SENSITIVITY TO INTERAURAL INTENSITY DIFFERENCES OF NEURONS IN PRIMARY AUDITORY CORTEX OF THE CAT: I. TYPES OF SENSITIVITY AND EFFECTS OF VARIATIONS IN SOUND PRESSURE LEVEL. Irvine, D. R. F., R. Rajan and L. M. Aitkin. Departments of Psychology and Physiology, Monash University, Clayton, Vic. 3168, Australia.
APStracts 2:0249N, 1995.
SUMMARY AND CONCLUSIONS
1. Interaural intensity differences (IIDs) provide the major cue to the azimuthal location of high-frequency narrow-band sounds. In recent studies of the azimuthal sensitivity of high-frequency neurons in the primary auditory cortex (field AI) of the cat, a number of different types of azimuthal sensitivity have been described and the azimuthal sensitivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity-dependence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sensitivity from a large sample of neurons with characteristic frequency (CF) >5.5 ? kHz in AI of anesthetized cats. IID- sensitivity functions were classified in a manner that facilitated comparison with previously obtained data on azimuthal sensitivity, and the effects of changes in the base intensity at which IIDs were introduced were examined. 2. IID-sensitivity functions for CF tonal stimuli were obtained at one or more intensities for a total of 294 neurons, in most cases by a method of generating IIDs that kept the average binaural intensity (ABI) of the stimuli at the two ears constant. In the standard ABI range at which a function was obtained for each unit, 5 types of IID sensitivity were distinguished. Contra- max neurons (50% of the sample) had maximum response (a peak or a plateau) at IIDs corresponding to contralateral azimuths, while ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maximum response (peak) in the range +/-10 ? dB IID, while a small group of trough neurons (2%) had a restricted range of minimal responsiveness with near maximal responses at IIDs on either side. A final 18% of AI neurons were classified as insensitive to IIDs. The proportions of neurons exhibiting the various types of sensitivity corresponded closely to the proportions found to exhibit corresponding types of azimuthal sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form of its IID-sensitivity function. Thus, neurons excited by monaural stimulation of only one ear but with either inhibitory, facilitatory, or mixed facilitatory/inhibitory effects of stimulation of the other ear had predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral monaural stimulation was excitatory). Neurons driven weakly or not at all by monaural stimulation but facilitated binaurally almost all exhibited near-zero-max IID sensitivity. The exception to this tight association between binaural input and IID sensitivity was provided by neurons excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to IIDs, our data were in agreement with two recent reports indicating that they can exhibit various forms of IID sensitivity: only 23 of 75 EE neurons were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more intensities (3-5 in most cases) for 84 neurons. The form of the IID-sensitivity function (defined in terms of both shape and position along the IID axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (approximately 15% if a strict criterion of invariance was employed), and for many of these neurons the spike counts associated with a given IID varied with ABI, particularly at near-threshold levels. When the patterns of variation in the form of IID sensitivity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimuthal sensitivity, there was a close correspondence between the effects of intensity on corresponding types of azimuthal and IID sensitivity. 5. In agreement with the report of Semple and Kitzes (1993a,b), the responses of a significant proportion of high-frequency AI neurons (30-40%) varied non-monotonically with both IID and ABI such that a restricted focus of maximal responsiveness was associated with a particular combination of limited IID and ABI ranges. 6. The close correspondence between the types of IID and azimuthal sensitivity and the effects of variation in intensity on the two types of sensitivity supports the view that the azimuthal sensitivity of high-frequency AI neurons is shaped by their IID sensitivity. The fact that IID sensitivity is invariant with changes in stimulus intensity (at suprathreshold levels) for only a small proportion of AI neurons has implications for the manner in which IID (and azimuthal location) are represented by populations of neurons in AI. If IID is represented by the distributed pattern of activity across a population of neurons, then the pattern of activity associated with a given IID would vary with intensity unless the population were restricted to the small proportion of neurons with intensity-invariant IID sensitivity. 7. Comparison of the types of IID (and azimuthal) sensitivity exhibited by high-frequency AI neurons with those seen in the inferior colliculus, and of the binaural interaction characteristics of high-frequency neurons at these two levels, indicates far greater diversity in cortex. Significant transformations in the binaural properties of high-frequency neurons occur in the projections from the midbrain to the cortex and/or as a consequence of mechanisms intrinsic to the cortex. 

Received 13 June 1994; accepted in final form 2 August 1995.
APS Manuscript Number J338-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.