4pPP8. New evoked cochlear responses: Double chirp-evoked distortion products and double click-evoked otoacoustic emissions.

Session: Thursday Afternoon, May 16

Author: Douglas H. Keefe
Location: Boys Town Natl. Res. Hospital, 555 N. 30th St., Omaha, NE 68131


A novel family of evoked OAE responses controls probe distortion using a stimulus presentation method well-suited to research and clinical applications. The family allows a complementary representation between DP measurements and double click-evoked OAE measurements. Each stimulus sequence includes three, equal-duration sub-sequences defined as follows: s[inf 1](t) is a single chirp or click, s[inf 2](t)=(epsilon)s[inf 1](t-(tau)) is a copy of s[inf 1] with relative amplitude (epsilon) and delay (tau) (0-5 ms), and their superposition s[inf 12](t)=s[inf 1](t)+s[inf 2](t). The pressure response to each sub-sequence is p[inf 1], p[inf 2], and p[inf 12], respectively. The double chirp-evoked distortion product (2ChDP) and double click-evoked OAE (2CEOAE) are defined by p[inf D]=p[inf 12]-p[inf 1]-p[inf 2]. This subtraction of click respones is similar to Kemp et al. (1986) in eliminating the linear response, but differs in that particular choices of {s[inf 1](t),(tau),(epsilon)} improve measurements of 2CEOAEs at high frequencies by eliminating time-gating, and at low frequencies by reducing probe distortion, especially when two acoustic sources are used. With a 16 kHz sample rate, 2ChDP and 2CEOAE responses have been obtained from 350-7900 Hz. A 2CEOAE special case is (tau)=0, resulting in a wide bandwidth, single click-evoked OAE measurement without time-gating. Methodological refinements in real-time artifact rejection and nonlinear coherence help reduce noise. [Work supported by NIH Grant No. P01 DC00520.]

from ASA 131st Meeting, Indianapolis, May 1996