Roles of the thalamic CM–PF complex—Basal ganglia circuit in externally driven rebias of action

The centromedian (CM)–parafascicular (PF) nuclear complex in the primate thalamus has reciprocal and specific connections with the basal ganglia. It has been argued that the thalamic CM–PF complex has a role in pain processing and attention. However, the functional relationship of this complex with the basal ganglia, which is considered to have a role in goal-directed movement, has not been well characterized. Here we present a hypothetical view that the thalamic CM–PF complex–basal ganglia circuit plays complementary roles in response bias. The basal ganglia are involved in creating ‘reward-based pre-action bias’, which facilitates the selection and execution of an action associated with a higher value. In contrast, when an action with a lower value is unexpectedly requested, the CM–PF induces an ‘externally driven rebiasing’ process in the striatum that aborts the pre-action bias and assists selecting and executing actions appropriate for unexpected situations. This model provides a framework for how the thalamic CM–PF complex and the basal ganglia function together in general for unexpected situations.