Abstract 137, Date 1:00 pm, Saturday, February 6, 2010 (48 hours)
Session Session D9:
Temporal Adaptation of the Click-Evoked Otoacoustic Emission Level-Curve Reveals Dynamic Properties of Human Cochlear Processing
*Sarah Verhulst, James M. Harte, Christopher A. Shera, Torsten Dau
The level of a click-evoked otoacoustic emission (CEOAE) depends on the level of the evoking click through the CEOAE level-curve. The CEOAE level grows linearly for clicks below 3040 dB and saturates for higher input levels. This study shows that the CEOAE level-curve for a test click can be shifted when a suppressor click is presented less than 10 ms before the test click. This effect is referred to as temporal adaptation of the CEOAE level-curve, and its strength is determined by the levels and the temporal separation of the two clicks. Two cochlear mechanisms could underlie the observed temporal adaptation: (i) temporal overlap of the basilar-membrane (BM) excitation patterns of the two clicks and/or (ii) a temporal change in cochlear nonlinearities caused by the presentation of the suppressor click.

To investigate the origin of temporal adaptation, we developed a nonlinear adaptive transmission line model of the cochlea. Reverse travelling waves (OAEs) were produced by adding irregularities to the BM impedance along the cochlear partition. The implemented nonlinearity modelled the compressive behaviour of BM impulse responses that preserves the intensity invariance of their temporal fine structure. These model features yielded realistic behaviour of the CEOAE level-curve and group delay of the simulated CEOAEs. Furthermore, model simulations successfully reproduced the characteristic features of temporal adaptation of the CEOAE level-curve. Because the model employed an instantaneous (i.e., time-invariant) nonlinearity, we conclude that temporal adaptation can be explained by the temporal overlap of the BM patterns of two closely spaced clicks. However, the model simulations do not rule out contributions to temporal adaptation from time-dependent processes, such as fast adaptation in the outer hair cells.