P. Heil et Drf. Irvine, FUNCTIONAL SPECIALIZATION IN AUDITORY-CORTEX - RESPONSES TO FREQUENCY-MODULATED STIMULI IN THE CATS POSTERIOR AUDITORY FIELD, Journal of neurophysiology, 79(6), 1998, pp. 3041-3059
The mammalian auditory cortex contains multiple fields but their funct
ional role is poorly understood. Here we examine the responses of sing
le neurons in the posterior auditory held (P) of barbiturate- and keta
mine-anesthetized cats to frequency-modulated (FM) sweeps. FM sweeps t
raversed the excitatory response area of the neuron under study, and F
M direction and the linear rate of change of frequency (RCF) were vari
ed systematically. In some neurons, sweeps of different sound pressure
levels (SPLs) also were tested. The response magnitude (number of spi
kes corrected for spontaneous activity) of nearly all field P neurons
varied with RCF. RCF response functions displayed a variety of shapes,
but most functions were of low-pass characteristic or peaked at rathe
r low RCFs (<100 kHz/s). Neurons with strong responses to high RCFs th
igh-pass or nonselective RCF response function characteristics) all di
splayed spike count-SPL functions to tone burst onsets that were monot
onic or weakly nonmonotonic. RCF response functions and best RCFs ofte
n changed with SPL. For most neurons, FM directional sensitivity, quan
tified by a directional sensitivity (DS) index, also varied with RCF a
nd SPL, but the mean and width of the distribution of DS indices acros
s all neurons was independent of RCF. Analysis of response timing reve
aled that the phasic response of a neuron is triggered when the instan
taneous frequency of the sweep reaches a particular value, the effecti
ve F-i. For a given neuron, values of effective F-i were independent o
f RCF, but depended on FM direction and SPL and were associated closel
y with the boundaries of the neuron's frequency versus amplitude respo
nse area. The standard deviation (SD) of the latency of the first spik
e of the response decreased with RCF. When SD was expressed relative t
o the rate of change of stimulus frequency, the resulting index of fre
quency jitter increased with RCF and did so rather uniformly in all ne
urons and largely independent of SPL. These properties suggest that ma
ny FM parameters are represented by, and may be encoded in, orderly te
mporal patterns across different neurons in addition to the strength o
f responses. When compared with neurons in primary and anterior audito
ry fields, field P neurons respond better to relatively slow FMs. Toge
ther with previous studies of responses to modulations of amplitude, s
uch as tone onsets, our findings suggest more generally that field P m
ay be best suited for processing signals that vary relatively slowly o
ver time.