Correlated amplitude fluctuations of spontaneous otoacoustic emissions

The frequency spectrum of a spontaneous otoacoustic emission (SOAE) recording may contain multiple narrow peaks. These peaks correspond to multiple SOAEs emitted by the inner ear. The individual SOAEs exhibit small amplitude fluctuations. The correlation between the amplitude fluctuations of multiple SOAEs in an individual ear was studied in 19 SOAE recordings from humans. For 424 pairs of SOAEs, the normalized amplitude cross-correlation function was computed. Significant correlation was found in 63 pairs (15%): 34 pairs showed positive and 29 pairs showed negative correlation. Some SOAE pairs were part of an equally spaced triplet of emission peaks, with one SOAE apparently generated as a distortion product of the other two "primary" SOAEs. The occurrence of correlation between a distortion product SOAE and a "primary" SOAE was 56%. This is significantly higher than the occurrence of correlation for the entire set of SOAE pairs. In addition, the spectral width of the distortion product SOAE was correlated with the spectral width of the "primary" SOAEs (R = 0.87, p < 10(-6)). For 79% of the correlated SOAE pairs, the cross-correlation function peaked at positive time delay. This indicates that for these pairs the fluctuation of the lower frequency emission lagged the fluctuation of the higher frequency emission. The average peak delay was 11.0 ms (s.d. 22.1 ms). The results demonstrate that interactions between SOAEs in an individual ear frequently occur and are asymmetrical.