Rereading The Hungry Brain, I notice my review missed one of my favorite parts: the description of the motivational system. It starts with studies of lampreys, horrible little primitive parasitic fish:

How does the lamprey decide what to do? Within the lamprey basal ganglia lies a key structure called the striatum, which is the portion of the basal ganglia that receives most of the incoming signals from other parts of the brain. The striatum receives "bids" from other brain regions, each of which represents a specific action. A little piece of the lamprey's brain is whispering "mate" to the striatum, while another piece is shouting "flee the predator" and so on.