Low-frequency bias-tone effects on auditory-nerve responses to clicks and tones : investigating multiple outer-hair-cell actions on auditory-nerve firing

Abstract

Active motility in outer hair cells (OHCs) amplifies basilar-membrane (BM) and auditory-nerve (AN) responses to low-level sounds. The recent finding that medial olivocochlear (MOC) efferents (which innervate OHCs) inhibit AN initial peak (ANIP) responses from mid-to-high-level clicks, but do not inhibit initial BM responses, suggests a coupling of OHC motility to inner-hair-cell (IHC) stereocilia that is not through the BM. The main thesis objective was to test whether different OHC mechanisms produce AN responses to low-level sounds versus ANIP from mid-to-high-level clicks by comparing the suppressive effects of low-frequency "bias-tones" on these responses. Bias tones suppress by pushing OHC stereocilia into low-slope regions of their mechanoelectric transduction functions thereby lowering OHC amplification, particularly for probe tones near an AN-fiber's characteristic frequency (CF). This suppression occurs at opposite bias-tone phases, with one suppression typically larger than the other. Bias-tone effects were measured on cat AN-fiber responses using 50 Hz bias tones. In the first thesis part, bias-tone suppressive effects on AN responses to low-level clicks and low-level CF-tones were found to be similar, as expected but never previously shown. Then, in the main thesis focus, bias-tone suppressions of AN responses to low-level clicks and ANIP responses were studied. Both responses were suppressed twice each bias-tone cycle, but their major suppressions were at opposite bias-tone phases, which indicates that both ANIP and low-level AN responses depend on the slope of OHCstereocilia mechanoelectric-transduction, but with some significant difference. In the last thesis part, bias-tone suppression effects on low-CF (<4 kHz) AN-fiber responses to low-level CF and off-CF (by >0.7 octaves) tones were studied. Previous work found differences in AN-response group delays between CF and off-CF frequency regions that might arise from two different IHC-drive mechanisms, and the objective was to test this hypothesis. Our results showed similar bias-tone effects in both regions. Overall, the results demonstrate differences and similarities in the OHC mechanisms that produce ANIP and traditional, low-level cochlear amplification, and the results are consistent with the ANIP drive coupling OHC motility to IHC stereocilia without going through BM motion.