Perhaps the simplest description of the human brain is Paul MacLean's triune brain,which distinguishes three regions: the neocortex, the limbic system, and the reptilian brain (see figure 13.5). Although it is greatly oversimplified, this description has influenced how people think about brain functions. For example, many have assumed that the physically highest level of the brain, the neocortex, dominates the other, lower levels. However, this assumption is contradicted by evidence that the physically lower limbic system can effectively hijack the brain; that is, emotions can overtake so-called higher mental functions when they need to. The limbic system - the primary seat of emotion, attention, and memory - contains such structures as the hypothalamus, hippocampus, and amygdala. It helps determine valence (e.g., whether you feel positive or negative toward something) and salience (e.g., what gets your attention); it also contributes to human flexibility, unpredictability, and creative behavior. It has vast interconnections with the neocortex, so that brain functions are not either purely limbic or purely cortical but a mixture of both.

We have all, of course, seen emotions overwhelm reason (at least in others), which is one reason why the word emotional has negative connotations. (For example, people who panic out of fear may cause more harm to themselves than if they had "kept a cool head" and made rational decisions.) Nonetheless, it is clearly beneficial for our survival that fear can hijack our brain and cause us to jump out of the way of a rapidly approaching object before we can consciously perceive and analyze that a bus is about to hit us.

These kinds of emotions, which seem to be hard-wired or innate, are sometimes called primary emotions. They include responses such as the fear example above and involuntary reactions to surprise. Other emotions, the secondary emotions, appear to develop as we mature. They connect cognitive events with lower-level physiological responses and occur as a result of joint neocortical and limbic activity. Such emotions play an especially important role in decision making, even in decision making that appears to be purely rational.

Findings on the importance of emotions for rational decision making seem paradoxical. They are based on a remarkable story told by A.R. Damasio about the patient "Elliot." Elliot, and patients like him, have a particular kind of brain damage that affects a circuit between the prefrontal cortex and the amygdala, a communication channel between the neocortex and limbic system that appears to be essential for secondary emotions. At first glance, Elliot appears to be like Star Trek's Spock - emotionally unexpressive, unusually rational. One might think that Elliot would therefore be superb at making rational decisions. However, unlike the fictitious half-human Spock, Elliot's lack of emotions severely impairs his decision-making ability and causes tragedies in his business and personal life.

Although Elliot's IQ and cognitive abilities are all normal or above average, when confronted with a simple decision, such as when to schedule an appointment, he disappears into an endless rational search of "well, this time might be good" or "maybe I will have to be on that side of town so this time would be better," and on and on. Although a certain amount of indecisiveness is normal, Elliot apparently doesn't experience the usual feelings of embarrassment when someone stares at him for taking so long to make up his mind. Nor is the indecision accompanied by the healthy limbic responses that normally associate positive or negative feelings with certain decisions, responses that help us limit a search by nudging us away from possibilities with bad associations. Instead, Elliot tends to search an astronomical space of rational possibilities and seems unable to learn the links between dangerous choices and bad feelings; so he repeatedly makes bad decisions. Elliot's lack of emotions severely handicaps his ability to function rationally and intelligently.

In other words, not only does too much emotion wreak havoc on reasoning, but also, paradoxically, too little emotion wreaks havoc on reasoning. Apparently, a balance is needed: not too much emotion, not too little emotion. Computers, except for HAL, do not have enough emotion. Artificial intelligence systems to date are not unlike Elliot: they have above-average knowledge (usually consisting of a huge set of rules) of some area of expertise, but are disastrous at making decisions. They are too rational; they cannot associate judgments of value and salience with their decisions. Little has been done to imitate these judgments, which are essentially products of the limbic system, in computers.