region of the ICD using either electrical stimulation only or electrical stimulation paired with acoustic stimulation with an 18 ms delay. These results will guide implementation in human patients. There are already deaf patients who suffer from tinnitus that are being implanted with a deep brain stimulator for hearing restoration called the auditory midbrain implant. Hardware modifications to the auditory midbrain implant have been completed that will allow us to stimulate the ICD and evaluate the effects on the tinnitus percept directly in patients.