Sound-evoked fMRI activation of the inferior colliculi (IC) was compared between tinnitus and nontinnitus subjects matched in threshold (normal), age, depression, and anxiety. Subjects were stimulated with broadband sound in an "on/off" fMRI paradigm with and without on-going sound from the scanner coolant pump.(1) With pump sounds off, the tinnitus group showed greater stimulus-evoked activation of the IC than the non-tinnitus group, suggesting abnormal gain within the auditory pathway of tinnitus subjects.(2) Having pump sounds on reduced activation in the tinnitus, but not the non-tinnitus group. This result suggests response saturation in tinnitus subjects, possibly occurring because abnormal gain increased response amplitude to an upper limit.(3) In contrast to Melcher et al. (2000), the ratio of activation between right and left IC did not differ significantly between tinnitus and non-tinnitus subjects or in a manner dependent on tinnitus laterality. However, new data from subjects imaged previously by Melcher et al. suggest a possible tinnitus subgroup with abnormally asymmetric function of the IC.The present and previous data together suggest elevated responses to sound in the IC are common among those with tinnitus and normal thresholds, while abnormally asymmetric activation is not, even among those with lateralized tinnitus.

Frequency selectivity in the inner ear is fundamental to hearing and is traditionally thought to be similar across mammals. Although direct measurements are not possible in humans, estimates of frequency tuning based on noninvasive recordings of sound evoked from the cochlea (otoacoustic emissions) have suggested substantially sharper tuning in humans but remain controversial. We report measurements of frequency tuning in macaque monkeys, OldWorld primates phylogenetically closer to humans than the laboratory animals often taken as models of human hearing (e.g., cats, guinea pigs, chinchillas). We find that measurements of tuning obtained directly from individual auditory-nerve fibers and indirectly using otoacoustic emissions both indicate that at characteristic frequencies above about 500 Hz, peripheral frequency selectivity in macaques is significantly sharper than in these common laboratory animals, matching that inferred for humans above 4-5 kHz. Compared with the macaque, the human otoacoustic estimates thus appear neither prohibitively sharp nor exceptional. Our results validate the use of otoacoustic emissions for noninvasive measurement of cochlear tuning and corroborate the finding of sharp tuning in humans. The results have important implications for understanding the mechanical and neural coding of sound in the human cochlea, and thus for developing strategies to compensate for the degradation of tuning in the hearing-impaired.auditory filters | comparative hearing S ound waveforms consist of pressure fluctuations in time and space. In the process of transducing mechanical vibrations into neural signals, the cochlea performs a mechanical frequency analysis that decomposes sounds into constituent frequencies (1, 2). The frequency tuning of the cochlear filters plays a critical role in the ability to distinguish and segregate different sounds perceptually. For example, sounds that radiate from different sources superpose in the air, and are thus "mixed up" before striking the eardrums. Based on the output of the cochlear filters, and by comparing responses from the two ears, the nervous system is capable of disentangling the various sounds, grouping related frequency components to identify auditory objects and localize their sources in space (3). The critical role of peripheral frequency selectivity is perhaps best illustrated by the consequences of damage to the inner ear, which typically leads to a degradation of the cochlear filters. The loss of sharp filtering results in an impaired ability to detect signals in noise and to separate different sounds (4). Frequency selectivity is therefore crucial to everyday human communication.The study of the cochlea is hampered by its fragility and inaccessibility. Direct measurements of mechanical or neural frequency tuning in healthy cochleae are only possible in laboratory animals. To date, measurements of the mechanical vibration of the cochlea's basilar membrane have been largely restricted to the basal high-frequency end of the cochlea, where surgical acce...

Many non-mammalian ears lack physiological features considered integral to the generation of otoacoustic emissions in mammals, including basilar-membrane traveling waves and hair-cell somatic motility. To help elucidate the mechanisms of emission generation, this study systematically measured and compared evoked emissions in all four classes of tetrapod vertebrates using identical stimulus paradigms. Overall emission levels are largest in the lizard and frog species studied and smallest in the chicken. Emission levels in humans, the only examined species with somatic hair cell motility, were intermediate. Both geckos and frogs exhibit substantially higher levels of high-order intermodulation distortion. Stimulus frequency emission phase-gradient delays are longest in humans but are at least 1 ms in all species. Comparisons between stimulus-frequency emission and distortion-product emission phase gradients for low stimulus levels indicate that representatives from all classes except frog show evidence for two distinct generation mechanisms analogous to the reflection- and distortion-source (i.e., place- and wave-fixed) mechanisms evident in mammals. Despite morphological differences, the results suggest the role of a scaling-symmetric traveling wave in chicken emission generation, similar to that in mammals, and perhaps some analog in the gecko.