Perceptual Consequences of Tinnitus: Effects of Sensory Deficits and Top-Down Attention
Tinnitus is the phantom perception of sound without an external acoustic source. Although tinnitus affects 10-15% of adults, with symptoms from mildly bothersome to debilitating, current treatments are limited by an inadequate understanding of its underlying pathophysiology. Improved understanding must address perception at all levels, from a peripheral deficit (e.g. hearing loss), to central auditory plasticity generating the internal signal, to higher-order brain networks governing its conscious awareness. The first study examined top-down mechanisms of attention in modulating tinnitus and potentially its loudness, pitch, or distress. Subjects with and without chronic tinnitus monitored one of two tonal steams, one with similar frequency as tinnitus and the other well-outside this region. Cortical evoked-potentials showed enhanced attentional gain when tinnitus subjects focused to the tinnitus frequency, which related to increased tinnitus loudness. A later enhancement to the non-tinnitus frequency suggested tinnitus also impacts selective processing of other sounds. A discrimination analysis indicated that attention-derived variables could serve as a biomarker to detect tinnitus from control subjects with similar age and hearing. The second study employed psychophysical methods to uncover sensory deficits in tinnitus and improve its diagnosis. Tinnitus subjects showed normal temporal acuity for detecting silent gaps in pure-tones, including those matching the tinnitus pitch. Although gap detection is widely used to assess tinnitus in animal research, assuming tinnitus “fills-in” the gap, the procedure appears ill-suited for clinical diagnosis in humans. Tinnitus subjects also showed normal frequency discrimination. However, slightly improved intensity discrimination of low-level sounds indicated a mechanism of increased auditory gain in the central pathways. Together, these results suggest that “bottom-up” sensory deficits in tinnitus have subtle impacts on basic auditory processing, while top-down attention plays a central role in gating tinnitus perception and may be developed as an objective clinical biomarker.