Figure 1.

Architecture of the basal ganglia model. The main circuit (centre) can be decomposed into two copies of an off-centre, on-surround network: a selection pathway (right) and a control pathway (left). Three parallel loops – channels – are shown in both pathways, with example activity levels in the bar charts to illustrate the relative contributions of the nuclei (the three channels are colour-coded black/grey/white, corresponding to the example bar charts). Note that, for clarity, full connectivity is only shown for the second channel. Briefly, the selection mechanism works as follows. Constant inhibitory output from substantia nigra pars reticulata (SNr) provides an “off” signal to its widespread targets in the thalamus and brainstem. Cortical inputs representing competing saliences are organised in separate channels (groups of co-active cortical neurons), which project to corresponding populations in striatum and STN. In the selection circuit, the balance of focussed (one-to-one) inhibition from striatum and diffuse (one-to-many) excitation from STN results in the most salient input suppressing the inhibitory output from SNr on that channel, signalling “on” to that SNr channel's targets. In the control circuit, a similar overlap of projections to GP exists, but the feedback from GP to the STN acts as a self-regulating mechanism for the activity in STN, which ensures that overall basal ganglia activity remains within operational limits as more and more channels become active. For quantitative demonstrations of this model, see Gurney et al. (2001b, 2004) and Humphries et al. (2006).