TOPOGRAPHY OF INTENSITY TUNING IN CAT PRIMARY AUDITORY-CORTEX - SINGLE-NEURON VERSUS MULTIPLE-NEURON RECORDINGS

1. We studied the spatial distributions of amplitude tuning (monotonic ity of rate-level functions) and response threshold of single neurons along the dorsoventral extent of cat primary auditory cortex (AI). To pool data across animals, we used the multiple-unit map of monotonicit y as a frame of reference. Amplitude selectivity of multiple units is known to vary systematically along isofrequency contours, which run ro ughly in the dorsoventral direction. Clusters sharply tuned for intens ity (i.e., ''nonmonotonic'' clusters) are located near the center of t he contour. A second nonmonotonic region con be found several millimet ers dorsal to the center. We used the locations of these two nonmonoto nic regions as reference points to normalize data across animals. Addi tionally. to compare this study to sharpness of frequency tuning resul ts, we also used multiple-unit bandwidth (BW) maps as references to po ol data. 2. The multiple-unit amplitude-related topographies recorded in previous studies were closely approximated the previously reported individual case maps when the multiple-unit monotonicity or the map of bandwidth (in octaves) of pure tones to which a cell responds 40 dB a bove minimum threshold were used as the pooling reference. When the ma p of bandwidth ( in octaves) of pure tones to which a cell responds 10 dB above minimum threshold map was used as part of the measure, the p ooled spatial pattern of multiple-unit activity was degraded. 3. Singl e neurons exhibited nonmonotonic rate-level functions more frequently than multiple units. Although common in single-neuron recordings (28%) . strongly nonmonotonic recordings ( firing rates reduced by >50% at h igh intensities) were uncommon (8%) in multiple-unit recordings. Inter mediately nonmonotonic neurons (firing rates reduced between 20% and 5 0% at high intensities) occurred with nearly equal probability in sing le-neuron( 28%) and multiple unit (26%) recordings. The remaining reco rdings for multiple units (66%) and single units (44%) were monotonic ( firing rates within 20% of the maximum at the highest tested intensi ty). 4. In ventral AI (AIv). the topography of monotonicity for single units was qualitatively similar to multiple units, although single un its were on average more intensity selective. In dorsal AI (AId) we co nsistently found a spatial gradient for sharpness of intensity tuning for multiple units: however, for pooled single units in AId there was no clear topographic gradient. 5. Response (intensity) thresholds of s ingle neurons were not uniformly distributed across the dorsoventral e xtent of AI. The most sensitive neurons were consistently located in t he nonmonotonic regions. The scatter of single neuron intensity thresh old was smallest at these locations and increased gradually toward mor e dorsal and ventral locations. 6. The existence of a specialized regi on for near-threshold stimuli along the AIv/AId border is revealed in these experiments. Neuronal recordings in this region are sharply tune d for frequency and amplitude. have low intensity thresholds, have low scatter in characteristic frequency and threshold, and selectively re spond to narrowband stimuli within 40 dB of the cortical intensity thr eshold. Nonmonotonic neurons have been shown to shift their spike coun t-versus-level functions linearly in response to a continuous noise ma sker. Neurons in the ventral nonmonotonic region thus might serve as f ine spectral/amplitude filters that only respond to frequency-banded c omponents with intensities just above the cat's threshold in the prese nce of background noise. 7. The results of this study support the parc eling of AI into at least two physiologically distinct subdivisions. T he ventral subdivision (AIV) has a complete single-unit topographic re presentation of stimulus intensity. Low-intensity signals elicit maxim al response at a signal detection region, located at the dorsal extrem e of AIv at the AIV-AId border. Neurons respond better to higher-inten sity signals progressively ventrally until the AII border is approache d. Aid is well suited for differential frequency analysis and contains a single-unit topograph for stimulus bandwidth, as previously reporte d.