High-frequency tinnitus without hearing loss does not mean absence of deafferentation

A broad consensus within the neuroscience of tinnitus holds that this audiologic condition is triggered by central deafferentation, mostly due to cochlear damage. The absence of audiometrically detectable hearing loss however poses a challenge to this rather generalizing assumption. The aim of this study was therefore to scrutinize cochlear functioning in a sample of tinnitus subjects audiometrically matched to a normal hearing control group. Two tests were applied: the Threshold Equalizing Noise (TEN) test and a pitch scaling task. To perform well on both tasks relatively normal functioning of inner hair cells is a requirement. In the TEN test the tinnitus group revealed a circumscribed increment of thresholds partially overlapping with the tinnitus spectrum. Abnormal slopes were observed in the pitch scaling task which indicated that tinnitus subjects, when presented with a high-frequency stimulus, relied heavily on input derived from lower-frequency inner hair cells (off-frequency listening). In total both results argue for the presence of a deafferentation also in tinnitus subjects with audiometrically normal thresholds and therefore favour the deafferentation assumption posed by most neuroscientific theories.