How Do We Study the Brain?
The brain may be subdivided into three main subdivisions (see Figure 1; also see Pinky & the Brain for a description of major brain areas in cartoon form). First, the brain stem is composed of a set of neural[∂] structures making up the extension of the spinal cord into the skull. Much research has shown that activity within the brain stem is associated with attention, a variety of reflexes essential to survival, and daily patterns of sleeping and waking (see Section 1-4 and Section 1-5). Second, the limbic system is composed of a set of neural structures surrounding the upper part of the brain stem. Activity within the limbic system is associated with the regulation and expression of emotions, memory formation, and biological drives. Third, the cerebral cortex, which is the largest subdivision of the human brain, is composed of the wrinkly tissue that covers the outermost parts of the brain. Activity in the cerebral cortex is associated with many of the so-called “higher functions” of humans, such as language ability, reasoning ability, the planning of actions, and perception.
Figure 1. The Three Subdivisions of the Brain and Some of Their Major Structures
(adapted from Kassin, 2001, p. 55)
The ultimate goals of brain research are:
- to analyze the brain into its component parts;
- to determine how these components are organized;
- and to determine how their activity is coordinated when perceiving, reasoning, planning, responding, and so on.
But, until about 1980, the human brain could be studied primarily as a sort of "black box" — a sealed container with "machinery" that, other than during neurosurgery, could be observed directly only by X-ray imaging, the recording of electrical activity in the outer layers of the cerebral cortex, or the dissection of brains during autopsies. All these methods for observing the brain, however, suffer from severe limitations: X-ray images show little detail; the EEG gives only a crude measurement of the summed activity of millions of neurons; and, after death, the brain changes very quickly. Researchers' inability to observe living brains in action meant that it was very difficult for them to find links between the activity of particular brain structures, on the one hand, and mental and behavioral functioning[∂], on the other.
The nature of brain research changed dramatically with the development of new technologies that provided detailed images of brain structures and their activity. We now are able to look at which areas of the brain are most active while performing tasks such as reading, evaluating, or deciding. Since 1980, the pace of discoveries about the brain and its functions has exploded. In this section you will learn about some of these functions and the kinds of research performed to discover them.
The Central & Peripheral Nervous Systems
The central nervous system (CNS) consists of the brain and spinal cord. It is the only part of the nervous system enclosed in bone — a fact that suggests that CNS activity is very important for the survival of organisms. The spinal cord is a structure about 18 inches long and one inch wide that runs through the spinal column. The brain, which weighs about three pounds and is made up of about 100 billion cells, sits on top of the spinal cord within the skull. Together, the brain and spinal cord have the following functions:
- CNS activity underlies the complex processing of sensory information in both sensory and working memory.
- CNS activity underlies judgement- and decision-making in working memory, which direct our responses to the sensory information.
The rest of the nervous system comprises the peripheral nervous system (PNS). The PNS has two major functions:
- Sensory stimuli from the external world activates sensory receptors in the PNS, which is sent through nerves in the PNS and into the CNS.
- CNS activity associated with making decisions about how to respond to sensory input activates and is sent through nerves in the PNS to the muscles, glands, and organs of the body involved in responding.
For example, when something touches the skin on your arm, tactile (touch) receptors in the skin are activated; this information is sent through nerves in your arm; and then this activity enters the CNS (first entering the spinal cord and then the brain). CNS activity is associated with processing this sensory informatio and, perhaps, forming a conscious perception that the skin is being touched. Based on the information processing, CNS activity is associated with deciding what to do and how to do it. This decision is constructed into a motor message[∂] that exits the CNS and enters the PNS. The motor message travels down your arm to the appropriate muscles, perhaps causing them to contract. In this way, the PNS and the CNS work together in adjusting (adapting) our behavior to internal and external demands. Martin (1986) described the interaction between the PNS and the CNS:
The brain and spinal cord play a role in virtually every physiological activity — from swallowing to sweating, from listening to ... music ... to making love. Yet without the many millions of minute [extremely small] fibers of the peripheral nervous system, fibers that supply every organ, every muscle, every scaly patch of skin, there would be no communication between brain and body. The brain would languish like an unprogrammed computer, and the body would be functionless — some marvelous machine that could never be powered up. (p. 174)
Therefore, any disruption of communication between the PNS and the CNS will lead to disturbances in our ability to sense or respond to the environment.
When the spinal cord is severed, any neural activity sent from the brain into the spinal cord cannot move past that point. Because of this, the parts of the body innervated[∂] by the motor pathways below the cut no longer are able to respond (for example, a person will show paralysis of the muscles below the cut). In addition, any sensory information that enters the spinal cord below the cut cannot move past it and, therefore, cannot be processed by sensory areas in the brain, which means that the person is unable to sense parts of the body below the cut.
The intact (undamaged) spinal cord, on the other hand, not only transfers information between the brain and all parts of the PNS, it also processes some of the incoming sensory information and sends out its own motor messages. These spinal-cord-induced movements make up several basic reflexes[∂]. An example of a spinal reflex is the withdrawal of the hand from a hot object. If your finger touches a hot stove, the sensory information travels up your arm and into the spinal cord where it is processed in a rapid and superficial manner (REFERENCE). If the temperature is extreme enough, your spinal cord will send out a motor message to the muscles of your hand and arm, which will cause them to quickly pull away from the stove. This happens before you feel any pain, and sometimes even when the temperature is very cold instead of hot. For example, you may remember having put your finger under very cold running water and pulling it away before realizing it wasn’t hot. This is because your spinal cord does not process sensory information in such a way that you are able to consciously feel pain. That sort of processing occurs in your brain. Because it takes a little longer for the information to get to the brain and be processed, you may already have withdrawn your hand before you consciously perceive pain.
Normal walking also is controlled mostly by spinal reflexes. Once you make a decision to walk and then begin walking, your brain no longer is involved (unless some problem arises): your spinal cord controls the walking movements. In cats, if the spinal cord is lesioned[∂] above the part that controls the hind legs — a technique that eliminates the brain’s influence— cats still are able to move their legs in a normal walking pattern when placed on a treadmill (REFERENCE). If these cats are not forced to walk, however, they simply lie on the floor. This is because any decision made in the brain to start walking no longer can be sent to the spinal cord. In a similar manner, the scratch reflex in dogs also is controlled by the spinal cord. When there is some sort of irritation (such as a flea) on a dog’s skin, it will move a hind leg up to the source of the irritation and begin a rhythmic scratching pattern. This is true even in dogs whose spinal cords have been experimentally lesioned above the part of the spinal cord that controls this reflex (REFERENCE).
More complex processing of information occurs within the brain. Since the 1600s, a great deal of evidence has accumulated that confirms that the normal functioning of the brain is essential for complex mental processing of information and the production of complex behavioral patterns (Zimmer, 2005). We talked a bit about the brain stem in Sections 1-4 and 1-5. In this section, we'll examine the limbic system and discuss in more depth how researchers design studies to look for causes.
Study Questions
- What are the three major subdivisions of the brain?
- What are the main functions of the brain stem?
- What are the main functions of the limbic system?
- What are the main functions of the cerebral cortex?
- What are the ultimate goals of brain research?
- What are the main functions of the CNS?
- What are the main functions of the PNS?
- How would you define the concept of a "motor message" in your own words?
- What is an example of a motor message not mentioned in the text?
- If the spinal cord were cut just below the point where it enters the skull, what problems would result?
- What are examples of spinal-cord reflexes?
What Does the Limbic System Do?
The limbic system (see Figure 2) is made up of structures whose activity is associated with biological drives (for example, eating, drinking, and sex), emotions (for example, anxiety and aggression), and memory formation. With respect to emotions, researchers have found that, when the entire cerebral cortex is removed in cats, but their limbic systems are left intact, the "decerebrated" cats exhibit exaggerated aggressive behaviors and postures in response to stimuli (REFERENCE). In humans, people who experience epileptic seizures involving excess activity in the limbic system sometimes exhibit strong aggressive impulses, strong fear responses, uncontrollable laughter, strong sexual arousal, or a feeling of bliss and “oneness” with the universe (REFERENCE). In the case study of Greg, first mentioned in Section 1-1, much of his limbic system was destroyed by a tumor, which was associated with an absence of emotionality and desires along with severe amnesia. In this section, we will examine the functions of structures in the limbic system associated with these functions.
Figure 2. Major Structures Within the Limbic System.
(The picture appears at this link.)
Amygdala. The amygdala is important for the ability to experience and express emotional responses to events. For example, the fear you feel when watching a horror film or remembering it later depends upon activity in the amygdala. The amygdala also allows us to interpret fear in other people's faces. Lesioning[∂] the amygdala on each side of the limbic system can make an animal unaggressive and emotionally unresponsive. For example, in research with monkeys and cats, those with damage to their amygdalas would attempt to eat burning matches, their own feces, and other objects that normally would elicit fear or disgust. Monkeys who had responded to snakes with terror before the lesioning of their amygdalas would, after the lesioning, show no fear of the same snakes. Some monkeys even placed the snakes inside their mouths!
In humans, electrical stimulation of the amygdala often causes feelings of fear and apprehension. And there is some evidence that abnormalities in the amygdala leading to increased activity of its neurons can lead to furious attacks on others (see this article for a discussion of violent behavior and the brain). In individuals suffering from epileptic seizures involving the limbic system, increased aggressiveness towards others sometimes is observed. One case involved a 34-year-old engineer, who suffered a ruptured peptic ulcer, which caused him to develop epileptic seizures involving the limbic system (REFERENCE). During these seizures, he sometimes would become violent towards others, including his wife. For example, he might interpret something she said as an insult and brutally attack her for several minutes. After doing this, he would fall asleep for about 30 minutes and then wake up feeling refreshed. After other treatments had failed, he agreed to undergo neurosurgery to have a small part on each of his amygdalas destroyed. After the operation, there were no further attacks of rage. The success of the treatment suggests, but does not prove, that the man's violent behavior was caused by increased activity in the amygdalas during his epileptic seizures.
Because of its association with emotional responsiveness, the amygdala is thought to be very important for memory formation, especially fear conditioning (discussed in Section 3), (see this article for a discussion of the amygdala and memory). Without our amygdalas, each event in the world is basically the same lifeless experience. The emotional “charge” linked to events facilitates the processes of encoding, storing, and retrieving memories about the events. The amygdala works in concert with the hippocampus, another major structure in the limbic system.
Hippocampus. In Section 1-1, you learned about Greg — a 25-year-old man who developed a large benign brain tumor that destroyed a large part of his limbic system (Sacks, 1995). Although the tumor was removed in 1975, Greg was by then suffering from many problems caused by the damage: blindness, obesity, baldness, lack of emotionality, severe amnesia, and bizarre behavior. As quoted in Section 1-1, his neurologist, Oliver Sacks, described Greg this way:
Lacking facial hair, and childlike in manner, he seemed younger than his twenty-five years. He was fat, Buddha-like, with a vacant, bland face, his blind eyes roving at random in their orbits, while he sat motionless in his wheelchair. If he lacked spontaneity and initiated no exchanges, he responded promptly and appropriately when I spoke to him, though odd words would sometimes catch his fancy and give rise to associative tangents or snatches of song and rhyme. Between questions, if the time was not filled, there tended to be a deepening silence; though if this lasted for more than a minute, he might fall into Hare Krishna chants or a soft muttering of mantras. (p. 45)
Greg's severe amnesia was, perhaps, his most incapacitating problem: he had forgotten almost everything about his life from about 1968 forward. During the late 1960s, Greg was a teenager who had been heavily involved in the “hippie” culture — "experimenting" with various drugs and eventually devoting himself to the practice of an Eastern-influenced religion. Greg neither remembered neither becoming ill nor developing severe physical and psychological impairments. Sacks evaluated the nature and severity of Greg’s amnesia in various ways:
If I gave him lists of words, he was unable to recall any of them after a minute. When I told him a story and asked him to repeat it, he did so in a more and more confused way, with more and more “contaminations” and misassociations — some droll, some extremely bizarre — until within five minutes his story bore no resemblance to the one I had told him. Thus when I told him a tale about a lion and a mouse, he soon departed from the original story and had the mouse threatening to eat the lion — it had become a giant mouse and a mini-lion. Both were mutants, Greg explained when I quizzed him on his departures. Or possibly, he said, they were creatures from a dream, or “an alternative history” in which mice were indeed the lords of the jungle. Five minutes later, he had no memory of the story whatever. (p. 47)
Greg showed both retrograde and anterograde amnesia (see Figure 3). Retrograde amnesia is the inability to retrieve long-term memories during a period of time before and up to the occurrence of a trauma[∂] or the onset of a brain disease. Greg could remember little from about 1968 to 1973, probably because of damage the tumor caused to parts of his cerebral cortex. Anterograde amnesia is the inability to store new long-term memories during a period of time beginning with the occurrence of a trauma or the onset of a brain disease and after. Anterograde amnesia often involves damage to the hippocampus, as is the case in the early stages of Alzheimer's Disorder. In Greg's case, the brain tumor had destroyed his hippocampi as well as his amygdalas.
Figure 3. Retrograde Amnesia and Anterograde Amnesia
Pleasure Centers. Several areas within the limbic system, when stimulated electrically, are associated with feelings of reward and pleasure in mammals. In the 1950s, James Olds and Peter Milner discovered the first evidence for what was to become known as “pleasure centers” in the brain (Olds, 1955; Olds, 1958; Olds & Milner, 1954; also see Hebb, 1955 and Thompson, 1999). They had accidentally placed an electrode in a particular area of a rat’s limbic system and noticed that the rat seemed to enjoy electrical stimulation of this area. When they implanted electrodes in this same area in other rats, they found that electrical stimulation served as a reward for the learning of new behaviors. In fact, they allowed rats to press a lever that stimulated this area in their own brains. They found that rats press the lever as much as 2000 times an hour (an average of one lever press every 1-2 seconds). Rats sometimes would continue pressing until they dropped from exhaustion. In 1962, James Olds repeated these experiments with monkeys and obtained similar results (sometimes with lever presses as high as 8000 times per hour — about two presses every second). Other studies have found pleasure centers in cats, dogs, and dolphins.
Activity involving two parts of the limbic system — the nucleus accumbens and the ventral-tegmental area (see Figure 4) — is associated with the rewarding effects of survival-related behaviors (such as eating) and sexual behaviors. When a survival-related or sexual need is being satisfied by current behavior, the ventral-tegmental area activates the nucleus accumbens, which typically produces a pleasurable feeling. The activation of the nucleus accumbens is accomplished by the release of a biochemical called "dopamine." Thus, the pathway from the ventral-tegmental area to the nucleus accumbens is referred to as the dopamine circuit. The pleasurable feeling produced by the activation of the nucleus accumbens by dopamine positively reinforces the behaviors satisfying the biological need. Because these behaviors are essential for the survival and reproduction of individuals, it seems likely that the dopamine circuit evolved in mammals in order to reinforce learned behaviors that increase survival and reproduction (see Section 3 on instrumental learning and operant conditioning).
Figure 4. The Locations of the Nucleus Accumbens and the Ventral-Tegmental
Area in the Brain. (The picture appears at this link.)
Addictions to psychoactive drugs[∂] involve activation of the dopamine circuit. The rewarding effects of addictive drugs is due to the activation of the nucleus accumbens, either directly by the drug or by activatingthe ventral-tegmental area, which then activates the nucleus accumbens. In fact, rats allowed to stimulate their own nucleus accumbens by pressing a lever act as if they are addicted to the brain stimulation. For example, in various studies, rats would choose brain stimulation over food, even when they were being kept on a starvation diet; and rat mothers would abandon their newborn pups in order to get to a lever that provided brain stimulation.
Although, in humans, direct stimulation of the dopamine circuit seems to be associated with pleasure, the feelings generally are not as intense as they seem to be in direct-stimulation studies using rats and monkeys. The initial evidence for this claim was obtained during the early 1960s by Robert Heath, who was working with depressed patients and patients suffering from severe pain (Heath, 1963). He thought that stimulation of the dopamine circuit (and other pleasure centers) would reduce or eliminate the depression and pain of these patients. Heath found that brain stimulation often produced pleasurable sensations in his patients — sensations that generally were sexual in nature. For example, a 36-year-old woman became sexually aroused by brain stimulation and would flirt with her therapist whenever it occurred. One person found the brain stimulation to be exceedingly pleasurable: he kept pressing a button that produced brain stimulation several hundred times an hour for up to several hours in a row. An 11-year-old boy stated simply, “Hey, you can keep me here longer when you give me these [stimulations]; I like those!” It seemed that patients who had the most severe depression or who were feeling the most pain experienced the greatest degree of pleasurable sensations when receiving brain stimulation. Patients who had mild to moderate symptoms experienced the least amount of pleasure. Thus, in most patients, the pleasurable sensations caused by stimulation of the dopamine circuit were milder than those experienced by rats and monkeys.
In some patients, in fact, brain stimulation produced either no response or unpleasant sensations. For example, one man stated that brain stimulation made him feel as if he were just about to have an orgasm; but, when the orgasm didn't occur, he felt only frustration. When he was allowed to stimulate his own brain, he pressed the button continuously in the hope of achieving an orgasm and, hence, being relieved from his intense frustration. But he never attained this goal (at least not with brain stimulation). Strangely, some patients felt as if they had a word on the “tip of their tongues,” and would keep pressing the button in order to retrieve the word. But of course, since there was no word to retrieve, the brain stimulation left them feeling very frustrated, too. (In their case, however, there were no other methods to use to end their frustration.)
Thalamus. The thalamus consists of a collection of areas that are responsible for various functions. But one function shared by all these areas is their role as the "router"[∂] for information to be sent to the cerebral cortex. in particular, sensory pathways from the body converge on the thalamus, which then sends this information up to the various sensory areas of the cerebral cortex. In addition, the thalamus performs some initial processing of this sensory information. For example, the thalamus is important for the perception of pain and may also allow us to perceive pressure and temperature to some extent. Formation of long-term memories also may involve activity of the thalamus: people who have damage to certain parts of the thalamus sometimes have difficulty with memory. Probably because of its sensory functions, activity in the thalamus seems to be important for self-awareness and awareness of the world. In fact, people who have severe damage to the thalamus may show normal arousal (sleep/wake) cycles but have no apparent awareness of themselves or their worlds.
Study Questions
- What happens in cats when their cerebral cortex is surgically removed?
- What happens in humans when there is excess electrical activity in the limbic system?
- What happens in humans when there is extensive damage to the limbic system?
- How would you describe in your own words the main functions of the amygdala?
- What is the role of the amygdala in interpreting the emotions of others?
- What happens in humans when the amygdala is electrically stimulated?
- What is the role of the amygdala in learning and memory?
- How would you describe in your own words the main functions of the amygdala?
- What happens in humans when the hippocampus on each side of the brain is damaged?
- How is antergrade amnesia similar to retrograde amnesia?
- How does antergrade amnesia differ from retrograde amnesia?
- What were the main findings of the studies by Olds & Milner during the 1950s?
- What are the two major structures in the dopamine circuit?
- What happens when the dopamine circuit is electrically stimulated?
- How is the dopamine circuit involved in the development of addictive behaviors?
- What did Robert Heath discover in his studies of psychiatric patients?
- In Heath's patients, with what set of emotions was stimulation of the dopamine circuit associated?
- How would you describe in your own words the functions of the thalamus?
What Causes Addiction?
xAddictionbNEUROTRANSMITTERS, NEURONS, AND NEURAL IMPULSES DISCUSSED DURING CLASS (2/14/06). ADD SECTION ON THIS FOR FUTURE SEMESTERS.
Dopamine was originally thought to serve as a kind of pleasure signal in the brain, telling us when something feels good or rewarding. But scientists now believe that dopamine is more a predictor of salience[∂] — that is, it tells us, and then helps us to remember, what we should focus on. When you see a person you are strongly attracted to, scientists can now see a spike of dopamine in your brain. If you are hungry and smell a food you like, dopamine also increases. But even unpleasant experiences — like physical pain or the fear of an intruder in the house — can cause a dopamine spike. (Denizet-Lewis, 2006)
Stimulant drugs, such as cocaine and methamphetamine, cause dopamine-containing neurons to release greater amounts of the neurotransmitter, which activates the reward center described earlier and leads to euphoria[∂]. The nervous system, however, is constructed in such a way that, when its normal activity is disturbed (by drugs, for example), it changes in ways that "try" to reattain normal activity. This process is known as compensation, and is dependent on the plasticity of the nervous system. It can do this in many ways, such as the following:
With regular, repeated "addictive" drug use, ... the brain eventually responds by reducing its normal release of dopamine. Studies also show a simultaneous decrease in the number of dopamine receptors created. That, in turn, makes the brain's reward system less likely to respond to behaviors (romance, a good meal, the company of friends) that produce a normal dopamine surge. The addicted brain essentially becomes pathologically selective, dependent on bigger and bigger blasts of, say, cocaine to feel rewarded. (Denizet-Lewis, 2006)
Compensation, therefore, leads to dependency (DEFINE) and tolerance (DEFINE).
When people addicted to a drug experience situations that remind them of the drug — such as seeing drug paraphenalia — they experience surges of dopamine to parts of the brain related to learning (see Section 3) and the initiation of actions, which causes not a feeling of euphoria but an intense craving for the drug (Volkow, et al., 2006). The amygdala is involved in this: it is important for the learning of behaviors associated with strong emotions, such as the emotions linked to drug intoxication. The craving is what drives most recovering addicts back to drug use. If the craving could be reduced or eliminated, recovering addicts would have a much better chance of abstaining from the drug completely. This is where much research now is focused.
FROM VOLKOW, ET, AL: We measured changes in dopamine by comparing the specific binding of [11C]raclopride when subjects watched a neutral video (nature scenes) versus when they watched a cocaine-cue video (scenes of subjects smoking cocaine). The specific binding of [11C]raclopride in dorsal (caudate and putamen) but not in ventral striatum (in which nucleus accumbens is located) was significantly reduced in the cocaine-cue condition and the magnitude of this reduction correlated with self-reports of craving. Moreover, subjects with the highest scores on measures of withdrawal symptoms and of addiction severity that have been shown to predict treatment outcomes, had the largest dopamine changes in dorsal striatum. This provides evidence that dopamine in the dorsal striatum (region implicated in habit learning and in action initiation) is involved with craving and is a fundamental component of addiction.
FROM Denizet-Lewis, 2006: Dopamine may also make some people more vulnerable to addiction. Recent studies in both animals and humans have indicated that those with low levels of dopamine D2 receptors, which regulate the release of dopamine in the brain, are more likely to find the experience of taking drugs pleasurable. Some researchers, like Volkow, suggest that people with fewer D2 receptors experience a less intense reward signal, causing them to overindulge in order to feel satisfied. In one experiment, Volkow increased the level of dopamine D2 receptors in rats that had low levels. After the increase, the rats significantly curtailed their intake of alcohol, which they had eagerly gulped down before. Unfortunately, we don't yet know how to safely increase the number of dopamine D2 receptors in humans.
GABA--inhibitory neurotransmitter
Glutamate--excitatory neurotransmitterFROM Denizet-Lewis, 2006: GABA (gamma-aminobutyric acid) is the brain's major inhibitory transmitter, and its role, in essence, is to keep glutamate, the main excitatory transmitter, from overwhelming us. In the extreme, too much glutamate can cause a seizure and too much GABA can put us in a coma. Researchers are particularly interested in the brain's critical balance of GABA and glutamate — some hypothesize that addictive craving is the result of too much glutamate or too little GABA. "We've been able to measure GABA in living brains for some time, but measuring glutamate in living human brains has just become feasible in the last few months," says Frank Vocci, the director of the division on pharmacotherapies and medical consequences at the institute on drug abuse. "What's been shown is that people with alcohol and cocaine problems have less GABA in their brains, and we do know that medications that increase GABA have shown some efficacy in treating addiction." (Vocci says that it isn't yet clear whether the absence of GABA is a cause of addiction or a result.)
What Are the Limitations of Scientific Theories?
In the above discussion, some theories about the functions of structures in the limbic system were described. All these theories are incomplete and probably many of them contain inaccuracies, both minor and major. This shows that, although theories represent our best current understanding of something, we know that most will require alterations as we learn more. Thus, we rarely say that a theory, even a good theory that leads to many confirmed predictions (see Section 1), is true. There are two major reasons for this:
- Reality is always more complex than we can describe.
- Theories consist of generalizations based on a sample of observations.
Reality is Complex. Think of a time when you had a disturbing dream. Perhaps you tried to describe the dream to someone or to write it down. When describing the dream verbally, you probably felt as if you were not quite describing the whole picture — that, instead, your description of the dream was missing much of the subjective detail essential to communicating your experience of the dream. This difficulty involved the fact that much of the dream was nonverbal (changing visual images, emotions, intuitions, etc.), whereas your description was verbal. Not only could you not describe adequately the nonverbal elements of the dream but, in using language, you also were attempting to describe the dream as a sequence of events, similar to a story you might read, whereas the dream itself often included events that were nonsequential — an experience in which images, emotions, and actions changed and melded into new combinations that had no logical connection to what came before or after.
Since your verbal description of the dream didn't capture fully the dream experience, would you conclude that your description was false? When put this way, you may be inclined to answer “yes.” Yet, for certain purposes, your description undoubtedly was "adequate" (although perhaps not "true" or "accurate"). For the purpose of getting across a basic idea of what happened in the dream, your description probably was adequate. For the purpose of describing why the dream disturbed you, your description may have been adequate. Furthermore, you know that, if you had spent more time reworking the description over and over again, you probably could have communicated more of the reality of your experience. In the end, however, no matter how hard you tried, your description would never have been able to capture fully your experience of the dream. Thus, your description was not really a true one, but neither was it a false one. Instead, your description of the dream worked for certain purposes but not for others.
The same can be said for scientific theories. Although scientific theories never reflect fully the reality we wish to describe and explain, they may be adequate with respect to achieving three major goals (functions) of theories:
- the organization of past and present data;
- the prediction of future data;
- the explanation of all data.
For example, the theory of evolution — the claim that the geographical distribution of, the physical/mental similarities among, and the physical/mental differences among present-day species are due to their descent from common ancestors and subsequent changes in the descendant species — is an adequate theory with respect to the three functions of scientific theories:
- Organization. We can order groups of organisms in terms of their degree of relatedness: how long ago they shared a common ancestor.
- Prediction. We can predict that closely related groups will share more gene variants or more similar mental characteristics than less closely related groups.
- Explanation. The similarities and differences among organisms can be explained, in part, by how long ago they shared a common ancestor.
Although, the theory of evolution is an adequate theory with respect to these functions, we also want to know the answer to another question: does the theory of evolution provide a true account of the histories of extant[∂] and extinct species? This is similar to the question of whether or not your verbal description of your dream provided a true account of it. The best answer probably is that, just as your description of the dream was true in terms of communicating its main elements, the theory of evolution is true in terms of describing the general outline of the histories and interrelationships of modern-day species. Furthermore, just as your dream description included inaccuracies because of, say, the addition of inaccurate details when reconstructing the dream, evolutionary theories include inaccuracies because of, say, the collection of incomplete or inaccurate data when researching the evolutionary histories of groups of species. Researchers know that, the more they continue to develop their various evolutionary theories through (a) the collection of new data to test predictions derived from the theory and (b) the interpretion (and reinterpretion) of analyses of these data, the more accurate these theories will become in terms of describing the true histories and interrelationships of species.
Making Generalizations. Theories are developed through observing the world around us and making generalizations from these observations. For example, will the sun come up tomorrow morning? You probably answered this question with a resounding, “Yes!” If I (the author of this text) ask you how you know this to be true, you probably will say something such as:“Everyone knows this is true: the sun has always done this.” But, just because the sun has always risen each morning for as long as anyone has been observing and recording this event, does it necessarily follow that the sun must rise tomorrow morning? Let's consider this question by looking at a related example. Let's say that you ask me the following question: "Will you wake up tomorrow?" I would answer this question with a resounding, "Yes!" If you ask me how I know this to be true, I probably will say something such as: "I know that this is true because, on each and every day of my life, I have awakened after sleep." But, just because I have always awakened after a sleep, does it necessarily follow that I must awaken tomorrow? Of course not. For example, I may experience injuries or the development of a disease that places me into a state of coma or that causes my death. Or, I may not sleep at all tomorrow, which means that I cannot awaken from sleep. Just because an event has always been observed to occur does not mean that it must continue to occur in the future. It is inevitable that, one "day," the sun no longer will rise over the horizon because the earth no longer will exist, just as it is inevitable that one day (much, much sooner than is the case for the earth), I no longer will awaken because I no longer will exist.
The conclusion that an event will continue to be observed in the future because it has been observed a number of times in the past is known as a generalization. Generalizations depend on a reasoning process known as induction, which may be defined as the mental processes involved in deriving a general conclusion about a phenomenon from a limited number of observations. For example, it has been observed many times that liquid water turns into a solid at 32º F (0º C) and into a gas at 212º F (100º C). Thus, chemists have concluded that pure liquid will always turn into a solid or gas at these temperatures. The conclusion (generalization) was reached through induction. To take another example, some theorists have concluded that dissociative identity disorder (formerly known as multiple personality disorder) always is caused by traumatic experiences, especially those occurring during early childhood. They base this generalization on clinical observations of people who exhibited the symptoms of dissociative identity disorder and who also retrieved memories of traumatic childhood abuse. Again, the conclusion was reached through induction.
But logicians (philosophers who study the principles underlying sound reasoning) have long argued that there is a fundamental problem with induction:
The problem is that it is both impossible to make all observations pertaining to a given situation and illogical to secure all relevant facts for all time, past, present, and future. However, only by making all relevant observations throughout all time could one say that a final valid conclusion had been made. On a personal level, this problem is of little consequence [because our everyday inductions usually are adequate for our purposes], but in science the problem is significant. Scientists formulate laws and theories that are supposed to hold true in all places and for all time, but the problem of induction makes such a guarantee impossible. “ (McComas, 1997, p. 91)
According to the philosopher, Julian Baggini (2005):
induction is a logical embarrassment. That is because, whether we base our generalisations on many instances or just one or two, we are still concluding that something is always the case on the basis of only a limited set of observations. Most fundamentally, we are also assuming that the future will be like the past, when we have no experience at all of what the future will be like.
When one is generalizing based on a small or biased set of observations, it is very easy to derive incorrect conclusions. For example, the theory of dissociative identity disorder just mentioned is based on observations of people being treated for psychological problems, typically with techniques used to retrieve implicit memories believed to be at the unconscious level because of the actions of defense mechanisms such as repression[∂] and dissociation[∂]. In other words, these observations consist of case studies that are biased in terms of the type of therapeutic techniques used — a set of techniques that are similar to procedures known to cause the construction of false memories — and the theoretical approaches adopted by therapists inclined to use these questionable techniques.
Study Questions
- What are the two major limitations of scientific theories?
- Why are theories capable of giving only a broad outline of reality and the causes of phenomena?
- What are the three major functions of theories?
- With respect to the existence of modern humans, which theory better fulfills the three functions of theories: the theory that humans were created by an "intelligent designer" approximately 6000 years ago or the theory that humans have evolved over millions of years from other primate species?
- How does your preferred theory (in the previous question) fulfill each of the three functions?
- What is an example of a generalization that you've made in your everyday life?
- Earlier in this section, I claimed that, one day (relatively soon), I no longer will exist. What generalization justifies this claim?
- What observations were used to make the inductive inference that "all humans (organisms) eventually stop existing (die)? Based on the discussion in this section, is it possible that this inductive inference is incorrect? Why or why not?
- What is the fundamental problem with induction?
In order to perform adequate tests of theories, researchers must include systematic observations in their studies — observations made according to a plan. The specifics of the plan depend upon the type of claim being tested. For example, college aptitude tests[∂] (such as the SAT) are used to predict how well a person will do in college, typically in terms of grade-point average (GPA). If we want to test the claim that a new aptitude test predicts future GPAs, then our systematic observations will include collecting a representative sample of students, giving them the aptitude test, having them take some number of college courses, and correlating scores on the aptitude test with GPAs. If a strong positive correlation is found, then we can conclude that the claim is likely to be correct. Other types of claims will require different plans. Let's look more closely at the topic of systematic observation.
How Does One Observe Systematically?
You learned above that the goal of research is to derive generalizations from observations, and that deriving generalizations involves induction. As you learned, the fundamental problem with inductive inferences is that they are based on a limited number of observations. In order to increase the likelihood that a generalization is accurate, the observations on which it is based must be sufficient in terms of both their relevance and their number.
Observations are relevant when they are appropriate to the generalization one wishes to make. For example, a pollster (a person who conducts or analyzes opinion polls) would not make an inductive inference about which of two political candidates was more likely to win based on interviews of a group of children. It is best to interview a group of registered voters (especially those considered most likely to vote). Furthermore, a large number of such voters would need to be interviewed: a pollster would not make an inductive inference about which of two political candidates was more likely to win after interviewing only five registered voters. It is essential that the group interviewed contains a large number of people from the particular population of people which a pollster wishes to generalize about.
Representative Samples. In deciding which of two candidates is most likely to win an election, the people interviewed (the sample) would need to be similar to all people likely to vote in the election (the population). A sample is the set of observations made by researchers. The sample is selected from a population, which is the total number of relevant observations that could be made if there were unlimited time and resources to do so. In most studies, the population is too large for researchers to observe each individual. Instead, they must select a sample from the population to observe. If researchers are to make an accurate inductive inference about the population, they must select a representative sample — a sample that is similar to the population with respect to essential characteristics. In predicting the outcome of an election, the sample of registered voters interviewed must be similar to the population in terms of age, race, ethnicity, gender, political affiliations, and so on. If one or more of these characteristics deviate significantly from the population, then the result is a biased sample. A biased sample will lead to generalizations about the population that are inaccurate.
A famous example of this occurred just before the 1936 U.S. presidential election. Franklin Roosevelt was the incumbent Democratic president running against a Republican challenger by the name of Alfred Landon. Landon was supported primarily by those who had survived the initial economic losses of the Great Depression and were still relatively well-off financially. Roosevelt was supported primarily by people hit hard by the economic collapse. In predicting the outcome of the election, a magazine called Literary Digest sent questionnaires to about 10,000,000 Americans (Classic Polling Surprises, 2002; Goodwin, 1995). Their sample included subscribers to the magazine as well as a large number of people selected from phone books and motor-vehicle registration records. The pollsters received responses from about 2.5 million people, which is an extraordinarily large number of observations, especially considering that the population of the United States was only about 130 million in 1936, compared to 295 million today. Almost 60% of the respondents[∂] stated that they were going to vote for Landon, whereas only about 40% stated that they were going to vote for Roosevelt. Based on this finding, the pollsters predicted that Landon would win in a landslide. The actual results of the election were reversed: Roosevelt won in a landslide when he received 60% of the popular vote.
What went wrong with the magazine's polling? It may not be immediately obvious today — a time period in which virtually everyone has at least one telephone and virtually every family owns at least one car. But in the middle of the Depression, car and telephone ownership were much less common because many people couldn't afford them. Thus, wealthier people were much more likely to appear in telephone books and car-registration records. What the Literary Digest study had done was poll primarily the well-off and Republican in a country that was primarily poor and Democratic. Another problem was that only about 25% of the original questionnaires were returned. It seems likely that there is a difference between the minority who would take the time to fill out a questionnaire and send it back, and the majority who probably tossed it in the garbage. So, even collecting a sample of 2.5 million people does not guarantee that the results will provide an accurate picture of the population. This very large number of people still made up a biased sample.
With respect to research on the brain, many studies have observed people with various types of brain damage or abnormalities of brain activity. For example, many studies of the cerebral cortex have included participants with epilepsy[∂]. If the brains of epileptic people are organized differently than those of people without epilepsy, then any sample made up of epileptic people would be biased and, therefore, could not be used to generalize about the population consisting of all humans.
Controlling Extraneous Variables. Most of you probably have wondered how much you need to study in order to do well in your courses. Some of you may have heard about the rule-of-thumb[∂] that states that you should study two (or three) hours outside of class for every hour you spend in class. We'll refer to this as the "2-for-1 Rule." The rule claims that, if you go to class for three hours each week, you should study six hours outside of class each week in order to do well in the course. How could you test this claim for its accuracy? Perhaps you could test the claim by remembering courses you have taken in the past. You might remember that you took an American history course last semester and received an A even though you rarely opened the textbook. Instead, you simply listened carefully in class and took good notes, which you reviewed just before each test. In fact, you now recall that you received all As and Bs last semester with very little studying outside of class.
Do these observations show that the 2-for-1 Rule is wrong? Not necessarily. It may be that the courses you took last semester were not a representative sample of courses offered at the college. They may have been less demanding than most other courses. Or it could be that you are misremembering how much you actually studied for your courses, as we learned in Section 2 is quite possible. In other words, you were not making systematic observations when you simply tried to recall what happened in a few courses that you took last semester. Let's look at a fictional research study that includes systematic observations capable of testing the claim.
In our study, let's say that we asked a sample of 80 students to take a week-long course that met every day (Monday through Thursday) for one hour, for a total of four hours of class time, with a test on the last day (Friday). Two variables were measured: the number of hours spent studying and test scores. The students were split into four groups (20 students in each group), and each group was asked to study a different number of hours outside of class for the test (adapted from Goodwin, 1995, pp. 135-36).
Now, let's say that we discovered that Group 4, which had studied two hours for every hour spent in class, did best on Friday's test, Group 3 was next, Group 2 followed them, and Group 1 did the worst on the test. They concluded that the more hours spent studying, the better that one will do on tests.
Is this a reasonable generalization to make? Although it may seem as if the study included systematic observations that support the generalization, you may have noticed a problem with the study. The four groups of students differed not only in the total number of hours that they studied, but also in the number of days between the last time that they studied and the time that they took the test (which is called the "retention interval"). Because of this, we cannot know if the differences observed among the groups in test scores were due to the different amounts of time spent studying, to the different retention intervals, or to both.
When making systematic observations to test a causal claim (such as the claim that two hours spent studying for every hour spent in class causes better test performance), the most important component of the plan is the need to "control for"[∂] the effects of extraneous variables[∂]. In our example, we were unable to generalize about the effect on test scores of the amount of time spent studying because we did not control for the effect of retention interval. When we exert control, we want to be left with only one possible explanation for the results of a study — an explanation that involves only the factor being investigated. In our study, however, there are three possible explanations for the results, none of which can be ruled out:
- Spending more time studying causes higher test scores.
- Studying closer to the time of a test causes higher test scores.
- Spending more time studying and studying closer to the time of a test together cause higher test scores.
In order to systematically observe, we needed to control for the extraneous variable of retention interval. How could we have controlled for the effects of retention interval? Perhaps we could have had the groups study according to the following schedule:
In this case, each group would study only the day before the test, which would control for the extraneous variable of retention interval. But would this schedule allow us to achieve our goal of observing in a systematic manner? No, because it would introduce another extraneous variable: anyone who studies for eight hours on one day will suffer much more fatigue and, thus, have more trouble learning the material than someone who studies only two hours.
In our study, we need to control for the extraneous variables of fatigue and retention interval at the same time. The following schedule would allow us to do this:
If we now find that the students in Group 4 receive the highest average test scores, Group 3 the second highest, Group 2 the third highest, and Group 1 the lowest test scores, we can make the generalization that spending more time studying causes students to receive higher test scores.
Nevertheless, no matter how much care researchers take to control for the effects of extraneous variables, it always is possible that they may miss one or more extraneous variables because they did not think of them. For example, if you hadn't already had a great deal of experience with studying, it may never have occurred to you that a person who studies eight hours in one day may become more fatigued than a person who studies only four hours. This is why it is so important for researchers to describe their procedures very carefully when publishing their studies. This allows other researchers more easily to detect the possible influence of unsuspected extraneous variables and then attempt to replicate the results with a better controlled study of their own.
Study Questions
- What does it mean to systematically observe?
- In general, what must be true of observations if they are to lead to accurate generalizations?
- How would you define a "sample" in your own words?
- How would you define a "population" in your own words?
- What is a representative sample?
- If you wanted to find out what most students at your college plan to do with their education, how would you go about obtaining a representative sample?
- How is a biased sample similar to a representative sample?
- How does a biased sample differ from a representative sample?
- What is epilepsy?
- When do researchers need to control for the effects of extraneous variables?
- How do researchers control for the effects of extraneous variables?
- Use the example above to design a study that tests the following claim: "In a six-row classroom, consistently sitting in the first three rows causes students to do better on tests than sitting in last three rows.