Where the Brain Recognizes Risks and Possible Rewards

Circuits in the limbic region at the center of the brain beneath the cortex manage the critical task of assigning emotional importance and priorities to incoming perceptions and to internally generated thoughts and plans. These circuits instantaneously assess what may be attractive, dangerous, or rewarding.

Many think of emotion as a quality added to a perception or thought on the basis of conscious reflection—for example, "I smelled something burning and began to worry about whether there was a fire in the house." "I heard that song playing and began to feel sad; it reminded me of a girlfriend who left me." In these two examples something is noticed, thought about, and then reacted to with an emotion, in this case fear or sadness.

But the process by which the brain links perceptions or thoughts and emotions tends to be much quicker and less reflective. Usually the brain assigns emotion and thereby importance in ways that are instantaneous and automatic. Antonio Damasio described this process:

Emotion is critical for the appropriate direction of attention since it provides an automated signal about the organism's past experience with given objects and thus provides a basis for assigning or withholding attention relative to a given object . . . first, processing of objects can take place; second, emotion can ensue; third, further enhancement and focusing can occur, or not occur, under the direction of emotion. (1999, p. 273, italics added)

The brain acts most quickly on perceptions that it judges as threatening. Joseph LeDoux (1996) and Elizabeth Phelps (2005) have described how the amygdala, a tiny structure in the midbrain (see Figure 3), screens incoming perceptions for any sign of potential danger. This structure, not much larger than a walnut, receives constant input from all of the brain's lobes, and rapidly screens incoming perceptions and thoughts to determine whether they present any threat. It functions something like an automatic missile-avoidance system on a military aircraft.

It is this mechanism that causes one quickly to jump aside to avoid a car that suddenly speeds around the corner as one is crossing the street. The unexpected car's approach, caught only in the periphery of vision, is reacted to with instantaneous movement before one is even aware of being afraid. The brain reacts without delay to unexpected objects moving quickly toward us, to objects touched that are too hot, to substances that taste rotten; such self-protective reactions do not wait for conscious thought.

The brain's recognition of some situations as dangerous, to be avoided, extends far wider than these simple examples of possible threats to physical safety. From earliest infancy the brain builds up a massive file of memories that can signal possible danger, not only from speeding cars or possibly poisonous foods, but also from situations that might elicit other kinds of pain, for example, ridicule, disgust, anger, or rejection from others. The networks of the brain are such that the amygdala and its connections will react to these acquired warnings with instantaneous release of chemicals that activate avoidance responses, often without awaiting reflection. It is these chemicals, quickly attached to the perception or thought, that create the fear and mobilize appropriate action.

While some fear reactions are based on instinct, many more are based on personal memories. Contents of these files used to identify potential dangers are highly individualized, gradually accumulated from the earli est years and throughout each individual's life. In countless interactions with our physical and social world, each person acquires memories of what is considered dangerous or wrong. Interactions with parents, siblings, teachers, and friends, as well as exposure to stories and photos through various media, shape one's personal reactions about what is dangerous or wrong to look at, taste, touch, want, and do. Ongoing experiences continually update these mental files: some experiences reinforce previous understandings and others dramatically alter how a particular individual reacts to a particular perception, thought, or action.

On the basis of such files, the brain reacts automatically to some incoming perceptions with instantaneous signals that register discomfort ranging from mild unease to total repugnance or panic. In ensuing moments, hours, or days, one's emotional reactions may become more elaborated and more conscious, but the core emotional reaction, particularly to potential dangers, emerges usually in an instant via the brain's early warning mechanisms in the amygdala and associated systems.

Positive emotional reactions— interest, attraction, and desire—are also "automatically" assigned by the brain. One example is sexual attraction. Many have had the experience of feeling suddenly attracted to a person with a certain physical appearance, mannerisms, or more subtle qualities. Such attraction is often attributed to mysterious "forces of the heart," but in fact, the brain is in charge. Psychoanalysis has offered psychological explanations for how such experiences may be linked to unconscious aspects of life experiences, but ultimately all of our responses to experiences are carried out within the brain's neural networks. Mark Baxter and Elisabeth Murray (2002) presented evidence that the amygdala and related brain structures continuously scan the environment not only for dangers, but also for indications of something interesting or potentially rewarding.

The brain uses one specific transmitter chemical, dopamine, to highlight important stimuli. Numerous studies have demonstrated that dopa-mine release in the brain acts as a powerful signal to indicate important stimuli, particularly those that bring pleasure. Dopamine is produced deep in the midbrain at rates and in amounts that vary according to the brain's moment-by-moment perceptions. Figure 4 shows the origins of the dopa-

Limbic Region

Substantia

Prefroi corte

Limbic Region

Substantia

Prefroi corte

Figure 4 Main pathways of the dopamine system. Dopamine pathways for executive functions originate primarily in the ventral tegmental area. They extend throughout the prefrontal cortex and into limbic centers to release dopamine in response to perceived danger or reward.

mine system for executive functions, in the ventral tegmental area, and its major projections to other areas of the brain.

Once released by the ventral tegmental circuits, dopamine is carried through two primary pathways. One pathway feeds dopamine to the prefrontal cortex, where action plans are selected. The other carries dopamine to other regions that map and monitor the ongoing state of the organism to reflect comfort, displeasure, and so on (Pennington 2002; Damasio 2003). Dopamine provided through these circuits provides incentive for the brain to act when something important is noticed. This incentive is likely to be provided without much input of conscious thought. Selection and intensity of arousal, based on the individual's personal history, usually occurs as an aspect of perception—how this individual perceives any given stimulus or situation at a given moment.

When an individual perceives a situation that for him or her appears potentially rewarding, the brain suddenly releases more dopamine into specific circuits. If no signals of possible frustration or danger contravene, signs of interest are likely to appear very quickly and the individual's actions are likely to move toward seeking the anticipated pleasure.

These mechanisms apply to basic rewards such as food, social interaction, and sexual pleasure. They also extend to perceived opportunities for fulfilling desires such as getting money, power, or social status. Eric Nestler and Robert Malenka (2004) have described the complicated processes by which this reward system can be disrupted by addiction. They have shown that an individual with a history of cocaine or heroin addiction, even after protracted abstinence from using the drug, is likely to demonstrate measurable levels of arousal simply in response to entering a neighborhood where he previously had purchased or used the drug. When anyone notices or is reminded of something that may bring them pleasure, arousal is likely to be mediated by rapid release of increased dopamine into relevant circuits, even without any conscious thought.

Dopamine plays an important role not only in signaling possible reward situations, but also in registering reward as it is being experienced. As the brain registers an ongoing experience of pleasure, it responds with a further release of dopamine that helps to sustain the rewarding action. The dual dopamine circuits play a critical role in mobilizing and sustaining effort to get what the individual wants or needs. These are crucial elements in the chemistry of motivation.

If dopamine is not released in these critical areas, the brain tends not to experience motivation to work, even for rewards that might otherwise be pleasurable. Roy Wise (1989) and other researchers have shown that in rats, monkeys, and humans, when effects of dopamine are blocked in specific areas of the brain, motivation and pleasure are also blocked. Blocking of dopamine release in specific areas of the brains of rats can cause them to give up working for food or for sexual pleasure. And damage to dopamine-producing cells in relevant areas of the human brain can eliminate cocaine addicts' craving for the drug. Dopamine does not itself produce the pleasure, but it creates the conditions under which sensations are recognized as pleasurable. Lacking anticipation or awareness of getting a "payoff," the organism, whether mouse, monkey, or human, tends quickly to abandon working and to ignore the task, even when the task may be essential to life (Wise and Rompre 1989).

The brain's reaction to the possibility of reward and to the threat of negative consequences like loss or punishment is subtle and complex. John O'Doherty and colleagues (2001) used imaging studies to show that the brain registers differing intensities of reward and punishment, even when the rewards and punishment are merely symbolic. They focused on a region in the front of the brain because previous studies had shown that damage there causes humans to lose normal sensitivity to rewards and punishment, whether potential or actual. When this area is damaged, even persons who were previously cautious tend to become reckless gamblers oblivious to negative consequences, ignoring information that would allow them to switch to more rewarding actions.

In the O'Doherty study, the brain activity of adults was imaged during repeated trials of a gambling game. On each trial they were to bet which of two stimuli was the "correct" one. If they chose the "correct" one, they were given a simulated money reward; the wrong choice caused them to lose (in simulation) a portion of their accumulated rewards. Images showed that one specific brain section increased activation when each reward was given while a different section increased activation in response to each punishment. Not only was activation observed in differing regions under these conditions, it was found to vary in intensity, proportional to the magnitude of each reward or punishment.

If the brain can so sensitively react to different levels of simulated rewards and punishment in a gambling game that participants knew in advance would yield no actual gain or loss of actual money, how much more does it react to the multiple rewards and punishments that are a part of daily life? This mechanism by which the brain registers subtle levels of reward and punishment is crucial in many aspects of human living. These circuits create shifting "equations for motivation" that guide operation of those executive functions that activate people to work, help them sustain effort, and coordinate their monitoring and self-regulation.

Mary Phillips and colleagues (2003a, 2003b) have described how working memory depends on these processes of emotional weighting not only to figure out what stimuli are important enough to remember and act on, but also to facilitate activation of memories from longer-term storage. Phillips showed that emotional significance plays an important role in culling memories related to specific tasks and situations as the individual moves more or less seamlessly from one situation to another. The brain's processes of assigning emotional importance are crucial elements underlying multiple executive functions.

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Responses

  • Kalervo
    What part of the brain recignises rewards?
    2 months ago

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