DRL performance in the weanling rat: a comparison with adult subjects

Weanling (21-day) and adult (3-month-old) albino rats were reinforced for lever pressing on a Differential Reinforcement of Low rates 20 seconds schedule. After 2 sessions at DRL 5, 10, 15 and 20 seconds, 40 DRL 20 seconds sessions were performed, at a pace of 4 sessions a day. Adult rats emit lower response rates, higher median Inter-Response-Times (IRTs) and obtain more reinforcers than weanling rats. Furthermore, their coefficients of variation (ratio between semi-interquartile range and median of the IRT distribution) are significantly lower than those of the young subjects. Age-related differences are discussed in relationship with general activity, food motivation and timing mechanisms. It is suggested that weanling and adult timing devices do not basically differ, but that age-related differences exist at the level of the translation of time estimates into overt behavior.