Behaviors of cubic distortion product otoacoustic emissions evoked by amplitude modulated tones

Abstract

Distortion product otoacoustic emissions (DPOAEs) were measured using sinusoidal amplitude modulation (AM) tones. When one of the primary stimuli (f(1) or f(2), f(1) < f(2)) was amplitude modulated, a series of changes in the cubic difference tone (CDT) were observed. In the frequency domain, multiple sidebands were present around the CDT and their sizes grew with the modulation depth of the AM stimulus. In the time domain, the CDT showed different modulation patterns between two major signal conditions: the AM tone was used as the f(1) or the f(2). The CDT amplitude followed the AM tone when the f(1) was amplitude modulated. However, when the AM tone acted as the f(2), the CDT showed a more complex modulation pattern with a notch present at the AM tone peak. The relatively linear dependence of CDT on f(1) and the nonlinear relation with f(2) can be explained with a variable gain-control model representing hair cell functions at the DPOAE generation site. It is likely that processing of AM signals at a particular cochlear location depends on whether the hair cells are tuned to the frequency of the carrier. Nonlinear modulation is related to on-frequency carriers and off-frequency carriers are processed relatively linearly.

Age-related shifts in distortion product otoacoustic emissions peak-ratios and amplitude modulation spectra

“…DP amplitude reduction is believed to resemble cochlear two-tone suppression (Oswald, Rosner et al 2006), which originates from the saturating nonlinearity in OHC transduction (Geisler, Yates et al 1990). The reduction is larger at 75 dB SPL because saturation normally occurs at higher levels (Bian and Chen 2011). As DP reduction is more apparent in aged animals than in the young, this indirectly suggests that aged animals have smaller OHC motile activities and OHC vibration becomes saturated relatively faster than young animals.…”

“…Decreased ability in detecting temporally modulated sounds accurately, e.g. difficulties in encoding small AM depth, could originate from changes in cochlear filtering properties and in cochlear mechanics due to aging (Bian and Chen 2011). However, few studies have examined cochlear mechanisms in AM coding.…”