Summary

Tinnitus is sound perceived in the absence of an external sound source. This abnormal auditory sensation, which can be in the form of buzzing, ringing or whistling, affects one or both ears. For a significant number of people with hearing loss, it can cause considerable deterioration in the quality of life and ability to work. In the two studies described in this report, a recent model, referred to as central auditory gain, has been used to integrate normal and pathological data into the same conceptual framework. It has been speculated that “central auditory gain” is a normal mechanism by which the auditory system modulates its response when acoustic conditions change. For example, auditory sensitivity increases with auditory deprivation, while it decreases with auditory stimulation. This phenomenon has been documented in adults with normal hearing. However, mainly subjective assessments of intensity have been measured, leaving questions about the existence and the localization of this central auditory gain mechanism unaddressed.

The objective of the first study was to demonstrate the existence of central auditory gain and to localize it functionally. Two groups of adults with normal hearing wore earplugs or noise generators for one week. They underwent tests before and after deprivation (earplugs) or stimulation (noise generators) with a hearing assessment battery that included measurements from the cochlea to the auditory cortex. The results demonstrate that the auditory system effectively modulates its response according to acoustic conditions (deprivation or stimulation, although less so for the latter), and that this modulation does not occur at the peripheral level (i.e., in the cochlea), but within the auditory cortex, i.e., in the highest level of the auditory system. In fact, in our study no change was observed below this level. Thus, the presence of auditory gain modulation of purely central origin is supported by our data.

The objective of the second study was to examine whether the central auditory gain could be modulated among adults with tinnitus. In fact, in this population, it has been suggested that the central auditory gain mechanism is maladaptive (in that it overreacts to stimuli), and that it could be responsible for tinnitus and hyperacusis, which are defined as auditory hypersensitivity. The model suggests that tinnitus reflects spontaneous neural hyperactivity, while hyperacusis reflects hyperactivity caused by external sounds. Essentially, the central auditory gain appears to be chronically altered among people with tinnitus and hyperacusis and constitutes the principal pathophysiological mechanism in hearing disorders. If such is the case, a return to normal of the gain adaptation mechanisms should be reflected by a decrease in the sensitivity observed in appraisals of loudness and even a decrease in the intensity of the tinnitus. In the second study, participants with or without hearing loss, and who had tinnitus, used noise generators for three weeks. Auditory and psychometric measurements were taken before the test, after one week of wearing the generators, after three weeks, and then one month after the end of the tests.

Our laboratory results suggest that wearing noise generators decreases sensitivity to external sounds and reduces the loudness of tinnitus. This decrease was more significant in the group without hearing loss. The subjective intensity of the tinnitus and the disturbance that it causes in daily life, as measured by visual analogue scales, also declines with treatment. The preliminary findings are the first resulting from a joint examination of two different tasks (the loudness of tinnitus and loudness functions) that involve a modulation of intensity (external sounds and tinnitus) possibly originating from a common mechanism, normal in one case and pathological in the other. Overall, our data suggest that the central auditory gain mechanism, present among the participants with normal hearing, could be used successfully to objectively measure improvement after the use of noise generators in people suffering from tinnitus and hyperacusis.