The brain's striped striatum lights up for surprise, not just reward
Deep beneath the cortex sits the striatum, named for its stripes of grey and white matter. It helps turn intentions into movement and cues into cravings, and brain scans show it reacting not only to rewards but to anything novel, unexpected, intense or even unpleasant. Its common thread appears to be salience.
The striatum is the largest component of the basal ganglia, a set of structures below the cortex, and acts as their main entrance. Its name refers to the striped look created where a band of white matter, the internal capsule, runs between two grey masses, the caudate nucleus and the putamen. Together those form the dorsal striatum. Below lies the ventral striatum, built from the nucleus accumbens and the olfactory tubercle, which curiously receives input from the smell centres yet has no demonstrated role in smelling.
The two halves divide the labour. The dorsal part handles movement, habits built from stimulus and response, and executive tasks such as holding back impulses; its inner zone takes signals from frontal and parietal cortex, its outer zone from sensorimotor cortex, and each plays a different part in learning skills. The ventral part, tied to the limbic system, specialises in reward, reinforcement and motivation. There is overlap: the dorsal striatum also helps encode new movements aimed at future rewards, such as reaching when a cue appears.
About 95 percent of striatal neurons are medium spiny neurons, which inhibit their targets using the messenger GABA. They come in two broad flavours, carrying D1-type or D2-type dopamine receptors, though roughly 40 percent express both. Excitatory glutamate inputs pour in from the cortex, with the densest projection from cortical layer V, while dopamine arrives from the substantia nigra and, via the mesolimbic pathway, from the ventral tegmental area.
When the dopamine supply to the dorsal striatum fails, the result is Parkinson's disease, and shrinkage of the striatum features in Huntington's disease and other movement disorders. After an ischaemic stroke, immature neurons that would normally travel from a nearby ventricle toward the olfactory bulb are diverted into the damaged striatum, though few of them survive. Researchers are also exploring a role for frontal-striatal circuits in language and in adolescent behaviour.
Source: Striatum
