Altered spectral localization cues disrupt the development of the auditory
space map in the superior colliculus of the ferret.
Jan W. H. Schnupp, Andrew J. King, and Simon Carlile.
University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, England.
Department of Physiology, University of Sydney, NSW 2006 Australia.
APStracts 4:233N, 1997.
ABSTRACT
Spectral localization cues provided by the outer ear are utilized in the
construction of the auditory space map in the superior colliculus (SC). The
role of the outer ear in the development of this map was examined by recording
from the SC of anesthetized, adult ferrets in which the pinna and concha had
been removed in infancy. The acoustical consequences of this procedure were
assessed by recording outer ear impulse responses via a probe-tube microphone
implanted in the wall of the ear canal. Both monaural and binaural spectral
cues normally show a number of asymmetric features within the horizontal
plane, which allow azimuthal locations on either side of the inter-aural axis
to be discriminated. These features were eliminated or altered by chronic
pinnectomy. The responses of auditory units in the SC to noise bursts
presented in the free field were examined at sound levels of approximately 10
and 25 dB above unit threshold. Following bilateral pinnectomy, the
representation of auditory space was severely degraded at both sound levels.
In contrast to normal ferrets, many units had bilobed azimuthal response
profiles, indicating that they were unable to resolve sound locations on
either side of the interaural axis. There was also much less order in the
distribution of best azimuths or elevations of those units that were tuned to
a single direction. Some units were tuned to locations that extended much
further into the hemifield ipsilateral to the recording side than the normal
range of best azimuths. Unilateral removal of the outer ear, which disrupts
the monaural spectral cues for one side only, had a much smaller effect on the
development of the auditory representation. At supra- and near threshold sound
levels, the representation of sound azimuth in the SC on both sides of the
brain was less scattered than that found following bilateral pinna removal.
Nevertheless, units with bilobed responses, broader tuning and inappropriate
best azimuths were observed in both the left and right SC of ferrets in which
the left pinna and concha had been removed in infancy. These data illustrate
that the localization cues provided by the outer ear play a critical role in
the development of the auditory space map in the SC. In contrast to other
manipulations of either auditory or visual inputs, the map does not appear to
adapt to the changes in spectral cues brought about by pinna removal,
suggesting that residual binaural cues are, by themselves, insufficient for
its normal maturation.
Received 22 January 1997; accepted in final form 4 September 1997.
APS Manuscript Number J61-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997