ENVELOPE CODING IN THE LATERAL SUPERIOR OLIVE. III. COMPARISON WITH
AFFERENT PATHWAYS.
Philip X. Joris, Tom C.T. Yin .
Department of Neurophysiology, University of Wisconsin Medical School
Madison, Wisconsin 53706, and Division of Neurophysiology, Medical School,
University of Leuven, B-3000 Leuven, Belgium.
APStracts 4:186N, 1997.
ABSTRACT
Binaural cues for spatial localization of complex high-frequency sounds are
interaural level and time differences (ILDs and ITDs). We previously showed
that cells in the lateral superior olive (LSO) are sensitive to ITDs in the
envelope of sinusoidally amplitude-modulated (AM) signals up to a modulation
frequency of only about 800 Hz. To understand the limitations in this ITD-
sensitivity, we here compare responses to monaural modulation in LSO and its
input pathways, derived from cochlear nucleus (CN) and medial nucleus of the
trapezoid body (MNTB). These pathways have marked functional and morphological
specializations, suggestive of adaptations for timing.
Afferent cell populations were identified on the basis of electrophysiological
signatures, and for each population average firing rate and synchronization to
AM tones were compared to auditory-nerve fibers and LSO cells. Except for an
increase in modulation gain in some subpopulations, synchronization of LSO-
afferents was very similar to that in auditory nerve fibers in their
dependency on sound pressure level (SPL), modulation depth and modulation
frequency. Distributions of cutoff-frequencies of modulation transfer
functions were largely coextensive with the distribution in auditory nerve.
Group delays, measured from the phase of the response modulation as a function
of modulation frequency, showed an orderly dependence on characteristic
frequency (CF) and cell type and little dependence on SPL. Similar responses
were obtained to a modulated broadband carrier.
Compared to their afferents, LSO cells synchronized to monaurally modulated
stimuli with a higher gain, but often over a narrower range of modulation
frequencies. Considering the scatter in afferent and LSO cell populations,
ipsi- and contralateral responses were well matched in cutoff-frequency and
magnitude of delays. In contrast to their afferents, LSO cells show a decrease
in average firing rate at high modulation frequencies.
We conclude that the restricted modulation frequency range over which LSO
cells show ITD-sensitivity does not result from loss of envelope information
along the afferent pathway, but is due to convergence or postsynaptic effects
at the level of the LSO. The faithful transmission of envelope phase-locking
in LSO-afferents is consistent with their physiological and morphological
adaptations, but these adaptations are not commensurate with the rather small
effects of physiological ITDs reported previously, especially when compared to
effects of ILDs. We suggest that these adaptations have evolved to allow a
comparison of instantaneous amplitude fluctuations at the two ears, rather
than to extract interaural timing information per se.
Received 7 May 1997; accepted in final form 5 August 1997.
APS Manuscript Number J374-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997