Responses of single neurons in posterior field of cat auditory cortex to tonal stimulation

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Abstract
In the auditory cortex of barbiturate-anesthetized cats, the posterior auditory field (field P) was identified by its tonotropic organization. Single neurons in that field were studied quantitatively for their sensitivity to the frequency and intensity of tonal stimuli presented via calibrated, sealed stimulating systems. Field P neurons had narrow, V-shaped, threshold frequency tuning curves. At suprathreshold levels, spike counts were generally greatest at frequencies at or close to the neuron''s threshold best frequency (BF). Of posterior-field neurons, 86% displayed spike counts that were a nonmonotonic function of the intensity of a BF tone. Of these, > 90% showed at least a 50% reduction in spike count at high stimulus levels, and .apprx. 20% of nonmonotonic cells ceased responding entirely at high stimulus intensities. The nonmonotonic shape of spike count vs. intensity profiles was typically preserved across the range of frequencies to which any given neuron was responsive. For some neurons, this had the consequence of generating a completely circumscribed frequency-intensity response area. These neurons responded to a tonal stimulus only if the stimulus was within a restricted range of both frequency and intensity. These response areas showed internal organizations that reflect 1 or both of 2 processes. For some neurons, the optimal sound pressure level for spike counts varied with tone frequency, roughly paralleling the threshold tuning curve. For other neurons, the optimal sound pressure level tended to be constant across frequency despite threshold variations of up to 20 dB. The minimum response latencies of posterior-field neurons were generally 20-50 ms, while cells in the primary auditory cortex (AI) in the same animals generally had minimum latent periods of < 20 ms. The cat''s auditory cortex might show an interfield segregation of neurons according to their coding properties.