Frequency organization of the 40-Hz auditory steady-state response in normal hearing and in tinnitus
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We used the 40-Hz auditory steady-state response (SSR) to compare for the first time tonotopic frequency representations in the region of primary auditory cortex (PAC) between subjects with chronic tinnitus and hearing impairment and normal hearing controls. Frequency representations were measured in normal hearing (n=17) and tinnitus (n=28) subjects using eight carrier frequencies between 384 and 6561 Hz, each amplitude modulated (AM) at 40-Hz on trials of 3 min duration under passive attention. In normal hearing subjects, frequency gradients were observed in the medial-lateral, anterior-posterior, and inferior-superior axes, which were consistent with the orientation of Heschl's gyrus and with functional organization revealed by fMRI investigations. The frequency representation in the right hemisphere was approximately 5 mm anterior and approximately 7 mm lateral to that in the left hemisphere, corroborating with MEG measurements hemispheric asymmetries reported by cytoarchitectonic studies of the PAC and by MRI morphometry. In the left hemisphere, frequency gradients were inflected near 2 kHz in normal hearing subjects. These SSR frequency gradients were attenuated in both hemispheres in tinnitus subjects. Dipole power was also elevated in tinnitus, suggesting that more neurons were entrained synchronously by the AM envelope. These findings are consistent with animal experiments reporting altered tonotopy and changes in the response properties of auditory cortical neurons after hearing loss induced by noise exposure. Degraded frequency representations in tinnitus may reflect a loss of intracortical inhibition in deafferented frequency regions of the PAC after hearing injury.
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