Evoked potential correlates of early conditioning in the rat ontogeny.

In rats aged 2-8 weeks cortical EP to CS (20 flash - tone combinations, 0,9/sec, reinforced since the 10th application by electric shocks to the hind leg) were studied within different kinds of behavioral responses during avoidance learning and extinguishing. In contrast to our results in freely moving rats no developmental trend was found in this kind of avoidance (lifting of the hind leg). Average EP within reinforced trials (with escape or no reactions) differed in isolated application of CS from those when both CS and US were acting together. In younger animals the EP to CS combined with US were characterized by an evident late negative wave which shifted later (5-6 weeks toward the early negative complex. The EP changes in the auditory cortex were more pronounced, whereas visual EP with CS-US combination were rather decreased. In the youngest animals (2 weeks) the auditory EP within trials with avoidance were characterized by a distinct short latency deflection of the first positive wave, whereas in EP to extinguished CS the second deflection of the first positive wave prevailed. Also in these phenomena, the typical changes were clearly revealed in the auditory cortex. At later developmental stages (starting the 3rd, more prominently the 4th and 5th week) the wave following primary positive - negative complex was shifted toward the negativity if the animal responded by an avoidance; on the contrary an ample positive, often a double-peak wave arose if the response was extinguished. The stimulus and reaction dependence in the cortical EP showed the role of not yet fully mature cerebral cortex in avoidance learning. Both, fast as well as with some delay running processes participated in the observed phenomena during the ontogenetical development.