Abstract
Tinnitus, a phantom auditory sensation, is associated with hearing loss in most cases, but it is unclear if hearing loss causes tinnitus. Phantom auditory sensations can be induced in normal hearing listeners when they experience severe auditory deprivation such as confinement in an anechoic chamber, which can be regarded as somewhat analogous to a profound bilateral hearing loss. As this condition is relatively uncommon among tinnitus patients, induction of phantom sounds by a lesser degree of auditory deprivation could advance our understanding of the mechanisms of tinnitus. In this study, we therefore investigated the reporting of phantom sounds after continuous use of an earplug. 18 healthy volunteers with normal hearing wore a silicone earplug continuously in one ear for 7 days. The attenuation provided by the earplugs simulated a mild high-frequency hearing loss, mean attenuation increased from <10 dB at 0.25 kHz to >30 dB at 3 and 4 kHz. 14 out of 18 participants reported phantom sounds during earplug use. 11 participants presented with stable phantom sounds on day 7 and underwent tinnitus spectrum characterization with the earplug still in place. The spectra showed that the phantom sounds were perceived predominantly as high-pitched, corresponding to the frequency range most affected by the earplug. In all cases, the auditory phantom disappeared when the earplug was removed, indicating a causal relation between auditory deprivation and phantom sounds. This relation matches the predictions of our computational model of tinnitus development, which proposes a possible mechanism by which a stabilization of neuronal activity through homeostatic plasticity in the central auditory system could lead to the development of a neuronal correlate of tinnitus when auditory nerve activity is reduced due to the earplug.
Highlights
Tinnitus is a phantom auditory sensation, the perception of a sound in the absence of a corresponding sound source
Our results demonstrate that unilateral auditory deprivation similar to a mild high-frequency hearing loss is sufficient to induce phantom auditory sensations in healthy normal hearing subjects without tinnitus
This suggests that auditory deprivation, and the concomitant reduction of auditory nerve activity, could be the dominant factor for the development of phantom sounds
Summary
Tinnitus is a phantom auditory sensation, the perception of a sound in the absence of a corresponding sound source. Animal models have demonstrated that cochlear damage can lead to behavioural evidence for tinnitus [3,4] and to the development of increased spontaneous activity of neurons in the central auditory system (see [5] for a review), which has been interpreted as a putative neuronal correlate for the phantom auditory sensation [6]. The development of neural correlates of tinnitus constitutes a side-effect of a stabilization of the mean activity in central auditory neurons through homeostatic plasticity: hearing loss reduces AN activity with a concomitant reduction in excitatory drive to the central auditory system. When homeostatic mechanisms restore average neuronal activity to normal levels by increasing excitation and reducing inhibition, the resulting increase in response gain in the central auditory system causes an amplification of spontaneous neural activity, leading to spontaneous neuronal hyperactivity and the generation of a tinnitus percept. It follows from the model that reduction of auditory nerve activity, and not cochlear damage as such, might be the determining factor for the development of tinnitus
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