R. R. Hoy
Sec. of Neurobiol. and Behavior, Cornell Univ., Ithaca, NY 14853-2702
R. N. Miles
State Univ. of New York, Binghamton, NY 13902-6000
Sound localization is a basic behavioral task of the auditory system. Incident sound waves arrive at the ears and generate interaural differences in time of arrival and in amplitude that are key cues for the computation of sound direction. In small animals, both cues can become vanishingly small, posing a challenge for directional hearing. Yet nearly all animals that hear can localize sound. In the fly Ormia ochracea, the two acoustic sensors are separated by only about 520 (mu)m, and are contained within an undivided air-filled chamber, an arrangement that results in minimal differences in interaural time (<2 (mu)s) and no intensity cues from an incident sound wave. Using laser vibrometry, it is shown that the mechanical response of the tympanal membranes has a pronounced directional sensitivity. Using probe microphones and neurophysiological recording techniques, it is demonstrated that this fly utilizes a novel mechanism for the detection of an incident sound wave. This mechanism relies on the mechanical coupling between the two tympanal membranes. In effect, the fly's ears operate by mechanical preprocessing, converting interaural acoustic time differences of 2 ms or less into neural time differences large enough to encode in the central nervous system.