Frequency distribution of synchronized spontaneous otoacoustic emissions showing sex-dependent differences and asymmetry between ears in 2- to 4-day-old neonates

Liu J, Wang N, Li J, Shi B, Wang H

Int J Pediatr Otorhinolaryngol. 2009 May;73(5):731-6

Department of Otorhinolaryngology Head and Neck Surgery, Beijing Chaoyang Hospital, Capital Medical University, PR China

Abstract:

OBJECTIVE: The mature pattern of frequency distribution of synchronized spontaneous otoacoustic emissions (SSOAEs) has been reported to be bimodal in adults and children between 5 and 11 years of age; however, little is known about the distribution in neonates between 2 and 4 days after birth. Furthermore, overall differences in frequency distribution resulting from difference in sex and asymmetry between ears have not been carefully examined. The aim of this study is to determine the frequency distribution of SSOAEs in neonates at 2 to 4 days of age, evaluate the maturity of the pattern of distribution in this age group, and to evaluate the effects of differences in sex and asymmetry between left and right ears on the frequency distribution.
METHODS: We evaluated 224 ears in 112 newborns (59 girls, 53 boys) whose ages ranged from 2 to 4 days. The SSOAEs were measured using ILO96.
RESULTS: Most of the SSOAEs (86.5%) appeared at frequencies between 1.01 and 4.50 kHz. The overall frequency distribution of the SSOAEs showed a 'peak-valley-peak' pattern when plotted. Two peaks with maxima at 1.41-1.60 and 3.01-3.20 kHz were separated by a valley with a minimum at 2.41-2.60 kHz. Both girls and boys had approximate monomodal patterns in the distribution of SSOAEs. Significant sex-dependent differences were noted with more SSOAEs at the lower frequencies (<or=2 kHz) in boys (46.1%) than in girls (32.0%) (P<0.05) and more SSOAEs at the higher frequencies (2.51- 4.50 kHz) in girls (50.9%) than in boys (37.5%) (P<0.05). Both the right and left ears showed the 'peak-valley-peak' pattern that was similar to the overall distribution pattern. But, compared with the peaks measured in the left ears at 1.01-1.50 and 3.01-3.50 kHz, the peaks of the right ears at 1.51-2.00 and 2.51-3.00 kHz were much closer to the valley.
CONCLUSIONS: The overall distribution of frequency of SSOAEs in 2- to 4-day-old neonates had the similar mature 'peak-valley-peak' distribution pattern seen in adults. Significant sex-dependent differences of the SSOAEs frequency distributions have been found. However, only slight ear asymmetries of the SSOAEs frequency distributions can observed in this age group. (Bold text emphasis by Martin Braun)

Comment:

Three years after the first report on the occurrence of the octave distance in the distribution of spontaneous otoacoustic emissions (SOAEs) by Braun (2006) we have now a replication of these findings by Liu et al. (2009). Here the frequency distribution of 769 SOAEs from 224 ears again showed a clear concentration in the 2-octave range between 1 kHz and 4 kHz, which corresponds to the range between C6 and C8 in musical terms. And again the distribution in this range was not normal, but bimodal with peaks at 1.5 kHz and 3 kHz, corresponding to F#6 and F#7 in musical terms.

Interestingly, the valley between the two peaks was not as deep in the new data from neonates as it was in the previous data from children and adults. Also the valley appeared at slightly higher frequencies in the new neonate data. This indicates that the mechanism which causes the bimodality is not yet fully mature at birth. Braun's hypothesis that the octave-spaced peaks at F#6 and F#7 are caused by descending neural input upon the SOAE generating outer hairs cells in the cochlea is thus supported by the new data, because this neural system is known to be fine-tuned after birth.

As shown in Figs. 2-4 of the new study, effects of side and sex modulate the distribution pattern. But they cannot be candidates for the cause of the bimodality, because this is present in all subgroups of the data. Also, effects of side and sex are fully absent in the previous data from children and adults (Braun, 2006). These effects are therefore a further indication of the immaturity of the neonate cochlea. Overall, the concept that SOAE probability in humans reflects octave-based processing in the auditory thalamus has gained further strength. (Comment Martin Braun)

NOM Home