The frontal cortex receives the largest portion of BG outputs (via the thalamus), suggesting a close collaboration between the BG and frontal cortex.23,24 DA acts not only on the PFC but
also on the BG. Importantly, the DA projections into the striatum (the input to the BG where cortical information converges) are much heavier than those to the PFC.25 Thus, DA teaching signals may play a stronger role in gating plasticity in the striatum in contrast to the PFC, where DA influence may be more subtle—shading, not gating, plasticity. This may explain our observation that during operant learning, learning-related changes in the striatum appear sooner and faster than those in the PFC.26 Thus, the trade-off Inhibitors,research,lifescience,medical between the advantages of slow plasticity versus fast plasticity may play out in interactions between the PFC and BG.27 The idea is that during learning, specific associations between cues and immediate actions are quickly acquired by the striatum, by virtue of its heavy inputs from midbrain DA neurons. The output of the basal ganglia trains the PFC26 where plasticity is “slower” (smaller changes Inhibitors,research,lifescience,medical in synaptic weights with each learning episode) because of the weaker DA influence. As a result, the PFC gradually builds up less error-prone, more elaborate, and generalized representations Inhibitors,research,lifescience,medical based on the patterns fed
to it by the BG. This may explain why the PFC and BG seem to operate based on different types of representational schemes.28 The fast striatal plasticity may be more suited for Inhibitors,research,lifescience,medical a quick stamping-in
of immediate, direct associations. But, as a consequence, the striatum learns complex behaviors in a piecemeal fashion, as a set of largely unconnected (cache) representations of which alternative was more successful at each decision Inhibitors,research,lifescience,medical point in the task.28 By contrast, the slow PFC plasticity may be suitable for building elaborate rule representations that gradually link in more information (ie, tree-like representations).28 The slow PFC plasticity may also find the commonalities and regularities among the simpler representations acquired by the striatum that are the basis for abstractions and general principles.29 In other words, the striatum learns the pieces of the Doramapimod puzzle while the PFC puts the puzzle together. 27 We recently witnessed this hand-off from the striatum to PFC as animals transitioned Thiamine-diphosphate kinase from simple, specific learning to more generalized, abstract representation.30 Each day, monkeys learned to associate two novel, abstract, dot-based categories with a right vs left saccade (Figure 3). At first, monkeys only saw a few examples of each category, and they could learn specific stimulus-response (S-R) associations by rote. But as more and more new category exemplars were added, the capacity to memorize specific S-R associations was exceeded. To solve the task, the monkeys then had to learn the categories, and extract the common essence that united exemplars from the same group.