Level dependence of optimal stimulus level difference for evoking DPOAEs in the gerbil

In distortion product otoacoustic emission (DPOAE) measurements the ear is stimulated with two pure tones f1 and f2. To maximize DPOAE levels and hence increase the sensitivity of DPOAE measurements, the separation of the two primary tone levels has been shown to play a crucial role. In contrast to conventionally used paradigms where the difference between the stimulus levels L1 and L2 is constant, Whitehead et al. [Whitehead, M.L. et al., 1995. J. Acoust. Soc. Am. 97, 2359-2377] found a variable level separation L1-L2, which is increasing with decreasing overall stimulus levels, to be optimal for evoking maximal DPOAE levels. This optimal level separation was quantified by Kummer et al. [Kummer, P. et al., 2000. Hear. Res. 146, 47-56] for humans. The aim of our study was to find out if such optimal level differences also exist in the gerbil Meriones unguiculatus in order to obtain an adequate animal model for determination of auditory sensitivity and its pathologies. The results clearly indicate that, as in humans, a variable level separation L1-L2 is optimal for generation of maximal DPOAE levels in the gerbil. This level separation strongly depends on the frequency relation between f1 and f2 and the deviation of the optimal level difference from L1=L2 increases with the frequency ratio f2/f1.