Mechanical responses to two-tone distortion products in the apical and basal turns of the mammalian cochlea.

Abstract

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 approximately 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 approximately 40-80 dB/octave) only when the frequency ratio F2/F1 increased beyond approximately 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 approximately 200 dB/octave. Optimal frequency ratios for the CDT increased from (F2 < 1.1F1) to (F2 approximately 1.2F1) with increasing SPL in the basal turn, but were stable at around F2/F1 approximately 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.