Discharge patterns of the primary auditory cortex in cats

Single unit responses of the primary auditory cortex of unanesthetized cats were studied using three standard sets of sound stimuli, i.e., clicks, white noise bursts, and pure tone bursts. The units studied were classified into eight categories according to whether they responded to all three, two, one or none of the standard sets of stimuli; there were no units responding only to clicks. This suggests that excitation of some cortical neurons requires a special sound spectrum as well as temporal factors. Frequency-modulated (FM) sounds were usually more effective eliciting responses than the standard sets of stimuli. Responses to FM in burst seemed to be classified into three groups, i.e., sharp monopeak post-stimulus time (PST) histograms, broad monopeak, and sharp multipeaks. The existence of the last group suggests a firm convergence from the lower auditory neurons. In some units, complex stimuli were more effective in evoking responses than simple FM sounds. These units responded only to sounds with complex temporal patterns. For example, a neuron did not respond to any FM sound with a constant modulation magnitude, but showed a transient response whenever the magnitude of FM varied from a continuous pure tone to a certain range of FM. These complex units often showed a response with hysteresis.