The Evolution of Motivation

At right you see, in animated sequence, the three main functional subdivisions of the brain.

First, the reptilian brain is shown, along with the top of the spinal cord and the cerebellum (the large bulge at top right of the structure). This region is responsible for our deepest instincts and behaviors for self-preservation and reproduction.

• At its top, from left to right, you'll see the pituitary gland (dangling down toward the left), the hypothalamus (looking something like a bird's beak pointed leftward), the tegmentum (the "dome" in the center), and the tectum (hooking upward and to the right). In general, this level is called the "midbrain."
• At the next level, just opposite the cerebellum, are the pons (left), and the medulla oblongata. The reticular activating system is not readily distinguishable in this image, existing to the right and left of the midline of the brain, at which this section (called a "saggital section") is taken, but it is hugely important -- filtering incoming sensory data for significance and directing our attention to the important stuff. The general name for this region is the "brain stem" or "hindbrain."

The second subdivision, sitting like a cap atop the reptilian brain, is the limbic system, also called the mammalian brain or the paleomammalian cortex. This region is responsible for our emotions, our social behaviors, and other "species-typical" behavior.

• The roughly oval structure straddling the tegmentum and tectum is the thalamus, which is the brain's "Grand Central Station." Through it passes data from all the senses, along with information sent from higher brain structures back out to the periphery. The tiny circle toward its front is the massa intermedia, a bundle of nerve fibers connecting the left and right lobes of the thalamus, which are barely separated by the cerebrospinal fluid of the third ventricle.
• The dark scythe in the middle of the limbic system is the corpus callosum, a much larger bundle of fibers connecting the left and right hemispheres of the neocortex.
• Wrapping around the top of the limbic system is the cyngulate gyrus, part of the limbic cortex, which is devoted to making associations between the limbic system and the surrounding areas of the neocortex.
• Most of the interesting structures of the limbic system are invisible here, for they exist to the right and left of the midline of the brain. These include the hippocampus, important in learning and memory, the amygdala, which apparently generates strong emotions, and the fornix, a bundle of nerve fibers that separates like a wishbone in front of the thalamus, then arches up and around behind it, along the hippocampus on each side, and to the amygdala at the front tip of the hippocampus.

Finally, wrapped around the rest at the very top is the neocortex. This is the part of the brain that humans tend to regard as bringing forth our intelligence (which it does) and other "uniquely human" qualities (which I argue are generic primate qualities instead, produced with much influence from the limbic system). The front portion is specialized for planning movements, working with concepts in various ways, learning and using language, and imagining relations among objects in space -- among many other things. At the top, it produces commands for muscular activity, and takes in sensory information. At the back, it is specialized for visual processing. If you could see the neocortex as it wraps around the left and right sides of the head, you'd see the parietal cortex, which makes sensory associations, and the temporal cortex, which governs our hearing and plays a role in memory.

This very general introduction to the human brain has made use of a number of gross simplifications. Any good textbook on neuroanatomy can provide better detail.