Investigating the suppressive effects of a low-frequency bias-tone on auditory-nerve responses to tail-frequency tones

Abstract

Sounds are amplified and frequency analyzed in the cochlea and the resulting signals are sent to the brain through the auditory nerve (AN). AN fiber threshold vs. frequency-response characteristics are band-pass shaped, with a sensitive, sharply-tuned tip centered at the "characteristic frequency" (CF) of the fiber. It is now well established that the sharp frequency tuning and high sensitivity of the tip is produced by mechanical feedback amplification from cochlear outer hair cells (OHCs). Specifically, sound vibrates the cochlear basilar membrane and this motion is transmitted to the OHC mechano-electric transduction function which produces a voltage change in the OHCs. This voltage change, through OHC electromotility, feeds back energy that amplifies basilar-membrane motion. For AN fibers with CF > 5 kHz (high-CF fibers), the sharp tuning-curve tip region is flanked on its low-frequency side with a broadly-tuned, less-sensitive "tailfrequency" region where the role of OHC motility has been thought as not significant. Contrary to this prevailing view, it has been experimentally found that the electromotility of OHCs is involved in generating AN responses in a narrow band of frequencies near 2.5 kHz in the tail. The main objective of this thesis was to investigate whether and how the mechano-electric transduction function of OHCs is involved in generating AN responses to tail-frequency tone at 2.5 kHz, and to determine any differences in the details of the mechano-electric transduction function in the tail-frequency responses versus in low-level CF-tone responses. The experimental strategy was to use the suppressive effects of a low-frequency bias-tone on AN responses driven by the active motility of OHCs. AN responses are affected by the mechano-electric transduction function of OHCs such that AN responses to low-level CF-tones are suppressed twice per bias-tone period by high-level bias-tones. This suppression is due to the saturation of the mechano-electric transduction as the bias-tone moves the operating point of OHC mechano-electric transduction into the nonlinear saturation regions twice per period of the bias-tone. Specifically, a bias-tone at 50 Hz was applied together with a second probe-tone, which was either a low-level CF-tone or a nearthreshold tail-frequency tone at 2.5 kHz, in order to characterize and compare the suppressive effects on these two response types. Single AN fiber recordings were made from 27 fibers from 6 cats. As expected, the characteristic pattern of twice-per-bias-tone-period suppression was found from the low-level CF-tone responses from all of the recorded fibers. As for the tail-frequency tone responses, significant suppression was found on 10 of the 27 fibers recorded. Among those 10 fibers, the twice-per-biastone- period suppression pattern associated with the non-linearity of the mechano-electric transduction function of OHCs was found for fibers with CF < 15 kHz. The lack of suppression for fibers with CF > 15 kHz may be due to the bias-tone sound levels not being high enough. These results directly show that the mechano-electric transduction function of OHCs is involved in generation of AN responses to 2.5 kHz tones for fibers with CF < 15 kHz in a similar way to how it is involved in producing cochlear amplification for low-level CF-tones. Further, comparisons of the details of the suppression pattern, e.g., most importantly the phase of major suppression, did not reveal any significant differences between the two response types. Overall these results indicate the detailed mechanisms of the OHC mechano-electric transduction function that are involved in producing the two responses types are similar. Additionally, the effects of a low-frequency bias-tone on the phase of AN responses to tailfrequency tones were compared with previous work on the effects of medial olivocochlear (MOC) efferent stimulation on the phase of AN responses to tail-frequency tones. Suppression of the gain of the OHC mechano-electric transduction function by a low-frequency bias-tone may affect the phase as well as the rate of AN responses driven by OHCs; however, effects of a low-frequency bias-tone on the phase of AN responses have not been reported in the literature. Bias-tone induced phase shifts were quantified as the difference in the phase of excitation to 2.5 kHz tones between bias-tone levels below and above the suppression threshold. Results were collected from five fibers. The shift in the phase of AN responses to 2.5 kHz tail-frequency tones induced by a bias-tone ranged from -45* (phase lag) to +5* (phase lead). A phase lag was found from 4 of the 5 fibers. These results are in general agreement with the MOC efferent effects on the phase of AN responses which ranged from -80* to +60* with an average of -15*, a phase lag. These results suggest that a lowfrequency bias-tone and MOC efferent stimulation affect the phase of AN responses to a tail-frequency tone through a similar mechanism by lowering the gain of the OHCs.