FUNCTIONAL-ORGANIZATION OF SOUND DIRECTION AND SOUND PRESSURE LEVEL IN PRIMARY AUDITORY-CORTEX OF THE CAT

1. The functional organization of neuronal tuning to the azimuthal loc ation and sound pressure level (SPL) of noise bursts was studied in hi gh-frequency primary auditory cortex (AI) of barbiturate-anesthetized cats. Three data collection strategies were used to map neural respons es: 1) electrode penetrations oriented normal to the cortical surface provided information on the radial organization of neurons' responses; 2) neurons' responses were examined at a few points in the middle cor tical layers in multiple normal penetrations across AI to produce fine -grain maps of azimuth and level selectivity; and 3) electrode penetra tions oriented tangential to the cortical surface provided information on neurons' responses along the isofrequency dimension. 2. An azimuth -level data set was obtained for each single- or multiple- (multi-) un it recording; this consisted of responses to noise bursts at five SPLs (0-80 dB in 20-dB steps) from seven azimuthal locations in the fronta l hemifield (-90 to +90 degrees in 30 degrees steps; 0 degrees elevati on). An azimuth function was derived from these data by averaging resp onse magnitude over all SPLs at each azimuth tested. A preferred azimu th range (PAR; range of azimuths over which the response was greater t han or equal to 75% of maximum) was calculated from the azimuth functi on and provided a level-independent measure of azimuth selectivity. Ea ch PAR was assigned to one of four azimuth preference categories (cont ralateral-, midline-, ipsilateral-preferring, or broad/multipeaked) ac cording to its location and extent. A level function obtained from the data set (responsiveness averaged over all azimuths) was classified a s monotonic if it showed a decrease of less than or equal to 25% (rela tive to maximum) at the highest SPL tested (usually 80 dB), and nonmon otonic if it showed a decrease of >25%. The percentage reduction in re sponsiveness, relative to maximum, at the highest lever tested (termed nonmonotonic strength) and the preferred level range (PLR; range of S PLs over which responsiveness was greater than or equal to 75% of maxi mum) of each response was also determined. 3. Normal penetrations typi cally showed a predominance of one azimuth preference category and/or level function type. The majority of penetrations (26/36: 72.2%) showe d statistically significant azimuth preference homogeneity, and approx imately one-half (17/36: 47.2%) showed significant level function type homogeneity. Over one-third (13/36) showed significant homogeneity fo r both azimuth preference and level function type. 4. Mapping experime nts (n = 4) sampled the azimuth and level response functions at two or more depths in closely spaced normal penetrations that covered severa l square millimeters of AI. Cortical sites containing neurons with the same azimuth preference and/or level function type were grouped toget her. The most extensive map revealed a contralateral-preferring band o f cortex that was elongated parallel to the frequency gradient and cov ered a frequency range from 8 to 20 kHz. Immediately ventral to this w as a midline-preferring region that covered the same frequency range. 5. Azimuth and level response functions were obtained at closely space d sites along 20 tangential penetrations oriented parallel to isofrequ ency contours, and most penetrations showed sequential responses that exhibited similar azimuth preferences and/or level function types. The majority (15/20: 75%) of penetrations showed significant clustering o f neurons with the same azimuth preference. There were often multiple, discontinuous groups of neurons with similar azimuth preferences (usu ally contralateral) along isofrequency bands. Systematic shifts in the azimuth preference of successive neurons or groups of neurons were no t commonly seen. Over one-half (12/20: 60%) the penetrations showed si gnificant clustering of monotonic and nonmonotonic response types. 6. The organization of neurons with differing degrees of azimuth sensitiv ity, as measured by azimuth function modulation, was also examined. Ne urons' responses were categorized as either high directional (HD; grea ter than or equal to 75% modulation) or low directional (LD; <75% modu lation). Only a minority of normal and tangential penetrations showed significant homogeneity or clustering of neurons with HD or LD respons es. 7. In an attempt to validate the use of the multiunit technique to study functional organization, comparisons of the azimuth and level r esponse properties of single and multiunits were made. This was achiev ed by 1) comparing the responses of multi/single-unit pairs (n = 114) recorded at the same location and single-unit pairs (n = 16) recorded at the same location; and 2) comparing the distribution of responses f alling into the different azimuth and level categories from a large mu ltiunit sample and a comparable single-unit sample collected during th ese experiments. Multi- and single-unit recordings showed differences in nonmonotonic strength and azimuth function modulation. However, mea sures of azimuth preference and selectivity (best azimuth) or level se lectivity (best SPL) that were derived from the range of maximum respo nsiveness of the azimuth or level function showed significant correlat ions.