A Monaural Spectral Contrast Mechanism for Neural Sensitivity to Sound
Direction in the Medial Geniculate Body of the Cat.
Thomas J. Imig, Pierre Poirier, W. Andrew Irons, and Frank K. Samson.
Department of Physiology, Kansas University Medical Center, Kansas City, KS
66160-7401.
APStracts 4:146N, 1997.
ABSTRACT
Central auditory neurons vary in sound direction sensitivity. Insensitive
cells discharge well to all sound source directions whereas sensitive cells
discharge well to certain directions and poorly to others. High-frequency
neurons in the latter group are differentially sensitive to binaural and
monaural directional cues present in broadband noise (BBN). Binaural
directional (BD) cells require binaural stimulation for directional
sensitivity; monaural directional (MD) cells are sensitive to the direction of
monaural stimuli. A model of MD sensitivity was tested using single unit
responses. The model assumes that MD cells derive directional sensitivity from
pinna-derived spectral cues (head related transfer function, HRTF). This
assumption was supported by the similarity of effects that pinna orientation
produces on locations of HRTF patterns (Young et al., 1996) and on locations
of MD cell azimuth function peaks and nulls. According to the model, MD
neurons derive directional sensitivity by use of excitatory/inhibitory
antagonism to compare sound pressure in excitatory and inhibitory frequency
domains, and a variety of observations are consistent with this idea. 1.
Frequency response areas of MD cells consist of excitatory and inhibitory
domains. MD cells exhibited a higher proportion of multiple excitatory domains
and narrower excitatory frequency domains than BD cells, features that may
reflect specialization for spectral-dependent directional sensitivity. 2. MD
sensitivity requires sound pressure in excitatory and inhibitory frequency
domains. Directional sensitivity was evaluated using stimuli with frequency
components confined exclusively to excitatory domains (E-only stimuli) or
distributed in both excitatory and inhibitory domains (E/I stimuli). Each of
13 MD cells that were tested exhibited higher directional sensitivity to E/I
stimuli than to E-only stimuli; most MD cells exhibited relatively low
directional sensitivity when frequency components were confined exclusively to
excitatory domains. 3. MD sensitivity derives from excitatory/inhibitory
antagonism spectral inhibition. Comparison of responses to best-frequency and
E/I stimuli provided strong support for spectral inhibition. Although spectral
facilitation could conceivably contribute to directional sensitivity with
direction-dependent increases in response, the results did not show this to be
a significant factor. 4. Direction-dependent decreases in responsiveness to
BBN reflect increased sound pressure in inhibitory relative to excitatory
frequency domains. This idea was tested using the strength of two-tone
inhibition which is a function of stimulus levels in inhibitory relative to
excitatory frequency domains. The finding that two-tone inhibition was
stronger at directions where BBN responses were minimal than at directions
where they were maximal supports the model.
Received 7 March 1997; accepted in final form 7 July 1997.
APS Manuscript Number J197-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997