Reaction to omission of reward in an operant conditioning situation in adult rats after undernutrition early in life

Rats were undernourished during the suckling and early post-weaning periods and were then fed a good quality diet ad lib from 43 to 100 days, prior to behavioural testing. Male, previously undernourished and control rats were exposed to a number of operant conditioning tasks involving partial or total omission of a food reward. In the first condition each lever press response was rewarded for five sessions, and then two sessions were given in which food was discontinued. Both groups showed an initial increase in response rate upon omission of reward, but did not behave differently. Following this, subjects were trained for 38 sessions on a fixed-interval 60 sec schedule. Under this schedule no differences were found between the groups in either post-reinforcement pause or response rate. After this, subjects received 10 sessions in which half the rewards at the end of the 60 sec intervals were randomly replaced by brief blackouts. Both groups of subjects paused for longer after food than after blackout but differed in that response rates of previously undernourished rats after both food and blackout increased markedly over sessions whereas those of the controls did not. Finally food was discontinued and both groups showed a rapid drop in response rate, as well as spontaneous recovery effects. The above results, in conjunction with those from other studies indicate that reward omission per se dose not produce higher than normal response rates in previously undernourished rats, but that unpredictable reward omission may do so.