CORTICAL SYNTHESIS OF AZIMUTH-SENSITIVE SINGLE-UNIT RESPONSES WITH NONMONOTONIC LEVEL TUNING - A THALAMOCORTICAL COMPARISON IN THE CAT

1. Azimuth and sound pressure level (SPL) tuning to noise stimulation was characterized in single-unit samples obtained from primary auditor y cortex (AI) and in areas of the medial geniculate body (MGB) that pr oject to AI. The primary aim of the study was to test the hypothesis t hat Al is an important site of synthesis of single-unit responses that exhibit both azimuth sensitivity (tendency for directionally restrict ed responsiveness) and nonmonotonic (NM) level tuning (tendency for de creased responsiveness with increasing SPL). This was accomplished by comparing the proportions of such responses in AI and MGB. 2. Samples consisted of high-best-frequency (BF) single units located in MGB (n = 217) and AI (n = 216) of barbiturate-anesthetized cats. The MGB sampl e was obtained mainly from recording sites located in two nuclei that project to AI, the ventral nucleus (VN, n = 118) and the lateral part of the posterior group of thalamic nuclei (Po, n = 84). In addition, a few MGB units were obtained from the medial division (n = 8) or uncer tain locations (n = 7). Each unit's responses were studied using noise bursts presented from azimuthal sound directions distributed througho ut 180 degrees of the frontal hemifield at 0 degrees elevation. SPL wa s varied over an 80-dB range in steps of less than or equal to 20 dB a t each location. Similarities and differences in azimuth and level tun ing were evaluated statistically by comparing the AT sample with the e ntire MGB sample. If they were found to differ, the AI, VN, and Po sam ples were compared. 3. Azimuth function modulation was used as a measu re of azimuth sensitivity, and its mean was greater in AI than in MGB. NM strength was defined as the percentage reduction in level function value at 75 dB SPL and its mean was greater in AI (showing a greater tendency for decreased responsiveness) than in MGB. Azimuth-sensitive (AS) NM units were identified by jointly categorizing each sample acco rding to both azimuth sensitivity (sensitive and insensitive categorie s) and NM strength (NM and monotonic categories). AS NM units were muc h more common in the AI sample than in any of the MGB samples, suggest ing that some such responses are synthesized in AI. 4. A vast majority of AI NM units have been reported to be AS, showing a preferential as sociation (linkage) between these two response properties. This findin g was confirmed in AI, but was not found to be the case in MGB. This s uggests that a linkage between these response properties emerges in th e cortex, presumably as a result of synthesis of NM AS responses. Alth ough the functional significance of the linkage is unknown, NM respons es may reflect excitatory/inhibitory antagonism that provides AS AI ne urons with sensitivity to stimulus features beyond that which is prese nt in MGB. 5. Breadth of azimuth tuning of AS cells was measured as th e portion of the frontal hemifield over which azimuth function values were >75% of maximum (preferred azimuth range, PAR). PARs were broadly distributed in each structure, and mean PAR was narrower in AI than i n MGB. A preferred level range (PLR) was defined for NM level function s as the range over which values were >75% of maximum, and mean PLRs w ere similar in each sample. There was a weak, but significant, positiv e correlation between PARs and PLRs in AI but not in MGB. This further suggests a linkage between azimuth and level tuning in AI that does n ot exist in MGB. 6. Best azimuth (midpoint of the PAR) was used to cla ssify cells as contralateral preferring, ipsilateral preferring, midli ne preferring, or multipeaked. Samples from AI and MGB exhibited simil ar distributions of these categories. Contralateral-preferring cells r epresented a majority of each sample, whereass midline-preferring, ips ilateral-preferring, and multipeaked cells each represented smaller pr oportions. This suggests that the azimuth preference distribution in A I largely reflects that in MGB. 7. A best SPL was defined as the midpo int of the PLR. This was broadly distributed with respect to best azim uths and BFs in both MGB and AI. Mean best SPLs in different structure s were similar. Best SPL was positively correlated with PLR size in bo th AI and MGB, showing that variation in best SPL reflected, at least in part, variation in breadth of level tuning.