Figure 1. Organization of intrinsic connections within the basal ganglia.

a ∣ Model based on the influential proposal by Albin and colleagues³, according to which the output of the basal ganglia is determined by the balance between the direct pathway — which involves direct striatonigral inhibitory connections that promote behaviour — and the indirect pathway — which involves relays in the external globus pallidus (GPe) and subthalamic nucleus (STN), and suppresses behaviour. The balance between these two projections was thought to be regulated by afferent dopaminergic signals from the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) acting on differentially distributed D1 and D2 dopamine receptors. b ∣ Recent anatomical investigations have revealed a rather more complex organization in which the transformations that are applied to the inputs to generate outputs are less easy to predict. GPe, globus pallidus pars externa; GPi, globus pallidus pars interna; SNr, substantia nigra pars reticulata. Figure is modified from REF. 10 © P. Redgrave (2007).