example, to doze off undisturbed by sounds of nearby conversation and steady jet engines, but to awake and become alert when the pitch of the engines changes and the plane tilts into its descent.

The limbic system (from the Latin limbus, for “hem” or “border”) is another assembly of linked structures that form a loose circuit throughout the brain. This system is a fairly old part of the brain and one that humans share with many other vertebrates; in reptiles, it is known as the rhinencephalon, or “smellbrain,” because it reacts primarily to signals of odor. In humans, of course, the stimuli that can affect the emotional brain are just about limitless in their variety.

The limbic system is responsible for most of the basic drives and emotions and the associated involuntary behavior that are important for an animal's survival: pain and pleasure, fear, anger, sexual feelings, and even docility and affection. As with the rhinencephalon, the sense of smell is a powerful factor. Nerves from the olfactory bulb, by which all odor is perceived, track directly into the limbic system at several points and are then connected through it to other parts of the brain; hence the ability of pheromones, and perhaps of other odors as well, to influence behavior in quite complex ways without necessarily reaching our conscious awareness.

Also feeding into the limbic system are the thalamus and hypothalamus, as well as the amygdala, a small, almond-shaped complex of nerve cells that receive input from both the olfactory system and the cerebral cortex. These connections are illustrated in an unusual way in the context of epilepsy. Perhaps because the amygdala is located near a common site of origin of epileptic seizures—that is, in the temporal lobe of the cerebral hemispheres—epileptics sometimes experience unidentifiable or unpleasant odors or changes of mood as part of the aura preceding a seizure. The limbic system is not thought to be involved in the causes of epilepsy, but it is indirectly stimulated by the electric discharge in the brain that sets off a seizure and gives evidence of the stimulation in its own characteristic ways.