COMPARISON OF NOISE AND TONE AZIMUTH TUNING OF NEURONS IN CAT PRIMARY
AUDITORY CORTEX AND MEDIAL GENICULATE BODY.
CLAREY, JANINE C., PASCAL BARONE, W. ANDREW IRONS, FRANK K. SAMSON, AND THOMAS
J. IMIG.
Department of Physiology, Kansas University Medical Center, Kansas City,
Kansas 66160-7401, U.S.A., and Vision, Touch and Hearing Research Centre,
Department of Physiology and Pharmacology, The University of Queensland,
Queensland 4072, Australia.
APStracts 2:0126N, 1995.
SUMMARY AND CONCLUSIONS
1. A comparison of the azimuth tuning of single neurons to broadband noise and
to best frequency (BF) tone bursts was made in primary auditory cortex (AI:
n=173) and the medial geniculate body (MGB: n=52) of barbiturate-anesthetized
cats. Observations were largely restricted to cells located within the
tonotopically organized divisions of the MGB (i.e., the ventral nucleus and
the lateral division of the posterior nuclear group) and the middle layers of
AI. All cells studied had BFs ³ 4 kHz. 2. The responses of each cell to sounds
presented from seven frontal azimuthal locations (-90[acute]i to +90[acute]i
in 30[acute]i steps; 0[acute]i elevation) and at five sound pressure levels
(SPLs; 0-80 dB or 5-85 dB in 20 dB steps) provided an azimuth-level data set.
Responses were averaged over SPL to obtain an azimuth function and a number of
features of this function was used to describe azimuth tuning to noise and to
tone stimulation. Azimuth function modulation was used to assess azimuth
sensitivity, and cells were categorized as sensitive or insensitive depending
on whether modulation was ³75% or <75% of maximum, respectively. The
majority (88%) of cells in the sample were azimuth sensitive to noise
stimulation, and statistical analyses were restricted to these cells which are
presumably best suited to encode sound source azimuth. Azimuth selectivity was
assessed by a preferred azimuth range over which azimuth function values
exceeded 75% (PAR75) or 50% (PAR50) of maximum response. Cells were
categorized according to the location and extent of their noise PARs.
Unbounded cells had laterally located PARs that extended to the lateral pole (
90[acute]i); bounded cells had PARs that were contained entirely within the
frontal hemifield, and a subset of these had PARs centered on the midline (
15[acute]i). A final group of cells exhibited multipeaked azimuth functions to
noise stimulation. 3. Azimuth functions to noise were generally more selective
and/or more sensitive than those to tones. Statistical analyses showed that
these differences were significant for cells in each azimuth function
category, and for the thalamic and cortical samples. With the exception of
multipeaked cells, responsiveness to noise was significantly lower than that
to tones in all categories, and for the thalamic and cortical samples. The
slope of the azimuth function, defined by the range of azimuths over which the
cell's response changed from 25% to 75% of maximum, tended to be steeper to
noise than that to tones; this difference was significant in the midline and
unbounded cell groups. The majority of cells (89.5%) showed best azimuths
(midpoint of the PAR75) to the two stimuli that differed by differed by
30[acute]i. This indicates that although many cells were more narrowly tuned
to the azimuth of noise than BF tones, they tended to have similarly located
noise and tone PARs. 4. Azimuth-level data sets were averaged over azimuth to
obtain a level function. The nonmonotonic strength of the level function was
defined by the percentage reduction in responsiveness at the highest level
tested. The effect of bandwidth on azimuth selectivity was slightly greater
for neurons that showed strongly nonmonotonic level functions than those with
weakly nonmonotonic functions. There was no relationship between the
nonmonotonic strength of the response to noise stimulation and differences
observed in azimuth function modulation to the two stimuli. 5. Fifty cells
were studied with reversible ear occlusion to obtain information on their
binaural inputs and interactions, and this was related to differences in their
azimuth tuning to noise and to tones. Cells were classified according to
whether their azimuth tuning depended upon monaural spectral cues (MD,
monaural directional cells) or binaural disparities (BD, binaural directional
cells) (Samson et al. 1993, 1994). Six MD cells received excitatory input from
one ear with no evidence of input from the other (MD-EO), and these cells
showed far broader and less modulated azimuth functions to tones than to
noise. Their azimuth tuning was apparently derived from spectral cues present
in broadband but not tonal stimuli. MD cells that received inhibitory input
from the non-excitatory ear (MD-EI; n=11) showed the same trends as for MD-EO
cells, although differences were less dramatic since the inhibitory input
shaped the cellÕs response to tones. 6. The azimuth tuning of 18 cells that
responded maximally to azimuths about the midline was found to be a product of
binaural facilitation. The other class of BD cells (n=15) received excitatory
input from one ear (usually the contralateral) and inhibitory input (or mixed
inhibitory-facilitatory input) from the other. These cells typically responded
well throughout most, or all, of one lateral hemifield. A substantial
proportion of cells within both groups showed greater selectivity and/or
sensitivity to noise than to tones, although the differences between azimuth
tuning to the two stimuli were less dramatic than those observed in either
group of MD cells. These data suggest that some aspect of a broadband stimulus
also contributes to azimuth tuning in binaural cells. 7. The consistency in
azimuth preference and the narrower tuning observed in many cells' responses
to noise compared to high-frequency pure tones is compatible with behavioral
studies in cats, monkeys, and humans that have shown that both these stimuli
can be localized in the horizontal plane but performance is more accurate to
noise than to tones.
Received 6 September 1994; accepted in final form 6 April 1995.
APS Manuscript Number J562-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 2 May 1995.