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The responses of 288 neurons with background activity in the first auditory cortex were investigated using glass microelectrodes in chronic experiments on cats at rest (123 neurons) and during elaboration of a defensive conditioned reflex to sound stimulation (165 neurons). At rest 43% of recorded neurons did not respond to the sound stimulus. Pronounced inhibition of background activity prevailed among responding neurons (about 60%). The conditioning caused an increase (to 72%) in proportion of neurons responsive to the sound stimulus, appearance of tonic reactions, as well as an increase in frequency of bursts and in proportion of activation type responses. It is suggested that these changes indicate the enhancement of neurons excitability in the zone under study during conditioning.

Using glass microelectrodes, the impulse activity of auditory cortex neurons was studied in chronic experiments on cats. It was shown that an increase in frequency of the background activity occurring in many neurons during conditioning was often observed prior to the appearance of conditioned motor reactions. Comparison of mean frequencies of the background activity and corresponding evoked activity shows, on the one hand, the nonlinear dependence between them and, on the other hand, the prevalence of activation reactions as compared with inhibitory ones. Some levels of background activity were distinguished corresponding to mean evoked activity frequency ranges. Background activity analysis performed prior to the presentation of the conditioned stimulus and after its cessation (aftereffect) shows that at stable conditioned reflex mean frequencies in these periods are close to each other.

In chronic experiments on cats the neuronal responses (recording by means of glass microelectrodes) of auditory cortex I were studied during elaboration of a defensive conditioned reflex to a sound stimulus. During conditioning the biphasic reactions with a rather short-latent (50-100 ms) and more long-latent (400-500 and 800-900 ms) activation were predominant. The neighbouring neurons whose responses were recorded by the same microelectrode were involved into the activity more intensively. A differentiating stimulus in 70% of applications caused distinct changes in neuronal activity (predominantly of activation type). Analysis of the dynamics of responses of the same neuron during a great deal of combinations and its comparison with the formation of a motor reaction showed no direct correlation between the two events.

The results of statistical computer analysis of 366 realizations of the background activity of 181 neurons from the primary auditory cortex (field 50) in vigil cats (14) at rest and during defensive conditioning are presented. In both states the parameters of background activity in most neurons differed from completely random succession. The conditioning evoked a stable elevation of the frequency of background activity during interstimulus intervals, an increase in stability of the background impulsation as regards its average frequency and variation coefficients of the duration of interspike intervals, as well as an enhancement of the number of neurons showing different types of association between interspike intervals.