The number of trial upon the effects of US duration in conditioned licking suppression with rats

Abstract

Four groups of thirsty rats experienced Pavlovian conditioning during licking water. The experimental design was a factorial one. Two groups of rats received a short (0.7 s) shock as an unconditioned stimulus (US), while the other two groups received a longer shock (4.9 s). The one of each shock duration group experienced a pairing of conditioned stimulus (CS) and US once in a session of 5 min long, while another of each experienced three pairings in a session of 5 min long. Despite of more pairings of CS-US in the three-trial groups than in the one-trial groups, conditioned suppression was more prominent in the one-trial groups than the three-trial groups. These results suggest that many CS-US pairings did not generate greater conditioned suppression, rather deteriorated it, which was accounted by the trials spacing effect. The baseline licking levels, which were regarded as reflecting the associative strength of the context with the US, were more suppressed in the three-trial groups than the one-trial groups. The conditioned suppression of the three-trial groups did not differ from each other, which is consistent with previous studies of US duration effect; Differences in US duration did not affect the level of conditioning under a between-subjects design. In contrast, the long US generated the greater conditioned suppression to the CS than the short US in the one-trial groups. These results suggest that the effects of US duration may be manifested under the condition, in which the associative value of the context is fairly weak.