Summary of thalamic terminal organization in cortical layers. Projections to the deep layers may interact with neurons that, in turn, project back to both the thalamus and striatum. These terminals can directly reinforce corticothalamic and corticostriatal inputs to specific cortico-BG circuits (A). In addition, through the non-reciprocal corticothalamic projection, terminals in layer V may also interface with other cortico-BG circuits via projection to a thalamic region that is part of another circuit system (B). Thalamocortical projections to the superficial layers may have a similar dual function. These projections may interact with the apical dendrites of layer V cells, further reinforcing each parallel circuit. In addition, through corticocortical projections from layer III, these terminals may influence adjacent circuits (modified from J. Neurosci., 2002, 22:8117–8132).