SENSITIVITY TO INTERAURAL INTENSITY DIFFERENCES OF NEURONS IN PRIMARYAUDITORY-CORTEX OF THE CAT .1. TYPES OF SENSITIVITY AND EFFECTS OF VARIATIONS IN SOUND PRESSURE LEVEL

1. Interaural intensity differences (IIDs) provide the major cue to th e azimuthal location of high-frequency narrowband sounds. In recent st udies of the azimuthal sensitivity of high-frequency neurons in the pr imary auditory cortex (field AI) of the cat, a number of different typ es of azimuthal sensitivity have been described and the azimuthal sens itivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity de pendence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sen sitivity from a large sample of neurons with a characteristic frequenc y (CF) >5.5 kHz in AI of anesthetized cats. IID sensitivity functions were classified in a manner that facilitated comparison with previousl y obtained data on azimuthal sensitivity, and the effects of changes i n the base intensity at which IIDs were introduced were examined. 2. I ID 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 whi ch a function was obtained for each unit, five types of IID sensitivit y were distinguished. Contra-max neurons (50% of the sample) had maxim um response (a peak or a plateau) at IIDs corresponding to contralater al azimuths, whereas ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maxi mum response (peak) in the range of +/-10 dB IID, whereas a small grou p of trough neurons (2%) had a restricted range of minimal responsiven ess 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 clos ely to the proportions found to exhibit corresponding types of azimuth al sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form o f its no sensitivity function. Thus, neurons excited by monaural stimu lation of only one ear but with either inhibitory, facilitatory, or mi xed facilitatory-inhibitory effects of stimulation of the other ear ha d predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral mo naural stimulation was excitatory). Neurons driven weakly or not at al l by monaural stimulation but facilitated binaurally almost all exhibi ted near-zero-max IID sensitivity. The exception to this tight associa tion between binaural input and IID sensitivity was provided by neuron s excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to nos, our data were in agreement with two recent reports indicating that the y can exhibit various forms of IID sensitivity: only 23 of 75 EE neuro ns were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more i ntensities (3-most cases) for 84 neurons. The form of the IID sensitiv ity function (defined in terms of both shape and position along the II D axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (similar to 15% if a strict criterion of inva riance was employed), and for many of these neurons the spike counts a ssociated with a given IID varied with ABI, particularly at near-thres hold levels. When the patterns of variation in the form of IID sensiti vity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimu thal 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, the responses o f a significant proportion of high-frequency AI neurons (30-40%) varie d nonmonotonically 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 th e types of IID and azimuthal sensitivity and the effects of variation in intensity on the two types of sensitivity supports the view that th e azimuthal sensitivity of high-frequency AI neurons is shaped by thei r LTD sensitivity. The fact that IID sensitivity is invariant with cha nges 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 acro ss a population of neurons, then the pattern of activity associated wi th a given IID would vary with intensity unless the population were re stricted to the small proportion of neurons with intensity-invariant I ID sensitivity. 7. Comparison of the types of IID (and azimuthal) sens itivity exhibited by high-frequency AI neurons with those seen in the inferior colliculus, and of the binaural interaction characteristics o f high-frequency neurons at these two levels, indicates far greater di versity in cortex. Significant transformations in the binaural propert ies of high-frequency neurons occur in the projections from the midbra in to the cortex and/or as a consequence of mechanisms intrinsic to th e cortex.