Development of Auditory Asymmetry in Transient Evoked Otoacoustic Emissions in Pre-Term Infants

We previously reported that transient evoked otoacoustic emissions (TEOAEs) continue to develop after the onset of cochlear function in pre-term infants from 34 to 39 weeks of conceptional age (Morlet et al., 1996). The time-course of development differed between genders. Reported here are findings of further analysis of data from the study cohort, comprising 1020 ears of 510 pre-term neonates (conceptional age ranging from 34 to 39 weeks), demonstrating developmental differences between right and left ears. The left ear of female pre-term infants showed enhancement of TEOAE amplitude at low and medium frequencies with age, whereas differences were less dramatic in the right ear. In male infants, TEOAE amplitude decreased in several frequency bands at high frequencies, above 4 kHz, between 34 and 39 weeks conceptional age; most developmental differences were found to be more dramatic in the right than in the left ear. It is tempting to speculate that these developmental features underlie well-known inter-aural asymmetries that have been demonstrated in the adult human.

Hearing asymmetry and auditory acuity in the Australian bushcricket Requena verticalis (Listroscelidinae; Tettigoniidae;Orthoptera)

SUMMARY Fluctuating asymmetry in bilaterally placed sensory structures of insects may be considered maladaptive; natural selection should favour a symmetrical system. Among bushcrickets or katydids, threshold hearing sensitivity is, in part, dependent on the size of the auditory spiracle in the prothorax. We tested the degree of natural asymmetry in the hearing system of the ensiferan orthopteran Requena verticalis by measuring the size of the auditory spiracle of females. Naturally occurring asymmetry approached 8%, which translates to a hearing bias at a threshold of only 0.8 dB. Auditory asymmetry of females was experimentally exaggerated by packing cotton wool into either the right or left auditory spiracle. We made neurophysiological recordings from the ascending auditory T-fibre in the neck connective and found that the left—right bias created by this operation approximated 5 dB; this is greater than that found in nature. For these experiments, sound was delivered to the operated side of the insect from a speaker placed at either 90° or 30° to the long-body axis. To test the influence of this induced auditory bias, free-moving females were allowed to orient towards a speaker emitting male calls at near-natural call intensities of 51 and 82 dB (SPL), 80 cm from the speaker on a flat arena. There was no variation in angle or vector between experimental and control insects, and there was no difference in acuity between intensity. We discuss the relevance of threshold measurements of bushcricket hearing systems in regard to sound localisation and, from these experiments question any role of asymmetry.