Factors shaping the tone level sensitivity of single neurons in posterior field of cat auditory cortex

1. The posterior field (field P) of the cat's auditory cortex contains a higher proportion of neurons whose response/level functions for characteristic frequency (CF) tones are nonmonotonic than does the primary field (AI). The general purpose of the present study is to assess whether the response/level functions of field P neurons are generated by the same mechanisms as those of cells in AI. All of the data came from single neurons in the cortices of barbiturate-anesthetized cats, to which we presented tonal stimuli through sealed, calibrated stimulating systems. 2. We obtained quantitative data from 123 neurons, of which 108 were located in field P. Of the 108 field P cells, 70% had nonmonotonic response/level functions for 5- ms rise time tones of CF. For cells of any given CF, both CF thresholds and best SPLs (i.e., SPLs associated with maximal responses) varied widely. A correlation analysis revealed that a linear relation between best SPL and CF threshold accounted for 73% of the data variance in the association between those response variables. An analysis of data from 83 nonmonotonic cells in AI revealed a similar relation. 3. Field P neurons whose response/level functions were nonmonotonic for 5-ms rise time CF tones became even more narrowly tuned to SPL when the rise time of the tone bursts was reduced to 1 ms. Lengthening the rise time to 20 ms reduced or eliminated the SPL tuning in almost all of these neurons. The general form of monotonic tone response/level functions was commonly unaffected by variation in signal rise time. In a few instances, cells with monotonic response/level functions for 5- and 20-ms rise time tones developed nonmonotonic functions for 1-ms rise time tones. 4. Field P neurons with nonmonotonic response/level functions for CF tones usually failed to respond to wideband noise pulses, or, less commonly, responded to noise only at low SPLs. In contrast, field P cells with a monotonic response to CF tones usually responded monotonically to noise. 5. The minimal mean first-spike latencies of field P neurons were generally longer than those of AI cells studied under similar conditions. The precision of first-spike timing, measured using the SD of the mean first- spike latency, was commonly poorer than that of AI cells. 6. The properties of field P cells followed the same rules as those seen in AI. The fact that best SPL was a systematic function of CF threshold suggests that the sensitivities of the excitatory input(s) that determine CF threshold, and of the inhibitory input(s) that determine the best SPL, normally covary and do so in both AI and field P. The responses to noise and the effects of rise time on the response/level functions of nonmonotonic field P cells are compatible with the view that the nonmonotonic form of the response/level function is usually associated with, or shaped by, lateral inhibitory processes, as it is in AI. The fact that minimal first-spike latencies of field P neurons are often longer than those of AI cells is compatible with, but not definitive of, field P deriving its effective input(s) from AI. The larger first-spike SDs of field P neurons suggest that this cortical territory probably has poorer resolution in its representation of sound time structure than does AI.