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The Journal of Neurophysiology Vol. 85 No. 4 April 2001, pp. 1575-1584
Copyright ©2001 by the American Physiological Society
Department of Neurobiology, Stanford University, Stanford, California 94305-5125
Gold, Joshua I. and
Eric I. Knudsen.
Adaptive Adjustment of Connectivity in the Inferior Colliculus
Revealed by Focal Pharmacological Inactivation. J. Neurophysiol. 85: 1575-1584, 2001. In the midbrain sound
localization pathway of the barn owl, a map of auditory space is
synthesized in the external nucleus of the inferior colliculus (ICX)
and transmitted to the optic tectum. Early auditory experience shapes
these maps of auditory space in part by modifying the tuning of the
constituent neurons for interaural time difference (ITD), a primary cue
for sound-source azimuth. Here we show that these adaptive
modifications in ITD tuning correspond to changes in the pattern of
connectivity within the inferior colliculus. We raised owls with an
acoustic filtering device in one ear that caused frequency-dependent
changes in sound timing and level. As reported previously, device
rearing shifted the representation of ITD in the ICX and tectum but not
in the primary source of input to the ICX, the central nucleus of the inferior colliculus (ICC). We applied the local anesthetic
lidocaine (QX-314) iontophoretically in the ICC to inactivate
small populations of neurons that represented particular values of
frequency and ITD. We measured the effect of this inactivation in the
optic tecta of a normal owl and owls raised with the device. In the normal owl, inactivation at a critical site in the ICC eliminated responses in the tectum to the frequency-specific ITD value represented at the site of inactivation in the ICC. The location of this site was
consistent with the known pattern of ICC-ICX-tectum connectivity. In
the device-reared owls, adaptive changes in the representation of ITD
in the tectum corresponded to dramatic and predictable changes in the
locations of the critical sites of inactivation in the ICC. Given that
the abnormal representation of ITD in the tectum depended on frequency
and was likely conveyed directly from the ICX, these results suggest
that experience causes large-scale, frequency-specific adjustments in
the pattern of connectivity between the ICC and the ICX.
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