Activity in Humans with Clinically Normal Hearing Sensitivity

38 Phantom sensations and sensory hypersensitivity are disordered perceptions that 39 characterize a variety of intractable conditions involving the somatosensory, visual, 40 and auditory modalities. We report physiological correlates of two perceptual 41 abnormalities in the auditory domain: tinnitus, the phantom perception of sound, and 42 hyperacusis, a decreased tolerance of sound based on loudness. Here, subjects with 43 and without tinnitus, all with clinically normal hearing thresholds, underwent (1) 44 behavioral testing to assess sound-level tolerance and (2) functional magnetic 45 resonance imaging to measure sound-evoked activation of central auditory centers. 46 Despite receiving identical sound stimulation levels, subjects with diminished sound47 level tolerance (i.e. hyperacusis) showed elevated activation in the auditory midbrain, 48 thalamus, and primary auditory cortex compared to subjects with normal tolerance. 49 Primary auditory cortex, but not subcortical centers, showed elevated activation 50 specifically related to tinnitus. The results directly link hyperacusis and tinnitus to 51 hyperactivity within the central auditory system. We hypothesize that the tinnitus52 related elevations in cortical activation may reflect undue attention drawn to the 53 auditory domain, an interpretation consistent with the lack of tinnitus-related effects 54 subcortically where activation is less potently modulated by attentional state. The 55 data strengthen, at a mechanistic level, analogies drawn previously between 56 tinnitus/hyperacusis and other, non-auditory disordered perceptions thought to arise 57 from neural hyperactivity such as chronic neuropathic pain and photophobia. 58 59