Measurement of acoustic distortion reveals underlying similarities between human and rodent mechanical responses

Abstract

The level of 2f1-f2 acoustic distortion product (ADP) measured in the meatus during two-tone stimulation was compared with N 1 thresholds measured at the round window for the guinea pig. A significant inverse relation was found between distortion level and N 1 threshold. A similar relationship has also been reported for ADP level and subjective thresholds in half the human ears measured [S.A. Gaskill and A.M. Brown, J. Acoust. Soc. Am. 88, 821-839 (1990)]. Guinea pig and human ADP levels behave similarly in response to varying stimulus parameters. The ADP levels grow to a maximum and decline with increasing stimulus separation. The decline is steeper in the human ear. In both species, ADP growth as a function of stimulus level is approximately 1 with covaried stimuli; more gradual with the level of f2 (L 2) alone increasing and steeper when the level of f1 (L 1) alone is increased. The latter slopes are strongly influenced by the level of the stationary L 2 and are less steep in the human ear. A link has been proposed between differences in ADP behavior and differences in auditory filter bandwidth in the two species. Guinea pigs show little intersubject variability in ADP level. They do not show the fine structure in distortion level across frequency or the variation in growth rate seen in human responses. Differences in organ of Corti fine structure may underly these differences.