The Journal of Neurophysiology Vol. 78 No. 1 July 1997, pp. 261-270
Copyright ©1997 The American Physiological Society

Mechanical Responses to Two-Tone Distortion Products in the Apical and Basal Turns of the Mammalian Cochlea

N. P. Cooper and W. S. Rhode

Department of Neurophysiology, University of Wisconsin-Madison, Madison, Wisconsin 53706

Cooper, N. P. and W. S. Rhode. Mechanical responses to two-tone distortion products in the apical and basal turns of the mammalian cochlea. J. Neurophysiol. 78: 261-270, 1997. Mechanical responses to one- and two-tone acoustic stimuli were recorded from the cochlear partition in the apical turn of the chinchilla cochlea, the basal turn of the guinea pig cochlea, and the hook region of the guinea pig cochlea. The most sensitive or "best" frequencies (BFs) for the sites studied were ~500 Hz, 17 kHz, and 30 kHz, respectively. Responses to the cubic difference tone (CDT), 2F₁  F₂ (where F₁ and F₂ are the frequencies of the primary stimuli), were characterized at each site. Responses to the quadratic difference tone (QDT), F₂  F₁, were also characterized in the apical turn preparations (QDT responses were too small to measure in the basal cochlea). The observed responses to BF QDTs and CDTs and to BF CDTs at each site appeared similar in many ways; the relative magnitudes of the responses were highest at low-to-moderate sound pressure levels (SPLs), for example, and the absolute magnitudes grew nonmonotonically with increases in the level of either primary (L₁ or L₂) alone. The peak effective levels of the CDT and QDT responses were also similar, at around 20 dB re L₁ and/or L₂. In other respects, however, the responses to CDTs and QDTs and to BF CDTs at each site behaved quite differently. At low-to-moderate SPLs, for example, most CDT phase leads decreased with increases in either L₁ or L₂, whereas most QDT phase leads increased with increasing L₁ and varied little with L₂. Most CDT responses also varied monotonically with equal-level primaries (i.e., when L₁ = L₂), whereas most QDT responses varied nonmonotonically. Different responses also varied in different ways when F₁ and F₂ were varied. Apical turn QDT responses were observed over a very wide F₁/F₂ range (F₁ =1-12 kHz), but were usually largest for stimuli <2-4 kHz. Apical turn CDT levels decreased (at rates of ~40-80 dB/octave) only when the frequency ratio F₂/F₁ increased beyond ~1.4-1.5. In the basal turn and hook regions, the CDT levels depended nonmonotonically on F₂/F₁, with the eventual rates of decrease being ~200 dB/octave. Optimal frequency ratios for the CDT increased from (F₂ < 1.1F₁) to (F₂  1.2F₁) with increasing SPL in the basal turn, but were stable at around F₂/F₁  1.05 in the hook region. CDT phase leads tended to increase with increasing F₂/F₁ in all three regions of the cochlea, particularly at low-to-moderate SPLs. These findings are discussed in relation to previous studies of cochlear mechanics, physiology, and psychophysics.

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