Basal ganglia · Grey Matter

The basal ganglia are a set of deep nuclei that decide which of the actions the cortex proposes is allowed to start, and that learn from reward which actions to repeat until they become habits.

Basal ganglia. The basal ganglia are a set of deep nuclei that decide which of the actions the cortex proposes is allowed to start, and that learn from reward which actions to repeat until they become habits.

Their input nucleus is the striatum (the caudate nucleus and the putamen, with the nucleus accumbens below them as the ventral striatum), which receives from almost the whole cortex. Their output nuclei (the internal globus pallidus and the substantia nigra pars reticulata) hold the thalamus under constant inhibition. A direct pathway through them releases that brake for the chosen action and an indirect pathway, through the subthalamic nucleus, strengthens it for the competitors, and the loop returns through the thalamus to the cortex. Dopamine from the substantia nigra pars compacta and the ventral tegmental area tunes the balance and marks outcomes better than expected, which is how the circuit learns.

Their failure shows in starting and stopping movements. Loss of the dopamine neurons in Parkinson's disease makes movements hard to start and small; degeneration of the striatum in Huntington's disease lets unwanted movements through.

They turn repeated actions into habits. With practice, control of a learned sequence shifts from the caudate (goal-directed) to the putamen (habitual), the core of procedural memory.

The ventral striatum is the reward hub. Dopamine release there accompanies pleasure and its anticipation, from food to the peak moments of music.

Questions: How do the basal ganglia let one movement start and hold the others back? Their output nuclei keep the motor thalamus under constant inhibition, a brake on every possible action. When the cortex proposes an action, the striatum's direct pathway inhibits the output nuclei for that action alone, releasing the brake so the thalamus and motor cortex can run it, while the indirect pathway reinforces the brake on the competing actions. Dopamine favours the direct pathway, so when the dopamine neurons die in Parkinson's disease the brake dominates and movements become hard to start, slow and small. How does a deliberate action become an automatic habit? At first an action is goal-directed: it depends on the prefrontal cortex and the caudate (the associative striatum), and it changes as soon as its outcome stops being wanted. With repetition, and with dopamine marking each success, control shifts toward the putamen (the sensorimotor striatum), which chunks the sequence into a single unit triggered by its context. The habit then runs with little attention and keeps running even when its reward has lost value, which makes habits efficient and also hard to break. What happens in the brain when you get a joke and laugh? Getting the joke is detecting an incongruity and resolving it, and imaging links that step to the temporo-occipital-parietal junction and the temporal lobe, with the prefrontal cortex helping to reinterpret the setup. Enjoying it engages the reward network: the amygdala, the ventral striatum with the nucleus accumbens, and the midbrain dopamine regions. Laughter itself is a motor act driven partly from the brainstem and the motor areas, which is why it can be hard to suppress once it starts. How does a piece of music produce chills of pleasure? The sound is analysed in the auditory cortex and the temporal lobe, which track melody and harmony and build expectations about what comes next. In 2011 Salimpoor and colleagues measured dopamine release with PET while listeners heard music that gave them chills: dopamine rose in the caudate during the build-up before the peak, and in the nucleus accumbens at the peak itself. Music thus engages the same reward circuit of the striatum as food or money, driven by expectation and its resolution, and the medial prefrontal cortex tracks music tied to personal memories.