Abstract
BackgroundAnimals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed. Learning with action-outcome delays is a complex problem, and little is known of the neural mechanisms that bridge such delays. When outcomes are delayed, they may be attributed to (or associated with) the action that caused them, or mistakenly attributed to other stimuli, such as the environmental context. Consequently, animals that are poor at forming context-outcome associations might learn action-outcome associations better with delayed reinforcement than normal animals. The hippocampus contributes to the representation of environmental context, being required for aspects of contextual conditioning. We therefore hypothesized that animals with hippocampal lesions would be better than normal animals at learning to act on the basis of delayed reinforcement. We tested the ability of hippocampal-lesioned rats to learn a free-operant instrumental response using delayed reinforcement, and what is potentially a related ability – the ability to exhibit self-controlled choice, or to sacrifice an immediate, small reward in order to obtain a delayed but larger reward.ResultsRats with sham or excitotoxic hippocampal lesions acquired an instrumental response with different delays (0, 10, or 20 s) between the response and reinforcer delivery. These delays retarded learning in normal rats. Hippocampal-lesioned rats responded slightly less than sham-operated controls in the absence of delays, but they became better at learning (relative to shams) as the delays increased; delays impaired learning less in hippocampal-lesioned rats than in shams. In contrast, lesioned rats exhibited impulsive choice, preferring an immediate, small reward to a delayed, larger reward, even though they preferred the large reward when it was not delayed.ConclusionThese results support the view that the hippocampus hinders action-outcome learning with delayed outcomes, perhaps because it promotes the formation of context-outcome associations instead. However, although lesioned rats were better at learning with delayed reinforcement, they were worse at choosing it, suggesting that self-controlled choice and learning with delayed reinforcement tax different psychological processes.
Highlights
Animals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed
To investigate whether the hippocampus contributes to learning with delayed reinforcement, we examined the ability of rats with excitotoxic lesions of the hippocampus to acquire instrumental responding with delayed reward, comparing them to sham-operated controls
We report that H lesions severely impaired rats' ability to choose the larger reward when it was delayed, but not when the delay preceding delivery of the large reward was removed, demonstrating that hippocampal lesions induce impulsive choice
Summary
Animals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed. Animals that are poor at forming context-outcome associations might learn action-outcome associations better with delayed reinforcement than normal animals. We tested the ability of hippocampal-lesioned rats to learn a free-operant instrumental response using delayed reinforcement, and what is potentially a related ability – the ability to exhibit self-controlled choice, or to sacrifice an immediate, small reward in order to obtain a delayed but larger reward. When an animal's own actions cause (and predict) some outcome, animals learn this relationship (an aspect of instrumental or operant conditioning). When animals act to obtain reinforcement, the final outcomes do not always follow the actions immediately; animals must learn instrumental action-outcome contingencies using delayed reinforcement. Individual variation in the ability to use delayed reinforcement may determine one aspect of impulsivity: an animal able to forgo short-term poor rewards in order to obtain delayed but better rewards may be termed self-controlled, whereas an animal that cannot tolerate delays to reward may be said to exhibit impulsive choice [12,13,14,15]
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