Development of human cochlear active mechanism asymmetry: involvement of the medial olivocochlear system?

To study the functional development of the medial oilivocochlear system, tr ansient-evoked otoacoustic emission suppression experiments were conducted in 73 ears of 38 pre-term and 11 full-term neonates. The continuous contral ateral stimulation was a broad band white noise, presented at 70 dB SPL. Ef ferent suppression was determined by subtracting the without-contralateral stimulation condition from the with-contralateral stimulation condition. Ac ross this population, a mean suppression effect of contralateral stimulatio n on transient-evoked otoacoustic emissions was found, with most of the sup pression effect observed after 8 ms. The amount of suppression is linearly, positively correlated with the conceptional age. In the subgroup of bilate rally tested neonates, the suppression of transient-evoked otoacoustic emis sions is similar in the right ear and the left ear in subjects whose concep tional age is less than 36 weeks and significantly higher in the right ear than in the left ear in older neonates. This last observation was seen at f requencies where transient-evoked otoacoustic emission amplitudes became hi gher in the right ear than in the left ear as the conceptional age increase d, a finding already reported in adults. This study shows that the function al adult pattern of the medial efferent system, probably involved in the de tection of signals in noise such as speech sounds, seems to appear graduall y in neonates and represents one of the several arguments in favor of funct ional auditory lateralization in humans, with a right ear advantage (C) 199 9 Elsevier Science B.V. All rights reserved.