Illustration of the behavioral task.

<p>Illumination of the chamber and insertion of both levers signaled the start of the trial. Choosing the safe lever always resulted in an immediate and constant reinforcement (0.01 ml). Choosing the risky lever resulted in a more variable outcome, while maintaining the same overall payoff (e.g. 50% chance of 0.02 ml). Following each choice, both levers were retracted and the light was turned off. The next trial started 10 seconds later (inter-trial interval, ITI).