I. Reward & Incentive Motivation

From a biological perspective, the early study of motivation relied on the concept of homeostasis, a term coined by Walter Cannon based on the work the French physiologist, Claude Bernard, which describes the tendency for physiological systems to remain within relatively narrow ranges, especially in regard to body temperature mammals, but also with regard to food and water intake in other animals. Bernard had written on the dynamic equilibrium which maintained the constancy of the animal body's internal milieu. From this concept, early in the last century a motivational model based on drive reduction developed that proposed when levels of a critical physiological parameter (such as warmth, food, water) drifted outside the narrow range the organism required a state of physiological need arose which generated a drive (such as hunger or thirst) which energized behavior to satisfy that need and return the physiological parameters within the required range. Simply satisfying that need was termed drive reduction. When a drive was reduced, the consequence was considered to be rewarding or reinforcing.

This basic drive reduction process was also proposed to be augmented by learning to yield a state called incentive motivation. Animals and people clearly show preferences for different sources or means to satisfy their needs and reduce their drives. These preferences are based on their experiences with different rewarding and reinforcing consequences. The external stimuli (sights, smells, sounds, tastes, touches) associated with the reduction of the drive and its rewarding consequences result in incentive motivation, which is the basis of such things as preferences among different means for satisfying a need as well as for larger magnitude rewards over smaller ones. These preferences based on experience can also trigger attraction, pleasure, or desires even in the absence of the preferred stimuli which reduce drive and lead to behaviors to seek those stimuli even in situations without an underlying drive.

In plain English, the saying some parents have used on their small children when they refuse to eat their vegetables, "You'll eat it if you're hungry enough," is based on drive reduction. When you are so hungry that you'll eat something that you'd ordinarily refuse (like for many of us, Brussels sprouts) that's eating purely on the basis of drive reduction. When you're hungry but you have a choice between Buffalo wings and Brussels sprouts and you pick Brussels sprouts because you're one of the rare people that really, really likes Brussels sprouts, that is incentive motivation-based eating. After you've gorged yourself on a Brussels sprouts to the point where you can't eat anymore, and then your mom pops out of the kitchen and says "Who's got room for pie?" and you reply, "I'll make room." Well, that's because of the incentive motivation generated desires and pleasures that exist in the absence of a drive.

However, this incentive based system can be turned on its head. What we've described until now is based on pleasant consequences to drive reduction. Sometimes the consequences are unpleasant and the learned incentive motivations can be negative and lead to repulsion being associated with a stimulus instead of attraction. For example, member of my family was in the hospital for several months, being treated for cancer. During that time, because of the radiation therapy and chemotherapy, the only meal they could keep down was Jell-O. So, for months, despite the nausea and discomfort associated with the cancer treatment, Jell-O was the only solid food that passed through their mouth. It satisfied their hunger drive repeatedly. That was in the fall of 1978. To this day, that person has never been able to eat Jell-O again.

II. Mesolimbic Dopamine System
From the standpoint of behavioral neuroscience, the old drive reduction and incentive motivation accounts first proposed by the behaviorist camp of psychology in the first half of the 1900's can be understood to some degree by the action of the limbic system and the Mesolimbic Dopamine System where dopaminergic cell bodies of the mid brain (the meso in mesolimbic) structure called the ventral tegmental area (VTA) are involved in producing essential dopamine for limbic system circuits involved in pleasure, reward and attention. When a rewarding event occurs the cells of the VTA fire and release dopamine from their terminal buttons in the frontal lobes (especially the prefrontal cortex and nucleus accumbens) and the temporal lobes (including the hippocampus and amygdala). The nucleus accumbens is the key to the rewarding experiences. It fires whenever a reinforcing event occurs. It is often referred to as the reward center of the brain. Whenever anything feels good (eating food, a recreational drug, drinking a beverage, kissing your sweetie, anything that feels good) the VTA releases dopamine into the nucleus accumbens and it fires. Rats with electrodes implanted in the nucleus accumbens or the VTA fibers leading into the nucleus accumbens will easily learn to lever-press for electrical stimulation of the electrode leading to dopamine release in the accumbens. The experience can be so rewarding that the rats will continue to relentlessly press the lever to the exclusion of food, water and sex; even to the point of death if the electrical current is strong enough.

Now, if we review the roles of the structures that receive dopamine from the VTA we can understand how the mesolimbic dopamine system participates in the reduction of drives and incentive motivation. The prefrontal cortex is involved in planning and working memory and selecting the appropriate behaviors in a given situation. The hippocampus is involved in transferring information from our short-term memory to our long-term memory. The amygdala adds emotional impact and significance to the facts and events being transferred into our long-term memories. And the nucleus accumbens is involved in the experience of the sensation of reward. So, hypothetically, if we came across a rewarding situation the dopamine would be released simultaneously by the VTA in the nucleus accumbens, prefrontal cortex, hippocampus and amygdala. The dopamine then affects each structure by facilitating their jobs during the rewarding event. In coordinated cooperative unison, the accumbens generates the pleasure of the reward, the hippocampus transfers the factual information about the event from short-term to long-term memory as the amygdala attaches the emotional significance of the pleasure to the facts while the prefrontal cortex processes those facts and their emotional content as part of the consequences of the actions that led to them. Through the execution of their roles, coordinated by the simultaneous release of dopamine triggered by the rewarding event, we can have some insight into the development of incentive motivation, preferences, and attraction (or repulsion), desires and pleasure to rewarding drive-reducing stimuli. So, for example, the next time I eat at a new restaurant and have one of the best meals I’ve had in along time, the pleasure I experienced, the memory of where, when and what I ate and the increased frequency of my visits to that restaurant for that meal can all be understood from the concepts of incentive motivation as well as the mesolimbic dopaminergic activation of the limbic system.

III. Homeostasis & Drives: Hypothalamic Thermostats & Set Points

We’ve been acquainted with the idea of homeostasis, the tendency and the mechanisms the body uses to stable, constant conditions in all of its physiological systems. Everything is regulated through homeostasis: levels of oxygen, carbon dioxide, salt, acidity, alkalinity, water, glucose, fats, proteins, waste products, body temperature and others. You can think of it as the individual having a set-point for every physiological and even psychological trait. When there is a deviation from the set point, the compensatory mechanisms that underlie homeostasis are engaged to restore the set-point.

The opponent-process theory of Richard Solomon suggests that even in psychological terms, the experience of one strong emotion will elicit the opposite emotion in order to reset the person's psychological state towards its norm. For instance, an event may trigger intense fear in us, but when that event terminates we are often very happy; the negative of the fear is compensated for by the positive feeling of happiness. As we face or endure a fearful situation, our fear often subsides. From an opponent process point of view, what is occurring is that a balancing homeostatic anticipatory mechanism engages the changes that lead to happiness earlier so that there is a kind of calming or minimization of the fear during the fearful event. When the event is over we are still happy but often not as elated as we were when we were the most fearful because the changes that underlie the happiness are also reduced to minimize the changes they would make from our normal relatively neutral state. These anticipatory changes are mediated by learning, just as incentive motivation was. The stimuli that in this example predict the start of the fearful event also predict and engage the opposing emotional response, just as the stimuli which predict the termination of the fearful event start to reduce the magnitude of the opposing emotional response. This theory may also be used as an explanation for how a long-term couple who care for each other very much, no longer shows the excitement commonly present in a new relationship. The excitement eventually came to be opposed by an opposite emotion to restore the emotional set-point. Should they be suddenly be separated due to situational circumstances, or even death, the opposing emotion to the excitement would be the only remaining emotion and might be manifested as grief, sadness or depression.

Solomon's theory was also adapted and proposed to explain drug addiction. Physiologically, the effects of a constantly administered drug typically produce tolerance, which mean that larger doses of the drug are required to get the prior drug effects. Tolerance is then seen as the result of an opposing homeostatic process to minimize the drug's disturbance of the body's physiological functioning. This tolerance also extends to the emotional and psychological effects of the addicting drug. If the drug produced a euphoric high early in the administration eventually that response is dulled due to the learned anticipatory engagement of the emotional opponent process. From this point of view the unpleasant and sometimes life-threatening effects of withdrawal can be seen as the unmasked compensatory opponent processes (physiological and psychological) being expressed unopposed in the absence of the drug.

From physiological view, the basic drives that can motivate us, like thirst or hunger, are also under control by the brain, in particular the hypothalamus. For instance, our need for maintaining a stable body temperature is governed by the preoptic area, which is involved in both helping to set our circadian rhythm and our body temperature. For instance, we are most likely to be our drowsiest at the same time that our body temperature is lowest and vice-versa. Temperature-sensitive cells in the preoptic area re able to sense body temperature via the temperature of the blood which passes through. For example in animals with probes implanted in the preoptic area, artificially cooling the preoptic area alone, independent of the body's true body temperature can lead to drowsiness as well as shivering behavior in an effort to generate more body heat. Artificially heating the area can lead to panting behavior in an effort reduce body temperature.

In contrast, thirst is also under neural control by the hypothalamus. But there are two types of thirst. In hyperosmotic thirst, loss of water or an increase in salt intake by the body leads to an imbalance between salts and water in the fluids and tissues. Even an imbalance of 1-2% from optimum can trigger this from of thirst, by causing a loss of water from inside these hypothalamic neurons into the extracellular space.. These osmotically sensitive cells in the hypothalamus trigger thirst but also can trigger hormonally-based responses from the pituitary that will slow down the production of urine by the kidneys to conserve water. In hypovolemic thirst, a loss of fluid volume from the blood lowers blood pressure which is sensed by receptors on the large arteries and veins entering and leaving the heart. This signal relayed to the hypothalamus triggers thirst. This can be due dehydration but hypovolemic thirst can also be triggered from a sudden blood volume loss, such as from heavy bleeding.

Hunger is triggered by drops in blood levels of nutrients, primarily glucose but also fatty acids. These levels are detected both by nerve endings (primarily the vagus nerve) of the peripheral nervous system that transmit the signals to the hypothalamus but the brain can also detect nutrient levels directly via the blood stream. Once a meal has been eaten a multitude of signals help to trigger the cessation of eating, again primarily via the vagus nerve. These include the physical sensations of stretching of the stomach, signals from the liver that it has absorbed nutrients, as well as hormones released during digestion and the storing of nutrients such as cholecystokinin (CCK) and leptin. These signals, once received by the hypothalamus, inhibit hunger.

However, the motivational value of food is also affected by the actual physical processes of consuming it, like chewing and swallowing. Food is not as satisfying or motivating if it is passed directly into the stomach through feeding tubes. The case is the same with the administration of nutrients via intravenous tubes. Even when an equivalently nutritious and calorically dense intake is consumed either in solid form or in liquid form, the solid form which must be chewed is more satisfying than the liquid form. This one of the problems with drinking sweet beverages, whether juice or soft drinks, with a substantial caloric content. They do not easily trigger satiety (the satisfaction of hunger) and thus do not inhibit hunger.

The hypothalamic centers involved in feeding are the lateral hypothalamus (LH) and ventromedial hypothalamus (VMH). The lateral is involved in initiating feeding and the ventromedial is involved in terminating feeding. If a rat has its LH damaged, it typically will not eat without force-feeding and may starve itself to death. If a rat's VMH is damaged it will not stop eating even when it is full and will become grossly and morbidly obese. However, they may also be involved in setting the body weight set point. If a rat is intentionally overfed to the point of obesity and the LH is damaged the rat will not eat, but will begin to eventually eat once it has lost a great deal of weight. Similarly, if a rat is placed on a very restricted diet and become thin and the VMH is damaged, the rat will eat constantly but will eventually stop once it has become fat but before it becomes as morbidly obese as full-weight rats with VMH damage.

IV. Emotion

Emotions have physiological and psychological consequences. These consequences of emotion can be reflected as arousal or activation of behavior. This arousal can have an influence on the performance on tasks, both mental and physical. The Yerkes-Dodson law states that there is an optimal (or best) level of arousal for that performance, and that the effects of arousal follow an inverted-U curve, that too little or too much arousal can impair task performance while a moderate level improves it. For example if your task is studying. Too low a level of arousal (boredom, fro instance) and you'll do a very poor job learning the information. Too high a level of arousal (jittery or anxious) and you'll also do a very poor job learning the information. Your best studying and learning effort occurs at a moderate level of arousal. Similarly, when taking the test, regardless of how well you studied, if your arousal level is low (fatigue) or high (worried or pressured) you'll do more poorly than if your arousal level was at some moderate level.

Different tasks may also have different levels of optimal arousal. The more difficult or mentally demanding a task, the lower the level of arousal needed for best performance, while simpler tasks are performed best at higher levels of arousal. For example, the optimal arousal level for assembling sensitive computer components would be lower than for moving in a warehouse.

There are several theories for understanding emotion and its effect on our mental states. However we’ll focus on the two oldest, the James-Lange (formulated by William James and Carl Lange in 1884) and the Cannon–Bard (formulated by Walter Cannon and William Bard in 1927), because the more recent theories extend and elaborate on their basic views and their principles still influence current thought. In the James-Lange theory the events we experience as emotional elicit innate reflexive physiological responses, often of the flight or fight variety. These responses include muscular movements, autonomic responses (such as heart rate, blood pressure, respiration rate) and hormonal responses. These responses help us deal with the event or situation. After dealing with the situation, the feedback from the responses is then interpreted by the brain and the experience is labeled with a given emotion based on the presumably unique pattern of physiological responses. And today, it's known that people with nerve damage that interferes with such feedback have difficulty estimating the intensity their emotional reactions. For example, you're in the woods, you see a bear, you have the appropriate physical responses and afterwards label it as fear. We can appreciate this to some degree. Often when we're startled we simply react and then afterwards are shaken or exhilarated by the experience. However, Cannon-Bard holds that the physiological responses between clearly qualitatively different emotions, such as anger and fear, are virtually indistinguishable. Their theory there is some degree of cognitive appraisal first, then simultaneously and in parallel we experience fear and exhibit our physical responses. For example, you're walking in the woods, you see a bear, you briefly assess the situation (oops, not a good thing) then separate systems simultaneously and in parallel generate both the physical reactions and the emotion of fear.

Both views have their strengths and weaknesses. James-Lange has the shortcoming of being unable to explain how virtually identical physiological responses can be attributed to very subjectively different emotions. But Cannon-Bard has difficulty explaining why autonomic feedback can and does occur to influence our perceived emotions.