Early life undernutrition and operant responding in the rat: The effect of the reinforcement schedule employed

Abstract

Rats were either undernourished from birth to 43 days and thereafter well fed (previously undernourished, PU) or well nourished throughout. When behaviour was tested in adulthood it was found that significant differences between the groups in rate of lever-pressing for food occurred when they were tested under a variable-interval 60-sec schedule of reinforcement, but not when reward was delivered according to a fixed-interval 60-sec or variable-ratio ten schedule. The results of a second experiment suggested that the rate difference might reduce or disappear with extended exposure to the schedule. The third experiment exposed rats to fixed-interval 60-sec and mixed fixed-interval 10-sec fixed-interval 110 sec schedules. Response rate differences between the PU and control groups occurred only under the mixed schedule, a result interpreted as showing that temporal irregularity of reward delivery plays some role in the genesis of more rapid operant responding in PU rats. When rats received a larger variable-ratio schedule, requiring 40 responses for reward, no significant rate differences between the groups were found over the whole experimental condition. It is suggested that schedules on which there are significant differences have some special characteristic, possibly sensitivity to differences in food motivation between the groups.