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The analysis of the reflex conditioned activity of rats at various age groups showed that there were differences in it. There was a considerable strength of the excitatory process in young rats, aged one and a half months. The positive reflexes to sound and light stimuli were formed compatively quickly (on the average after 8,6 and 4,8 combinations of the conditioned stimulus with the unconditioned stimulus) in comparison with the animals of mature age (respectively 16,5 and 8 combinations). In the old rats the author observed a lowering of the strength of the excitatory process and slower formation and stabilization of the positive conditioned reflexes. A gradual formation of the inhibitory function occurs during the process of ontogenetic development and this finds an expression in the unpossibility for the fromation of differentiation in young animals and the presence of such in 10-month old rats. The level of the differentiations in these animals reached 60,1% and 76,9%. The latent period changed with the advancement of age. In adult animals there was some shortenint ofthelatent period in comparison with the young animals although the difference was statistically insignificant. There was an increase of the absolute values of the latent period again after the occurrence of the old age (5,2'' and 4,52''). The indicated peculiarities in the formation and course of the conditioned reflexes probably were connected with various degree in the fluctuations of the strength of the exitiatory and inhibitory processes in the central nervous system during ontogenesis.
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Alimentary conditioned reflexes were studied in dogs during electrical stimulation of different zones of the caudate nucleus head with a 80 to 100 microampere current, of 5 c/s frequency and 0.5 msec pulse duration. The inhibitory influences came from the ventrolateral, and the activating ones, from the dorsal and central zones of the caudate nucleus head. Changes of conditioned reflexes resulting from its stimulation depended on the strength of the conditioned signals: the conditioned reflex to a weak stimulus (light) was more readily subject to changes as compared with the one to a strong signal (a metronome). The magnitude of unconditioned reflexes did not change in these experiments.
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The monkeys were trained to draw a lever by the left or right hand. Cortical motor evoked potentials (MEP) exhibited the highest amplitude in the cortical motor representation of the "working" hand during the period of learning. The extent of interhemispheral asymmetry of the MEP was the same during operation by the left and right hands. After formation of the conditioned reflex, the interhemispheral asymmetry was significantly less during manipulation by the left hand than by the right one. The decrease of asymmetry was due to relatively high amplitude of the MEP in the left hemisphere during instrumental manipulation of both the right and left hands. The data obtained indicate the formation of functional differences in symmetrical motor areas of the cortex in monkeys during their symmetrical instrumental conditioning by the left and right extremities.
Conditioned reflexes to different time intervals (from 0.25 to 90 min) were elaborated in Wistar line rats by motor food-procuring method. A conditioned reflex to time is formed easier and attains highest precision to intervals from 1 to 4 min. With shorter (0.5 to 0.75 min) and longer (6 to 10 min) intervals, the reflex to time is unstable and not precise. To intervals beyond the 0.5 to 10 min range the conditioned reflex to time could not be formed by the above method. Experimental data are corroborated by theoretical estimations of the shortest and longest intervals to which it is possible to elaborate a conditioned reflex to time by trace reflex mechanism. The data obtained are considered as a confirmation of Pavlov's hypothesis on the physiological mechanisms of the process of formation of a short-interval conditioned reflex to time.
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