Distortion product otoacoustic emission (2f(1)-f(2)) amplitude as a function of f(2)/f(1) frequency ratio and primary tone level separation in human adults and neonates

Distortion product otoacoustic emission (DPOAE) (2f(1)-f(2)) amplitude is dependent upon both the frequency ratio and level separation of the eliciting primary tones. In adults it has been established that, on average, a f(2)/f(1) ratio of 1.22 is optimal for evoking the most robust DPOAE; DPOAE amplitude is systematically reduced when f(2)/f(1) ratio is either increased or decreased, thus forming a bandpass function (Harris et al., 1989). The frequency ratio function (DPOAE amplitude x f(2)/f(1) ratio) is thought to reflect the filtering properties of the cochlea (Alien and Fahey, 1993; Brown et al., 1993). Primary tone level separation also influences DPOAE amplitude, with a 10- to 15-dB level difference between the primary tones (L(1)>L(2)) typically generating largest amplitude in adults. Equivalent studies have not been conducted in neonates. The present study evoked the 2f(1)-f(2) DPOAE in adults, term and premature neonates to define the optimal f(2)/f(1) ratio and L(1)-L(2) level separation and to investigate the filtering properties of the developing cochlea. Two f(2) frequencies were investigated: 1500 and 6000 Hz. F-2 was held constant while f(1) was varied to produce 13 frequency ratios. Primary tone level separation varied from 15 to 0- in 5-dB intervals. ANOVA were conducted on the resulting f(2)/f(1) frequency ratio and level separation data. Results showed that the mean optimal frequency ratio for DPOAE generation is comparable in adults and neonates. Also, either a 15- or 10-dB level separation (L(1)>L(2)) produced the largest DPOAE amplitude for adults and term neonates whereas DPOAEs from premature neonates appeared to be relatively insensitive to primary tone level separation. The f(2)/f(1) frequency ratio functions were similar in shape, slope, and bandwidth for adults and neonates, suggesting adult-like cochlear filtering prior to term birth. This finding is in agreement with previous work from our laboratory reporting adult-like DPOAE suppression tuning curves in term-born neonates [Abdala et al., Hear. Res. (1996)]. (C) 1996 Acoustical Society of America.