Psychophysical and Physiological Measures of 2f1-f2 Cochlear Distortion

Father Flanagan'S Boys' Home, Boys Town NE

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Abstract

? DESCRIPTION (provided by applicant): The proposed research will investigate the influence of cochlear distortion on sound detection using physiological and behavioral measures. During tone-on-tone simultaneous masking when the frequency of the probe tone (fp) is approximately 1.2 times the frequency of the masker tone (fm), detection of the probe is enhanced by the generation of the cubic difference tone (CDT; fCDT = 2fm-fp). Generation and detection of the CDT presumably has its origin in cochlear nonlinearity, which is intrinsic to cochlear amplification. Sensory hearing loss (HL) reduces cochlear amplification and, potentially, the ability of a listene with HL to benefit from audible cochlear distortion. The first aspect of the proposed work will investigate the effect of HL on behavioral detection of the CDT during a simultaneous masking task. Generation of the 2f1-f2 distortion-product otoacoustic emission (DPOAE; f1 and f2 denote the stimulus frequencies used to evoke the DPOAE) necessitates cochlear amplification at the same cochlear places as those underlying CDT generation and detection. Therefore, the DPOAE is hypothesized to provide a non-invasive, objective assay of the cochlear mechanisms underlying perception of the CDT. Specifically, the DPOAE is expected to be sensitive to HL at the cochlear places of CDT generation and detection, and account for inter-subject variability in behavioral threshold of the CDT. The second aspect of the proposed work will test this hypothesis by comparing DPOAE- based measures to behavioral CDT threshold in listeners with normal-hearing (NH) and HL. Results will demonstrate how HL affects the ability of listeners to detect cochlear distortion and provide a framework from which signal-processing techniques may be developed to compensate for the loss of beneficial distortion. Findings are also expected to contribute to future work seeking to separate the cochlear contribution to the CDT measured at higher levels in the auditory system. The CDT has been measured at the brainstem and cortex; however, the extent to which these measures reflect cochlear processing has not been established. Simultaneous masking has been implicated to be primarily a consequence of cochlear two-tone suppression through work comparing the difference in amount of masking between forward- and simultaneous masking to DPOAE suppression (Rodriguez et al. 2010). The difference in masking between conditions was approximated by the change in the f2 stimulus level required to achieve a constant DPOAE magnitude across suppressor levels. This definition presumes that the listener relies on the same auditory cue during simultaneous masking, which may or may not be true depending on the contribution of combination tones (such as the CDT) to masked threshold. The third aspect of the proposed work will examine the relationship between DPOAE level and masked behavioral probe threshold during on- and off-frequency masking in listeners with NH. Results will provide insight into the cochlear mechanisms underlying simultaneous masking while highlighting factors that must be considered when comparing behavioral and physiological measures.