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

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.

Department dc:contributor.department

Harvard University--MIT Division of Health Sciences and Technology.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2011

Author dc:creator

• Nam, Hui S., Ph. D. (Hui Sok) Massachusetts Institute of Technology

Advisor dc:contributor.advisor

• John J. Guinan, Jr.

• Harvard University--MIT Division of Health Sciences and Technology.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/68455

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/68455