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Abstract
The major cues for the sound localization are the interaural differences in the timing
and intensity of acoustic information. This poses a difficult coding problem for animals
with relatively small heads, such as birds, because interaural time differences (ITDs)
would have a small range and magnitude and interaural intensity differences (IIDs)
would be significant for only high frequency sounds. It has been suggested that this
coding problem is mitigated in birds by an enhancement of ITDs and IIDs resulting
from the acoustic coupling of the two middle ear cavities through an interaural canal.
In this report, the functional ITDs for sounds at different azimuthal locations were
recorded in young chicks, and the contribution of middle ear acoustic coupling was
evaluated. ITDs were calculated from simultaneous cochlear microphonic (CM) recordings
evoked by pure tone stimuli. These effective ITDs were larger than predicted by the
physical separation of the two ears, and this enhancement was more pronounced at low
(0.8 and 1 kHz) than at high (2 and 4 kHz) frequencies, reaching maximum values of
approximately 180 and 100 microseconds, respectively. The amplitude of the CM also
varied as a function of sound source location. This variation was as much as +/- 30%,
even for the low frequency tones. This suggests that IID cues are also available to
the chick. To determine the contribution of middle ear acoustic coupling to the timing
and amplitude of the CM response, the CM in one ear was measured prior to, and following
occlusion of the contralateral external auditory canal. The cochlear microphonic from
the ear distal to the sound source advanced in time and increased in amplitude when
the ear proximal to the sound source was sealed. These effects were more pronounced
for low frequency sounds. These findings confirm that acoustic coupling of the middle
ear cavities plays a role in enhancing sound localization cues in the chick.