Hippocampal damage does not impair instrumental appetitive conditioning with delayed reinforcement

Feedback timing modulates brain systems for learning in humans

The ability to learn from the consequences of actions--no matter when those consequences take place--is central to adaptive behavior. Despite major advances in understanding how immediate feedback drives learning, it remains unknown precisely how the brain learns from delayed feedback. Here, we present converging evidence from neuropsychology and neuroimaging for distinct roles for the striatum and the hippocampus in learning, depending on whether feedback is immediate or delayed. We show that individuals with striatal dysfunction due to Parkinson's disease are impaired at learning when feedback is immediate, but not when feedback is delayed by a few seconds. Using functional imaging (fMRI) combined with computational model-derived analyses, we further demonstrate that healthy individuals show activation in the striatum during learning from immediate feedback and activation in the hippocampus during learning from delayed feedback. Additionally, later episodic memory for delayed feedback events was enhanced, suggesting that engaging distinct neural systems during learning had consequences for the representation of what was learned. Together, these findings provide direct evidence from humans that striatal systems are necessary for learning from immediate feedback and that delaying feedback leads to a shift in learning from the striatum to the hippocampus. The results provide a link between learning impairments in Parkinson's disease and evidence from single-unit recordings demonstrating that the timing of reinforcement modulates activity of midbrain dopamine neurons. Collectively, these findings indicate that relatively small changes in the circumstances under which information is learned can shift learning from one brain system to another.

Hippocampal lesions facilitate instrumental learning with delayed reinforcement but induce impulsive choice in rats

BACKGROUND

Animals 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.

RESULTS

Rats 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.

CONCLUSION

These 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.

Effects of fimbria-fornix lesions on avoidance tasks with temporal elements in rats

Sidman schedule active avoidance, passive avoidance, and multiple avoidance (continuous alternation of active and passive avoidance) tasks were given to fimbria-fornix (FF)-lesioned (n = 10) and control (n = 10) rats to assess the effects of fimbria-fornix lesions on unsignaled avoidance learning with temporal cognition requirement. Active avoidance required subjects to make running responses, and passive avoidance required them to stop running and remain immobile on a running wheel. The tasks could be achieved purely by temporal cues, and no spatial elements were involved. Animals in the FF group performed the active, passive, and multiple avoidance tasks as well as control animals, showing no deficits by lesions in either the number of running responses nor the number of shocks received, although animals in the FF group displayed a greater negative transfer in passive avoidance when they received the active training before the passive training. The results indicate that fimbria-fornix lesions do not impair avoidance tasks when the tasks do not require spatial information, even if temporal information and/or inhibition are necessary to perform the tasks.