First, let’s ask where dopamine comes from and what it does. The great majority of dopamine in the brain is produced in two small nuclei deep in the middle of the brain: the substantia nigra (specifically, a portion of this area called pars compacta) and the ventral tegmental area (see Figure 2.510). These neurons send projections to much of the brain, but the projections to the basal ganglia are especially strong. The number of dopamine neurons in the brain is tiny—about 600,000 in humans11—which belies their outsized effect on nearly every aspect of our thought and behavior. Dopamine is a
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