Otoacoustic emission theories and behavioral estimates of human basilar membrane motion are mutually consistent

When two pure tones (or primaries) of slightly different frequencies (f (1) and f (2)) are presented to the ear, new frequency components are generated by nonlinear interaction of the primaries within the cochlea. These new components can be recorded in the ear canal as otoacoustic emissions (OAE). The level of the 2f (1)-f (2) OAE component is known as the distortion product otoacoustic emission (DPOAE) and is regarded as an indicator of the physiological state of the cochlea. The current view is that maximal level DPOAEs occur for primaries that produce equal excitation at the f (2) cochlear region, but this notion cannot be directly tested in living humans because it is impossible to record their cochlear responses while monitoring their ear canal DPOAE levels. On the other hand, it has been claimed that the temporal masking curve (TMC) method of inferring human basilar membrane responses allows measurement of the levels of equally effective pure tones at any given cochlear site. The assumptions of this behavioral method, however, lack firm physiological support in humans. Here, the TMC method was applied to test the current notion on the conditions that maximize DPOAE levels in humans. DPOAE and TMC results were mutually consistent for frequencies of 1 and 4 kHz and for levels below around 65 dB sound pressure level. This match supports the current view on the generation of maximal level DPOAEs as well as the assumptions of the behavioral TMC method.