Brain structures and their functions may be investigated through research using case studies, correlational studies, and experimental studies. Case studies and correlational studies help us to locate parts of the nervous system that might have particular functions (the word function here refers to the characteristic action of something—what that thing does). Experimental studies, on the other hand, help us to confirm that a part of the nervous system does, in fact, have a particular function (that is, that it causes particular cognitions, emotions, and/or behaviors to occur). You already have learned about the use of case studies. Therefore, in this section, you will learn about the use of correlational and experimental studies in brain research.

Correlational Studies of the Brain

In correlational studies, researchers measure two variables in a group of subjects in order to determine whether the two variables are correlated — whether the variables change together in the same direction (a positive correlation) or in opposite directions (a negative correlation).

In correlational studies of the brain, researchers measure, in a large group of subjects, a characteristic of a brain structure (such as its size, its activity, or the amount of damage it has sustained) and a cognitive, emotional, or behavioral event (such as the ability to speak, the amount of anxiety experienced, or the ability to move one's arm). For example, people who have had a stroke (a disruption of blood flow to a part of the brain) typically develop disturbances of brain functioning because the cells that make up the brain die at the location of the stroke. These disruptions of brain functioning often are correlated with cognitive, emotional, or behavioral impairments, such as difficulties with speaking, writing, reading, and/or understanding the speech of others. Correlational studies have shown, for instance, that people who have damage to the outer layer of the left side of the brain near the front are likely to have language difficulties. A study measuring the language abilities of stroke patients would be a correlational one because the researchers have not manipulated either the functioning of the brain or the language difficulties. Instead, they simply have measured these variables as they are expressed currently in people who have had strokes relative to those who have not had strokes.

In general, there are two types of correlational study used to investigate the functions of the nervous system:

• Studies of brain damage. In a damaged nervous system, researchers measure the amount of damage to particular neural[∂] structures and correlate this damage with specific abnormalities of behavior and mental processes. In order to use this method, we need to be able to see the structures of the nervous system. Autopsies often are performed after subjects die in order to find damaged structures. There also are several techniques that allow us to get images of the brains of living subjects. Although X-rays often have been used for this purpose, there are much better techniques now available — techniques that give much clearer pictures of the brain (such as magnetic resonance imaging, or MRI).
• Studies of brain activity. In an undamaged nervous system, researchers measure disturbances of neural activity (both electrical and biochemical activity) and correlate these disturbances with specific abnormalities of behavior and mental processes. The EEG is one method for measuring changes in the electrical activity of the brain. There also are other techniques now used that give us much more information about brain activity (such as positron emission tomography, or PET scans).

Because correlational studies are limited with respect to making cause-and-effect interpretations, you might wonder why we ever would use such studies to investigate brain functions. There is one major reason why such correlational studies are used: they allow us to detect associations that then might be studied experimentally. Furthermore, in human research, correlational studies often are the only kind of study we can perform because ethical considerations make experimentation very difficult or impossible (for example, we cannot purposely damage the brain of a human just to see what will happen). Reserachers, however, can discover causal relationships by performing experiments on nonhuman animals, and these findings may help us to better understand what is happening in humans.

Experimental Studies of the Brain

In experimental studies, researchers manipulate independent variables in order to see their effects on dependent variables; and they use methods, such as random assignment, to make the experimental and control groups as similar as possible (with respect to the effects of extraneous variables) at the start of the experiment.

In experimental studies of the brain, researchers manipulate the activity of a particular brain structure in order to show that activity of that brain structure causes particular cognitions, emotions, and/or behaviors to occur. For example, correlational studies of stroke victims have shown that damage to the left-frontal part of the brain is correlated with difficulties in language production. Researchers have tested experimentally the hypothesis that this area of the brain is important for language production by using sodium amobarbital, which is a drug that, when injected into an artery supplying blood to one side of the brain, will anesthetize that side of the brain for several minutes (this is referred to as the Wada Test). The procedure allowed researchers to perform experiments testing the following prediction (see Table 1): anesthetizing the left side of the brain will cause language difficulties.

Table 1. Design of an experiment testing the hypothesis that anesthetizing
the left side of the brain will cause language difficulties

In the experimental group, participants are subjected to anesthesia of the left hemisphere because that is the condition predicted to cause language difficulties. Just to make sure that it is not the case that anesthesia anywhere in the brain causes language difficulties, participants in the control group are subjected to anesthesia of the right hemisphere.

Rasmussen and Milner (1977) performed this experiment with 134 right-handed participants and 122 non-right-handed (left-handed and mixed-handed) participants. Their results appear in Table 2. Almost all of the right-handed participants (96%) experienced problems in language production when the left sides of their brains were anesthetized, whereas only 4% experienced problems in language production when the right sides of their brains were anesthetized.

Table 2. The association between handedness and hemispheric dominance
for language based on results from the Wada Test

Of much interest are the results of non-right-handed participants (those who either used their left hands for most tasks or used their rights hands for some tasks and their left hands for other tasks). Exactly 70% experienced problems in language production when the left sides of their brains were anesthetized (less than the 96% of right-handed people who experienced similar problems), 15% experienced problems in language production when the right sides of their brains were anesthetized (more than the 4% of right-handed people who experienced similar problems), and 15% experienced no problems when either side of their brains were anesthetized.

The results of this study allow us to conclude that, on average, activity in the left hemisphere of the cerebral cortex is necessary (it causes) the human ability to produce language:

• in the vast majority of right-handed people (96%);
• in a (smaller) majority of left-handed people (70%).

Results such as these show why it is important to consider both the average and the amount of variation around the average when interpreting the results of research studies.

Several experimental methods may be used to investigate the causal effects of nervous-system activity on cognitions, emotions, and behavior.

• Lesioning studies. Neural structures or pathways are lesioned[∂]— surgically removed or cut — and the effects of these lesions on mental functions and behaviors are observed. Human participants can be used for such studies only in cases where they are being treated for a medical disorder with neurosurgery.
• Electrical Stimulation Studies. Electrical and chemical activity in brain structures and pathways can be increased or decreased by direct electrical stimulation and the effects of these manipulations on mental functions and behaviors are observed. Again, human participants can be used for such studies typically only in cases where they are being treated for a medical disorder with neurosurgery.
• Magnetic Stimulation Studies. During the late 1980s, a new technology called transcranial magnetic stimulation (TMS) was developed. TMS uses magnetic fields to change electrical activity in localized areas of the brain. Because surgery is not needed to open the skull and peel away the protective tissues surrounding the brain, TMS is a safer way to directly stimulate neural structures. TMS not only is being used for basic research on the brain, it also is being studied for possible use as a biological therapy for mental disorders (see Couturier, 2005, for a review of research on the use of TMS for the treatment of major depression.)
• Drug Studies. Drugs are used to change brain activity and the effects of these drugs on mental functions and behaviors are observed. These studies provide the best information when it can be determined where in the brain they are having their effects, which is easiest to do with nonhuman animals (such as rats), especially when a drug is injected directly into a specific area.

Study Questions for Section 3-8

1. What are the two types of correlational study used to investigate the functions of brain structures?
2. Given that we cannot make cause-and-effect interpretations of the results of a single correlational study, why are correlational studies of the brain performed?
3. What are the four types of experimental study used to investigate the functions of brain structures?
4. Which of these methods seem to you to be the least dangerous? Why?
5. Which of these methods seem to you to be the most dangerous? Why?
6. How would you describe the Wada Test in your own words?
7. According to the results of Rasmussen and Milner (1977), activity in which side of your brain is necessary for the production of language?
8. Why were you unable to give a single answer to the previous question

Go to Quiz 3-8 questions

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