All animals, large and small, slow and fast, have mechanisms that allow them to adapt to changes in their local environment. Even single-celled organisms use chemotaxis as a means of guiding themselves along chemical gradients toward nutrient-rich environments. We typically call these
motivated behaviors
in that they refer to any adjustments, internal or external, made by an organism in response to environmental changes. Often, these adjustments are regulatory, designed to maintain homeostasis, and they can include modifications to endocrine, autonomic, immune, or behavioral processes.
When Olds and Milner made their original discovery, motivation was largely thought to be a simple matter of drive or “need” reduction. This theoretical perspective works fine if we limit our discussion to thermoregulatory or ingestive functions—for instance, perspiring to dissipate body heat, or thirst to satisfy the need for water. However, it fails to explain other behaviors such as aggression, sex, or novelty-seeking, all of which can be triggered by an external stimulus, yet have no identifiable deficit state. It also fails to explain why in many cases normal homeostatic mechanisms can be overridden by strong external incentives, such as occurs during drug binges, while gorging ourselves on a delicious meal, or flying down a snow-capped mountain on two thin slabs of fiberglass. Instead, we typically explain these sorts of behaviors as an attraction to external stimuli or events that have appetitive or rewarding properties.
Unfortunately for most animals, food is not just splayed out for the taking; sexual partners are not lined up and waiting; and there is not always natural spring water nearby. All animals have to actively seek out these sources. Thus, motivated behavior is not simply eating a meal or engaging in sex; these consummatory responses are typically only the end points of a long and complex sequence of actions, guided in part by drives and in part by the appetitive features of an incentive. So if appetitive features of an environment—things that are inherently attractive or rewarding to animals—are not simply tied to essential needs or drives that ensure survival, how do they emerge? Are they innate or learned?
The short answer is both. Behavioral scientists have used an impressive variety of strategies for studying motivation, many of which take advantage of the fact that animals exhibit both classically conditioned (Pavlovian) reflexes and goal-directed instrumental (also called operant) behaviors. An often-used operant conditioning paradigm involves placing a hungry rat in a test chamber and making the delivery of food contingent upon some behavior. Say, for example, that whenever a small lever is pressed by the rat, a food pellet is automatically dispensed into the testing chamber. The rat, of course, has no innate or implicit knowledge of this relationship—it has to discover it by exploring the environment.
When placed in a novel situation, mammals typically exhibit a period of freezing behavior, where they stay in one place and examine their surroundings followed by a gradual increase in exploratory activity. During the exploratory phase, a rat will eventually press the lever, often accidentally, and, after a few co-occurrences of lever press- food appearance, gradually discover that a relationship exists between the two. This process is known as
associative learning
, and occurs in virtually all animals, from sea slugs to primates. It is the foundation on which most forms of learning are based. In our example, learning the link between the lever being pressed and the subsequent appearance of food is facilitated by the fact that food, in this case, is a positive reinforcer, meaning its appearance increases the likelihood of repeating the behavior that preceded it.
Understanding how associative learning works has preoccupied the minds of psychologists, philosophers, and biologists for decades. Of particular importance to the present discussion is that not all associations are learned with the same accuracy and speed. In general, associative learning occurs most easily when one of the components of the pair involves an evolutionarily important variable. For instance, rats usually learn the association between a specific food item and the sickness it induces after only a single exposure, and they use this learning to avoid these foods in the future. Such
conditioned taste aversion
has obvious benefits to the survival of an organism, allowing it to avoid ingesting dangerous and potentially lethal toxins. The same is true of fear. Rats often learn to avoid places where they have experienced foot shock after just one trial.
Contrasting this, rats typically need hundreds of trials to learn an arbitrary association between two neutral stimuli, for example an odor and a small object, if neither of the objects belongs to a broader class of stimuli that have had a significant impact on the evolution of the species. Indeed, some associations seem impossible to learn if the components are in opposition to species-specific tendencies. In their classic (and mirthful) paper “The Misbehavior of Organisms,” Keller and Marion Breland reviewed a series of failed attempts at using operant conditioning techniques to teach animals to perform simple tasks.The title was a playful jab at their teacher, B. F. Skinner, whose book
The Behavior of Organisms
is widely considered a seminal work in the field of behavioral analysis.
In their first example they asked, “What makes Sammy dance?” Sammy, it turns out, is one of many adult bantam chickens that have been trained to emerge from a holding compartment and stand on a platform for twelve to fifteen seconds, after which food is automatically dispensed. In this task the only requirement for reinforcement is that each chicken must depress the platform and wait. Simple enough; however, most of the chickens developed a pronounced tendency to scratch at the platform, a behavior that became even more persistent when the waiting period was lengthened. Although the Brelands could not train chickens to perform the original task,“we were able to change our plans so as to make use of the scratch pattern, and the result was the ‘dancing chicken’ exhibit.” The point of this article was not to demonstrate that capable trainers can outsmart poultry, but rather that after being conditioned to perform a specific response, animals can gradually drift into entirely different behaviors that seem to go directly against reinforcement contingencies. “It can clearly be seen that these particular behaviors to which the animals drift are clear-cut examples of instinctive behaviors having to do with the natural food-getting behaviors of the particular species. The dancing chicken is exhibiting the gallinaceous birds’ scratch pattern that in nature often preceded ingestion.” Reinforcing the chickens with food thus led to the emergence of innate behavior that anticipated, but was not in itself rewarded by, the arrival of food.
Some stimuli, as noted before, have supernormal features, meaning their reinforcing value cannot be accounted for simply in terms of drive reduction. For instance, although bland foods such as normal rat chow do not reinforce the behavior of a satiated rat, sweet foods high in natural sugars do. In fact, sugars rated most sweet-tasting by humans, such as sucrose and fructose, are stronger reinforcers of behavior in satiated rats than those ranking lower on the sweetness scale, such as lactose and glucose. Sugar is obviously an evolutionarily powerful stimulus. Knowing how to detect it in the environment and having a taste preference for it so that it would be ingested provided clear survival benefits for our hunter-gatherer ancestors who possessed these traits.We now realize the biological importance of sugar molecules as a precursor source of ATP, which powers so many of the biochemical reactions critical for cell functioning. Our ancient brethren certainly had no understanding of these processes—they needn’t have for survival. For individuals to gain a selective advantage, they simply needed a hedonic preference for foods that contained digestible sugars and a means for their detection and ingestion.
Hedonic preference
refers to a stimulus property that is innately reinforcing or, speaking more colloquially, naturally rewarding. Psychologists often use the terms
primary positive reinforcer
or
unconditioned reinforcer
to describe this type of stimulus, emphasizing the notion that their rewarding properties are usually in place at or before birth. Human newborns, for example, prefer sweet-tasting liquids rather than plain water immediately after birth, before ever being exposed to sugar in the external environment (a preference that facilitates breast-feeding, since mothers’ milk is high in lactose).Two other primary positive reinforcers that work through taste include saltiness and a newly discovered gustatory dimension called umami, an indicator of protein content that is produced by monosodium glutamate (MSG). Both can serve as primary positive reinforcers of behavior and are discussed in the chapter on the development of taste preferences (see chapter 6).
There are primary positive reinforcers in every sensory domain—touch, smell, taste, vision, and hearing. As we shall see, many behaviors such as kin identification, parent-offspring attachment, and some forms of communication are motivated by compound reinforcers—particularly attractive combinations of primary positive reinforcers from several sensory modalities.
Another rich source of incentives that motivate complex behavior depends on
conditioned positive reinforcers
(also known as higher-order reinforcers)—initially neutral stimuli or behaviors that become rewarding through an association with a primary positive reinforcer. For instance, if our hungry rats learn that lever-pressing brings food, then this behavior itself acquires incentive value.
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After learning the association, rats press the lever constantly, often to the exclusion of other behaviors such as exploration and grooming. However, they obviously have no innate fondness for this behavior, since before learning they seldom exhibit it, and only then at random. Clearly the incentive value of lever-pressing is contingent on the subsequent appearance of food, because this behavioral tendency terminates once food is no longer available—a phenomenon known as extinction.
Hedonic preferences, in conjunction with the thoughts, perceptions, and actions that become conditioned to them, provide fertile ground for studying pleasure and other emotions. Yet, it’s important to realize that this theoretical foundation does not stem from a behaviorist school of thought, in which we are born as blank slates waiting to be writ upon. Quite the contrary, it assumes there are evolutionary and developmental constraints that shape
what we are likely to learn, when we are likely to learn it, and how such learning takes place
. Before we discover the ways in which each sensory modality contributes to the hedonic palate, however, we will first examine how this ancient circuitry evolved in our species and consider how the process parallels the embryological development of the human nervous system.
Essential Hardware
Since soft tissue does not fossilize, our understanding of human brain evolution comes mainly from comparative studies of living species that are closely related to
Homo sapiens
.There have been discoveries of early primate skulls that happened to fossilize with an imprint of the former owner’s brain, allowing an endocast to be made of the cortical surface; however, artifacts such as these tell us nothing about the inner circuitry of these ancient brains and very little about their gross anatomy. Instead, scientists have used a different approach.
The relationships among the major vertebrate classes have long ago been organized by examining the anatomical characteristics that distinguish species.This system of classification has been refined through the years by making further comparisons based on fossilized bone remains and DNA fragments in an effort to reconstruct the phylogenetic history of living forms. Powerful new methods have been used recently to compare DNA fragments of living animals from different branches of the phylogenetic tree and have found that in many cases (but not all) the phylogenetic reconstructions based on genetic evidence correspond well with those based on studies of fossilized remains.
These reconstructions can be used to determine whether similarities between two animals are the result of a shared evolutionary history or rather co-evolved independently in both species. For example, since chimpanzees, apes, and humans evolved from a common ancestor some four million years ago, it is likely that the brain structures common to all of these species were present in that ancestor. Brain regions that are not common to all three species are most likely to be evolutionarily newer structures derived from earlier predecessors. Similarly, brain areas that are common to all Old World monkeys (for example, Macaca) and hominoids (for example, apes and humans) are most likely even older in that they stem from the common ancestor class of Old World anthropoids that existed more than twelve million years ago.
Understanding approximately when (in terms of evolution) a brain region came into existence helps us in several ways. First, if we can show that other anatomical changes co-appeared at the same time as the brain region in question—for instance, the development of bipedalism or forward-facing eyes—it may be possible to reconstruct the selection factors that contributed to its evolution. Second, since we have a growing understanding of the relationship between brain and behavior, information about the general evolutionary history of the human brain allows us to develop theories about the evolutionary history of our cognitive and emotional capacities.
The evolution of mammals from reptilelike ancestors about three hundred million years ago brought the gradual accumulation of several distinctive features: the appearance of hair; sweat glands; mammary glands and suckling behavior; specialized teeth for grinding, slicing, and piercing new food sources; and physiological mechanisms for maintaining a constant body temperature (thermoregulation). All of these adaptations suggest that early mammals led a predatory lifestyle. During this period, the brain also went through a number of significant transformations.