Some features of learning in the Morris water test in rats selected for responses to humans

The characteristics of learning in the Morris water test were studied in gray rats subjected to prolonged selection for elimination (the tame strain) and enhanced (the aggressive strain) aggressivity towards humans. Blood corticosterone levels at different stages of learning were also estimated. Tame rats learned to locate the invisible platform better than aggressive rats. The time spent seeking the platform by aggressive rats increased because they spent more time at the periphery of the basin. The duration of vertical investigative activity while on the platform was greater in tame rats than in aggressive rats. Fixation of the memory trace was demonstrated by the observation that rats of both strains spent more time in the sector in which the platform had been located during the training period. Rats of the two strains showed essentially no difference in terms of the time spent seeking the platform when it was placed in the opposite sector. After one day of training, blood corticosterone was significantly lower in tame than in aggressive rats. On subsequent training days, hormone levels in tame animals increased and were no different from those in aggressive rats. It is suggested that decreased emotionality and stress reactivity facilitated the learning process in tame rats in the Morris water test.