  For decades the external globus pallidus (GPe) has been viewed as a passive
way-station in the indirect pathway of the cortico-basal ganglia-thalamic
(CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs.
According to this model, one-way descending striatal signals in the indirect
pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe
activity. Here we revisit this assumption, in light of new and emerging work on
the cellular complexity, connectivity, and functional role of the GPe in
behavior. We show how, according to this new circuit-level logic, the GPe is
ideally positioned for relaying ascending and descending control signals within
the basal ganglia. Focusing on the problem of inhibitory control, we illustrate
how this bidirectional flow of information allows for the integration of
reactive and proactive control mechanisms during action selection. Taken
together, this new evidence points to the GPe as being a central hub in the
CBGT circuit, participating in bidirectional information flow and linking
multifaceted control signals to regulate behavior.
