Dp. Phillips et al., FACTORS SHAPING THE TONE LEVEL SENSITIVITY OF SINGLE NEURONS IN POSTERIOR FIELD OF CAT AUDITORY-CORTEX, Journal of neurophysiology, 73(2), 1995, pp. 674-686
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.