The low -frequency limit of distortion product otoacoustic emissions generation in kangaroo rats

Lauren Ann Shaffer, Purdue University

Abstract

Distortion product otoacoustic emissions (DPOAE) were measured for f1 frequencies ranging from 200 to 1000 Hz in two species of kangaroo rats, Dipodomys merriami and Dipodomys spectabilis. Primary frequency ratio was fixed at 1.10 and primary levels were equal (65 dB SPL). Levels of the 2f 1 − f2 DPOAE remained near 30 dB SPL for frequencies above 500 Hz. Below 500 Hz, the 2f1 − f2 level rolled off. An interspecies difference was observed in the −3 dB corner frequency of the roll off. A lower 2f1 − f2 corner frequency was observed in D. spectabilis, consistent with predicted amplitude differences based on species size. The decline in 2f1 − f2 level below 500 Hz was in agreement with published threshold data for D. merriami. A comparison of DPOAE orders, including 2f 1 − f2, 3f1 − 2f2, 2f2 − f1, was used to evaluate the origin of the decline in level. The high pass filter shape of the level functions was observed across animals, species, DPOAE orders and primary ratios. All orders were observed to roll off in a range between 300 and 500 Hz. Differential filtering of DPOAE orders apical and basal to the primaries was observed in both species suggesting that the roll off was cochlear in origin. The decline in DPOAE level below 500 Hz is consistent with reports from other researchers that the apex of the cochlea shows decreased nonlinearity and that helicotrema effects dominate at very low frequencies. While ecological studies in kangaroo rats have implicated differences in hearing as having a causal relationship to interspecies differences in communication ability and anti-predator behavior, these are the first data to demonstrate a difference in auditory capacity at low frequencies.

Degree

Ph.D.

Advisors

Long, Purdue University.

Subject Area

Biophysics|Zoology|Physiology

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