FUNCTIONAL SPECIALIZATION IN AUDITORY-CORTEX - RESPONSES TO FREQUENCY-MODULATED STIMULI IN THE CATS POSTERIOR AUDITORY FIELD

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.