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 cha racteristic frequency (CF) tones are nonmonotonic than does the primar y field(AI). The general purpose of the present ent study is to assess whether the response/level functions of field P neurons are generated by the same mechanisms as those of cells in Al. All of the data came from single neurons in the cortices of barbiturate-anesthetized cats, to which we presented tonal stimuli through sealed, calibrated stimula ting systems. 2. We obtained quantitative data from 123 neurons, of wh ich 108 were located in field P. Of the 108 field P cells, 70% had non monotonic 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 a ssociated with maximal responses) varied widely. A correlation analysi s revealed that a linear relation between best SPL and CF threshold ac counted for 73% of the data variance in the association between those response variables. An analysis of data from 83 nonmonotonic cells in Al revealed a similar relation. 3. Field P neurons whose response/leve l 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 r educed to I ms. Lengthening the rise time to 20 ms reduced or eliminat ed the SPL tuning in almost all of these neurons. The general form op monotonic tone response/level functions was commonly unaffected by var iation in signal rise time. In a few instances, cells with monotonic r esponse/level functions for 5- and 20-ms rise time tones developed non monotonic functions for 1-ms rise time tones. 4. Field P neurons with nonmonotonic response/level functions for CF tones usually failed to r espond to wideband noise pulses, or, less commonly, responded to noise only at low SPLs. In con trast, field P cells with a monotonic respon se to CF tones usually responded monotonically to noise. 5. The minima l mean first-spike latencies of field P neurons were generally longer than those of Al 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 Al cells. 6. The properties of field P cells followed the same rules as those seen in Al. The fact that best SPL was a systematic function of CF threshold suggests that the sensitivities of the excitatory input(s) that determine CF thresh old, and of the inhibitory input(s) that determine the best SPL, norma lly covary and do so in both Al and field P. The responses to noise an d the effects of rise time on the response/level functions of nonmonot onic field P cells are compatible with the view that the nonmonotonic form of the response/level function is usually associated with, or sha ped by, lateral inhibitory processes, as it is in Al. The fact that mi nimal first-spike latencies of field P neurons are often longer than t hose of Al cells is compatible with, but not definitive of, field P de riving its effective input(s) from Al. The larger first-spike SDs of f ield P neurons suggest that this cortical territory probably has poore r resolution in its representation of sound time structure than does A l.