Journal of Neuroscience, Vol 13, 3647-3668, Copyright © 1993 by Society for Neuroscience
Bilateral inhibition generates neuronal responses tuned to interaural level differences in the auditory brainstem of the barn owl
R Adolphs
Division of Biology, Caltech, Pasadena, California 91125.
I investigated the neural algorithms by which neurons gain selectivity for
interaural level difference in the brainstem of the barn owl (Tyto alba).
Differences in the timing and in the level of sounds at the ears are used
by this owl to encode, respectively, azimuthal and vertical position of
sound sources in space. These two cues are processed in two parallel neural
pathways. Below the level of the inferior colliculus, all neurons in the
pathway that processes level differences show responses to this cue that
are monotonic, and thus not selective for a particular level difference.
Only in the inferior colliculus, which contains a map of auditory space,
are neurons sharply tuned to specific interaural level differences. How are
these response properties generated from those of the nuclei that provide
input to the inferior colliculus? I show that the posterior subdivision of
the nucleus ventralis lemnisci lateralis (VLVp) projects bilaterally to the
lateral shell of the central nucleus of the inferior colliculus, the input
stage to the map of auditory space. Both these nuclei are part of the
pathway that processes interaural level differences. Manipulations of the
responses in VLVp affected the responses to level differences in the
inferior colliculus; responses to time differences were unaffected. By
systematically increasing or decreasing neural activity in VLVp, I show
that the VLVp on each side provides inhibition to the colliculus at large
level differences. This results in a peaked response that is tuned to level
differences in the inferior colliculus. Some cells in the lateral shell of
the inferior colliculus appear to receive direct GABAergic inhibition from
VLVp. I suggest that this circuitry and the algorithms it supports are the
neural substrates that allow the barn owl to exploit level differences for
computation of sound source elevation.
