TOPOGRAPHY OF EXCITATORY BANDWIDTH IN CAT PRIMARY AUDITORY-CORTEX - SINGLE-NEURON VERSUS MULTIPLE-NEURON RECORDINGS

1. The spatial distribution of the sharpness of tuning of single neurons along the dorsoventral extent of primary auditory cortex (AI) was studied. A sharpness of tuning gradient was initially obtained with multiple-unit recordings, and in combination with the cochleotopic organization, served as a frame of reference for the locations of single neurons. The frequency selectivity or "integrated excitatory bandwidth" of multiple units varied systematically along the dorsoventral extent of AI. The most sharply tuned unit clusters were found at the approximate center of the dorsoventral extent. A gradual broadening of the integrated excitatory bandwidth in both dorsal and ventral directions was consistently seen. 2. The multiple-unit measures of the bandwidth 10 (BW10) and 40 dB (BW40) above minimum threshold, pooled across several animals and expressed in octaves, were similar to those described within individual cases in cats. As in the individual animals, the bandwidth maps were V shaped with minima located at the approximate center of the dorsal-ventral extent of AI. The location of the minimum in the multiple-unit bandwidth map (i.e., the most sharply tuned area) was used as a reference point to pool single-neuron data across animals. 3. For single neurons, the dorsal half of the BW40 distribution showed a gradient paralleling that found for multiple units. For both single and multiple units, the average excitatory bandwidth increased at a rate of approximately 0.27 octaves/mm from the center of AI toward the dorsal fringe. Differing from the dorsal half of AI, the ventral half of Al showed no clear BW40 gradient for single units along its dorsoventral extent. At 40 dB above minimum threshold, most ventral neurons encountered were sharply tuned. By contrast, the multiple-unit BW40 showed a gradient similar to the dorsal half with 0.23 octaves/mm increasing from the center toward the ventral border of AI. 4. For single neurons, BW10 showed no clear systematic spatial distribution in AI. Neither the dorsal nor the ventral gradient was significantly different from zero slope, although the dorsal half showed a trend toward increasing BW10s. Contrasting single neurons, both dorsal and ventral halves of AI showed BW10 slopes for multiple units confirming a V-shaped map of the integrated excitatory bandwidth within the dorsoventral extent of AI. 5. On the basis of the distribution of the integrated (multiple-unit) excitatory bandwidth, AI was parceled into three regions: the dorsal gradient, the ventral gradient, and the central, narrowly tuned area. In ventral AI, single units were significantly more sharply tuned than multiple units for BW10 and BW40. In dorsal AI, single units were not statistically different from multiple units for BW40. In central AI, single units were significantly sharper for BW40, but not BW10. 6. Estimates of the scatter of characteristic frequency (CF) of single neurons in the dorsoventral extent domain were obtained relative to the frequency organization determined with multiple units. The central narrowly tuned region showed the least CF scatter. The CF scatter increased toward the dorsal, and in particular, the ventral end of the dorsoventral extent of AI. 7. The combined single- and multiple-unit results suggest that AI is composed of at least two functionally distinct subregions along the dorsoventral extent/isofrequency domain on the basis of the bandwidth properties of tuning curves. The dorsal region (AId) displays a global gradient of BW40 expressed in single- and multiple-unit measurements, and contains broadly and sharply tuned single-peaked as well as multipeaked neurons. The ventral region (AIv) predominantly contains neurons narrowly tuned at 40 dB above threshold. The increase in integrated (multiple-unit) excitatory bandwidth in AIv can be related to a progressively larger CF scatter at more ventral locations. The transition between AId and AIv is delineated by a reversal in the BW40 gradient for multiple units coinciding with a region of small CF scatter and sharp tuning of the underlying single neurons.