Psychophysical estimates of level-dependent best-frequency shifts in the apical region of the human basilar membrane

Abstract

It is now undisputed that the best frequency (BF) of basal basilar-membrane (BM) sites shifts downwards as the stimulus level increases. The direction of the shift for apical sites is, by contrast, less well established. Auditory nerve studies suggest that the BF shifts in opposite directions for apical and basal BM sites with increasing stimulus level. This study attempts to determine if this is the case in humans. Psychophysical tuning curves (PTCs) were measured using forward masking for probe frequencies of 125, 250, 500, and 6000 Hz. The level of a masker tone required to just mask a fixed low-level probe tone was measured for different masker-probe time intervals. The duration of the intervals was adjusted as necessary to obtain PTCs for the widest possible range of masker levels. The BF was identified from function fits to the measured PTCs and it almost always decreased with increasing level. This result is inconsistent with most auditory-nerve observations obtained from other mammals. Several explanations are discussed, including that it may be erroneous to assume that low-frequency PTCs reflect the tuning of apical BM sites exclusively and that the inherent frequency response of the inner hair cell may account for the discrepancy.