According to a new study by Pasupathy and Miller, the neural correlates of associative learning appear in the caudate nucleus — part of the basal ganglia — earlier than in the prefrontal cortex (PFC). The findings shed new light on the interactions and relationships between the basal ganglia and cortex.

The authors used a simple conditional association learning task to investigate the roles of the two brain areas. Monkeys had to learn to make a leftward eye movement in response to one visual cue, and a rightward eye movement in response to another. Correct responses were rewarded with fruit juice, and once a pair of cues had been learned the required responses were reversed so that the monkeys had to learn the new associations. New cues were used each day, and Pasupathy and Miller recorded the activity of neurons in both the dorsolateral PFC and the caudate nucleus during the task.

Both the dorsolateral PFC and the caudate nucleus are known to show task-related activity in this sort of task, and they are joined as part of a corticostriatal loop, but until now the temporal relationship between their activity has not been investigated. The authors found that neurons in both areas showed changes in activity during associative learning, but that the neurons in the caudate nucleus showed these changes much earlier, and more abruptly, than did neurons in the dorsolateral PFC.

These results might support the idea that the basal ganglia can learn new associations rapidly, and then 'train' the PFC. Interestingly, the monkeys' behaviour showed slower learning of the new cue–response associations than did the activity of caudate neurons. Whereas caudate activity showed a sudden switch to the correct association, the monkeys' performance improved more gradually, and was correlated more strongly with the changes in activity seen in the PFC. The authors suggest that although the caudate nucleus might 'train' the PFC, behaviour depends more on PFC activity than on that of the basal ganglia.