Response timing and development: Fixed-interval performance in precociously weaned rats

Young rats weaned at 16 days were taught to press a lever by shaping at 18 days and trained for 11 days (from 20 to 30 days of age) on a fixed-interval 60-sec schedule, at a rate of 5 half-hour sessions per day. Comparison with rats weaned at 16 or 20 days, trained at 21 days, and subjected for 8 days (from 23 to 30 days of age) to the same procedure showed that the temporal regulation of behaviour, as evaluated by the Curvature Index, was mostly training-dependent. Post-reinforcement pause durations depended upon the mixed influence of age and training. The evolution of running rates mostly depended upon training, whereas trends in overall rates seemed to depend essentially upon the age of the subjects. Precocious weaning per se or raising conditions prior to weaning did not significantly influence the temporal regulation of behaviour under FI. The results support previous ideas that overall response rate and response timing were dissociated and indicate high sensitivity to periodic stimuli in weanling rats. Finally, they show that, within an information processing model of timing, FI data of weanling rats might be compatible with their having an “internal clock” that runs at a higher rate than in older subjects.