Tree representation of the devaluation experiment with two levers available concurrently.

<p>(A) In the training phase, either pressing lever one or pressing lever two , if followed by entering the magazine , results in acquiring one unit of either of the two rewards, or , respectively. The reinforcing value of the two rewards is equal to one. Other action sequences lead to no reward. As in the task of <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002055#pcbi-1002055-g002" target="_blank">Figure 2</a> , this task is also assumed to be cyclic. (B) In the devaluation phase, the outcome of one of the responses () is devalued (), whereas the rewarding value of the outcome of the other response () has remained unchanged. After the devaluation phase, the animal's behaviour is tested in extinction (for space consideration, this phase is not illustrated). Similar to the task of <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002055#pcbi-1002055-g002" target="_blank">Figure 2</a> , neither nor is delivered to the animal in the test phase.