Figure 2. Corticobasal ganglia–cortical loops in animals and humans.
a ∣ The connections between the cerebral cortex and the basal ganglia can be viewed as a series of parallel-projecting, largely segregated loops or channels29 conveying limbic (shown in red), associative (shown in yellow–green) and sensorimotor (shown in blue–white) information. Functional territories represented at the level of cerebral cortex are maintained throughout the basal ganglia nuclei and thalamic relays. Note, however, that for each loop the relay points in the cortex, basal ganglia and thalamus, offer opportunities for activity inside the loop to be modified or modulated by signals from outside the loop. Black arrows indicate glutamatergic projections, grey arrows indicate GABA (γ-aminobutyric acid)-ergic projections. b ∣ The spatially segregated ‘rostral caudal gradient’ of human frontal cortical connectivity in caudate, putamen and pallidum. The colour-coded ring denotes limbic (shown in red), associative (shown in yellow–green) and sensorimotor regions of the cerebral cortex in the sagittal plane (colour coding after Haber173). Using probabilistic tractography on magnetic resonance-diffusion weighted imaging data, Draganski et al. identified the regions of the striatum that receive the strongest input from the identified cortical regions (sagittal oriented images contained within the ring)37. For better visualization, segmented basal ganglia nuclei are superimposed on T1-weighted structural images in the far right of the diagram (the rostral part is shown on the left, the caudal part is shown on the right). PFC, prefrontal cortex. Part a is modified, with permission, from REF. 174 © (1995) MIT Press. Part b is reproduced, with permission, from REF. 37 © (2008) Society for Neuroscience.