DPOAE group delays versus electrophysiological measures of cochlear delay in normal human ears

Group delays of 2 f1-f2 distortion product otoacoustic emissions (DPOAEs) were determined using both f1- and f2-sweep paradigms in 24 normal-hearing subjects. These DPOAE group delays were studied in comparison with cochlear delays estimated from derived band VIIIth nerve compound action potentials (CAPs) and auditory brainstem responses (ABRs) in the same subjects. The center frequencies of the derived bands in the electrophysiological experiment were matched with the f2-frequencies in the DPOAE recording to ensure that DPOAEs and derived CAPs and ABRs were generated at the same places along the cochlear partition, thus allowing for a direct comparison. The degree to which DPOAE group delays are larger in the f2- than in the f1-sweep paradigm is consistent with a theoretical analysis of the so-called wave-fixed model. Both DPOAE group delays are highly correlated with CAP- and ABR-derived measures of cochlear delay. The principal result of this study is that "roundtrip" DPOAE group delay in the f1-sweep paradigm is exactly twice as large as the neural estimate of the "forward" cochlear delay. The interpretation of this notion in the context of cochlear wave propagation properties and DPOAE-generating mechanisms is discussed.