One way to study the characteristics of the conscious level is to look for relationships between brain activity and altered states of consciousness. Such relationships can be measured by calculating correlations. A correlation is an estimate of the degree to which two factors are associated — the degree to which two “things” change together. For example, it should be obvious that the number of hours spent studying for a test (Factor 1) is correlated with scores on the test (Factor 2). On average, the more hours spent studying for a test, the higher are test scores. Test scores also are correlated with the amount of time spent watching television. On average, the more time spent watching TV, the lower are test scores, probably because people can’t study much if they’re watching TV. These two examples show that factors can be correlated in two ways:
- as the value of one factor increases, the value of the other factor increases (of course, this is the same as saying ‘as one decreases, the other decreases’);
- as the value of one factor increases, the value of the other factor decreases.
We’ll return to the topic of correlations in Chapter 3. For now, just remember that, in the behavioral sciences, an important type of research study involves looking for correlations among factors.
Our modern understanding of the brain is based to a large degree on correlational studies. For decades, researchers have looked for correlations between brain activity and mental/behavioral functioning. In Section 2-5, for example, you learned that damage to different parts of the brain is correlated with various impairments in cognitive, emotional, and/or behavioral functioning. In these cases, the damaged areas of the brain showed little or no activity. Other correlational studies use intact brains (i.e., brains that have no obvious damage). In these studies, researchers measure changes in brain activity directly and correlate these changes with changes in mental/behavioral functioning. Thus, correlational studies of the brain are of two types:
- Correlational Studies of Brain Damage. Researchers correlate the location of damage with the severity of mental/behavioral impairments. This type of study requires the ability to observe damaged brain structures directly. One way to do this is to measure mental/behavioral impairments while people are alive and then examine their brains after death. Other techniques allow researchers to see damaged structures in living people, such as X-rays, computed tomography (CT) scans, and magnetic resonance imaging (MRI).
- Correlational Studies of Brain Activity. Researchers correlate changes in brain activity with changes in mental and/or behavioral functioning. As you may already know, brain activity involves biochemical activity in billions of brain cells called neurons. These biochemical changes result in electrical activity within and between neurons. There are a number of techniques that can measure biochemical or electrical activity in the brain.
A technique that is often used to measure electrical activity in the brain during sleep is the electroencephalograph (EEG). Much of the discussion that follows is based on EEG research, so it’s important to know some limitations of the EEG. First, the EEG primarily measures the electrical activity of only those neurons on the brain’s surface, which means that researchers can’t observe the activity of neurons deeper in the brain. Second, the EEG measures the summed electrical activity of millions of neurons at a time, which means that researchers can’t observe single neurons or small groups of neurons. Even with these limitations, however, the EEG has helped brain researchers learn a great deal about the relationships between states of consciousness and brain activity.
Types of EEG Activity
Right now, you probably are in a state of consciousness characterized by alert wakefulness. If you were hooked up to an EEG machine, it would indicate a high level of electrical activity in your cerebral cortex — the wrinkly outer part that makes up most of the human brain. If you currently were in a coma, on the other hand, the EEG would indicate a very low level of electrical activity in your cortex. The EEG measures brain activity by transforming the electrical activity of millions of neurons into patterns called brain waves. Changes in a person’s state of arousal (i.e., whether a person is awake, asleep, comatose, etc.) are correlated with changes in brain waves. The most important types of brain waves for our purposes are:
- Beta waves. These indicate that a person is actively attending to (is aware of) both internal (mental) and external events.
- Alpha waves. These indicate that a person is awake but in a relaxed mental state in which he or she is not paying much attention to the external world (i.e., his or her “mind is wandering”). Alpha waves usually are observed when people are daydreaming or just about to fall asleep.
- Theta waves. These indicate that a person is in a light sleep from which he or she can be awakened easily.
- Delta waves. These indicate that a person is in a deep sleep from which he or she cannot be awakened easily.
Figure 1 illustrates these four patterns of brain activity.
The Science of Sleep
In Section 1-2, you learned about two precepts (rules or principles) of science: the scientific approach to testing claims requires thinking that is (1) empirical and (2) skeptical. Sleep research is scientific when it does the following.
- It tests claims empirically. Claims about sleep are tested by making direct observations of individuals during sleep. For example, researchers interested in dreaming might wait unti people are dreaming and then wake them up in order to describe what they were dreaming about.
- It subjects claims to skeptical inquiry. Claims about sleep are doubted until tests of these claims provide evidence adequate for accepting them. Skeptical inquiry repeats itself whenever new evidence suggests that a claim may be wrong.
Before researchers can empirically test claims, they need to solve a major problem: what should they observe? Should they observe changes in bodily position, blood pressure, digestion, heart rate, brain activity, mental events? Many changes occur during sleep and it’s difficult to know which of them to observe. It should come as no surprise that what scientists choose to observe depends on what prior research has suggested to be important causal factors. In other words, in order to know what to observe, scientists must already have a theory in mind — a theory that specifies the likely causes of whatever it is they’re trying to understand, even if that theory is not yet supported by much evidence.
Let’s use an everyday example to examine this isse. If you want to understand why your car won’t start, what kinds of direct observations should you make? The answer depends on what you believe to be important proximal causes of starting a car’s engine. If your theory states that cars start because of invisible elves in the glove compartment who run into the engine and move various parts by pushing on them, then you’ll probably make detailed observations of the glove compartment to determine if anything is preventing the elves from running into the engine. The normal functioning of a car, of course, does not depend on the actions of invisible elves: the correct theory states that a car’s engine starts because of the combustion of fuel, which provides the energy to activate various engine parts. Anyone accepting this theory will make careful observations of the engine’s components — those that the theory states are most important for starting cars. Invisible elves, of course, still may live in the glove compartment, but you don’t need to worry about them if you’re trying to fix the car.
In the rest of this chapter, you’ll learn about research involving direct observations of (a) electrical activity in the brain and (b) mental events during sleep (e.g., dreaming). The reason for observing electrical activity in the brain should now be obvious: brain activity is the most-proximal cause of mental events and behaviors. The reason for observing mental experiences during sleep should also be obvious: such experiences have been a major focus of psychological research since its beginning, and they are what makes sleep most interesting to all of us.
Changes in Brain Activity During Sleep
In 1924, a German physician named Hans Berger was the first to record electrical activity in the human brain using a primitive EEG machine he had invented (Millet, 2001). Berger began publishing a series of article in 1929 describing, among other things, changes in EEG during sleep. In the 1930s, Loomis, Harvey, and Hobart (1935, 1937, 1938) reported their observations of EEG activity in sleeping subjects. They found that sleep consisted of several different “states,” each of which has its own characteristic pattern of electrical activity. They also found that, during sleep, changes from one state to another tended not to be gradual: state-to-state changes often occurred abruptly, which led them to conclude that “the old idea of a continuous curve of sleep needs modification” (1937, p. 136).
In the early 1950s, two researchers at the University of Chicago — Eugene Aserinky (1921-1998) and Nathaniel Kleitman (1895-1999) — observed EEG activity during sleep using much more-sophisticated techniques than had earlier researchers. Kleitman, who began his investigations of sleep in the 1920s, was the first researcher to focus exclusively on the scientific study of sleep. Aserinsky was a graduate student in Kleitman’s laboratory when he began the research that was to revolutionize sleep science. Aserinsky and Kleitman (1953) observed different states or stages, one of which had never been noticed before — a stage characterized by jerky eye movements and very high EEG activity. This discovery triggered a flood of scientific research on sleep and dreaming that continues to the present.
Up until the publication of Aserinsky and Kleitman’s 1953 paper, many people assumed that after we fall asleep, brain activity decreases more and more, and that it doesn’t increase start to wake up again. Aserinsky and Kleitman, therefore, were not interested in studying EEG activity during sleep. Instead, Aserinsky’s research was to focus on eye movements during sleep (Dement & Vaughan, 1999), although he needed to measure brain waves in order to know what stage of sleep his subjects were in. Before starting the research, he tested the EEG machine on his eight-year-old son because he thought that it might be having some problems (Brown, 2003). To his surprise, he observed his son’s eyes moving back-and-forth rapidly several times during the night, and noticed that these eye movements were correlated with increased brain activity — activity that was very similar to that of people who are wide awake. He confirmed these observations when he tested a group of adult subjects. Aserinsky and Kleitman named this stage rapid-eye-movement (REM) sleep because of the rapid bursts of eye movements that helped to distinguish it from other stages of sleep. Dement and Kleitman (1957a) confirmed that there is a strong correlation between rapid eye movements and dreaming: 93% of all dreams reported by their subjects occurred during REM sleep (see Table 2 in Dement & Kleitman).
Over the years that followed, Kleitman and his colleagues continued to study the stages of sleep. Among many other discoveries, they found that (a) the time at which each stage of sleep occurs during the night, and (b) how long each stage lasts, varies in two ways:
- Each person tends to show differences in the timing and duration of sleep stages from one night to the next.
- Each person tends to differ from other people in the timing and duration of sleep stages.
These results, however, are not surprising. Virtually every biological and psychological phenomenon shows variations similar to these. Before continuing the discussion of sleep research, therefore, let’s look at the issue of variation within and between individuals.
Study Questions for Section 2-6
- How would you define in your own words a correlation?
- What are examples from your own life of the two types of correlation described above?
- What are the two types of correlational study used in brain research?
- What is the EEG and what can it tell us about altered states of consciousness?
- What are the four types of brain wave described above? What state of consciousness is each correlated with?
- What are two main tenets of science?
- How do these two tenets apply to research on sleeping and dreaming?
- What did Aserinsky and Kleitman (1953) initially believe about changes in brain activity during sleep?
- How did the observations made by Aserinsky and Kleitman (1953) show that this belief was incorrect?
- What is rapid-eye-movement (REM) sleep? Why was it a surprising discovery and what made it so important for sleep research?
- What are the two kinds of variation seen in the timing and duration of sleep stages?
Practice Quiz for Section 2-6
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