THINKING CRITICALLY What Can fMRI Tell Us About Behavior and Mental Processes
58 Chapter 2 Biology and Behavior FIGURE 2 .7 Combining a PET Scan and Magnetic Resonance Imaging Researchers have superimposed images from PET scans and MRI to construct a three-dimensional view of the living brain. This ﬁgure shows the brain of a young epileptic girl. The picture of the outer surface of the brain is from the MRI; the pink area is from the PET scan and shows the source of epileptic activity. To the right of the ﬁgure are separate MRI and PET images taken at one plane, or “slice,” through the brain (indicated by the line on the brain at the left). A T H I N K I N G C R I T I C A L LY picture may be worth a thousand words, but the pictures of brain activWhat Can fMRI Tell Us About ity offered by fMRI are generating Behavior and Mental millions of them. As of 2005, more than 4,000 scientiﬁc articles have reported the results of Processes? fMRI scans taken while people engaged in various kinds of thinking or experienced various emotions. Neuroscientists who use brain imaging techniques are now to be found in psychology departments around the world, and, as described in other chapters, their work is changing the research landscape in cognitive, social, and abnormal psychology. Excitement over fMRI is not conﬁned to scientists, however. Popular and scientiﬁc magazines routinely carry fMRI pictures that appear to “show” people’s thoughts and feelings as they happen. ■ What am I being asked to believe or accept? In the early 1800s, similar excitement surrounded phrenology, a technique that involved feeling bumps and depressions on the skull. It was claimed that these contours reﬂected the size of 27 structures on the brain’s surface that determine personality traits, mental abilities, talents, and other characteristics. Although wildly popular with the public (Benjamin & Baker, 2004), phrenology did not survive the critical thinking of nineteenth-century scientists, and the technique has long been discredited. Today, some scientists wonder whether fMRI is a twenty-ﬁrst-century version of phrenology, at least in the sense that their colleagues might be accepting its value too readily. These scientists point out that, although fMRI images can indicate where brain activity occurs as people think and experience emotion, there is no guarantee that this activity is actually causing the associated thoughts and feelings (Aldridge, 2005). Questions are also being raised about the assumption that particular thought processes or emotions occur in a particular brain structure, or set of structures. It is easy to talk about “thinking” or “attention,” but these psychological terms might not correspond to speciﬁc biological processes that can be isolated and located by any technology. In short, critics claim that the results of fMRI scans can be misleading and that they don’t necessarily tell us much about how the mind works (Uttal, 2004). Perhaps it would be better to focus on how the brain produces thoughts and feelings instead of searching for their locations. ■ Is there evidence available to support the claim? When the participant in an fRMI experiment thinks or feels something, you can actually see the colors in the brain scan change, much like the color changes you see on The Central Nervous System: Making Sense of the World 59 weather radar as a rainstorm intensiﬁes or weakens. Looking at an fMRI scan, you get a clear impression that the brain areas that “light up” when a person experiences an emotion or performs a mental task are the ones involved in that emotion or task (see Figure 1.5). These scans are not as precise as they seem, though, because fMRI doesn’t directly measure brain cell activity. The colors seen in an fMRI scan reﬂect instead the ﬂow of blood in the brain and the amount of oxygen the blood is carrying. Changes in blood ﬂow and blood oxygen are related to changes in the ﬁring rates of neurons, but the relationship is complex and not yet fully understood (Buxton et al., 2004). Further, when brain cells process information, their ﬁring rates may either increase or decrease (Gonsalves et al., 2005). If the increases and decreases in a particular brain region happen to cancel each other out, an fMRI scan will miss the neuronal activity taking place in that region. In fact, compared with the direct measurement of brain cell activity that can be done in research with animals, fMRI technology is still rather crude. It takes coordinated changes in millions of neurons to produce a detectible change in the fMRI signal. Critics also argue that the results of fMRI research can depend too much on how experimenters choose to interpret them. In a typical fMRI experiment, participants are shown some kind of display, such as pairs of photos, and asked to perform various tasks. One task might be to press a button if the photos are exactly the same. A second task might be to press the button if objects in the photos are arranged in the same way. In this second task, a participant should press the button if one photo shows, say, a short man standing to the left of a tall woman and the other photo shows a small dog standing to the left of a giraffe. Both versions of the task require the participant to compare two images, but only the second of them requires considering whether things that look different are actually similar in some way. The fMRI scans taken during these tasks might show certain brain areas “lighting up” only during the second task. If so, the researcher would suggest that those areas are involved in recognizing analogies, or the similarities between apparently different things (Wharton et al., 2000). The researcher would base this conclusion on a computer program that compares fMRI scans taken during two tasks, subtracts all the “lighted” areas that are the same in both scans, and keeps only those that are different. But what the computer classiﬁes as “different” depends on a rule that is set by the experimenter. If the experimenter programs the computer to display only big differences between the scans, not many “lit up” areas will remain after the comparison process. But if even tiny differences are allowed to count as “different,” many more “lighted” areas will remain after the subtraction process. In our example, then, there could be large or small areas apparently associated with recognizing analogies, all depending on a rule set by the researcher. These problems aside, critics wonder what it really means when fMRI research shows that certain brain areas appear activated during certain kinds of tasks or experiences. Their concern focuses on studies such as one from the new ﬁeld of neuroeconomics that suggests that excessive activity in a particular brain area leads to bad investment decisions (Kuhnen & Knutson, 2005). Another fMRI study claimed to show the “neural basis of romantic love.” In this study, investigators scanned people’s brains as they looked at pictures of their romantic partners and compared these scans to those taken while the same people viewed nonromantic friends (Bartels & Zeki, 2000). According to the “difference” rule established by the experimenters, four brain areas were more active when viewing a romantic loved one than when viewing a friend. But does this result tell us anything about how or why these areas became active or what results this activity might have? In other words, do we know more about love? Critics of fMRI would say no. ■ Can that evidence be interpreted another way? Supporters of fMRI disagree. They believe that the colorful areas seen on fMRI scans can provide vital new information that will eventually allow scientists to answer 60 Chapter 2 Biology and Behavior EXPLORING BRAIN FUNCTIONS WITH fMRI As this participant performs a mental task, a functional magnetic resonance imaging scanner records blood ﬂow and blood oxygen levels in her brain. The resulting computer analysis shows as “lit up” areas the parts of the brain that appear to be activated during the task, but critics doubt that fMRI scanning is as clear or accurate as its proponents suggest. important questions about behavior and mental processes. They point, for example, to fMRI research on brain mechanisms that help us to appreciate what other people are feeling—that is, to experience empathy—and to learn by watching others. These mirror neuron mechanisms were discovered accidentally by scientists who had been using surgical techniques to directly record the activity of brain cells in monkeys’ brains (Rizzolatti et al., 1996). They found that neurons in an area called F5 are activated not only when a monkey plans to reach for an object, such as a peanut, but also if the monkey sees an experimenter reach for a peanut! After fMRI scanning became available, researchers could begin looking for mirror mechanisms in the human brain. And, in fact, some of the mirror systems they found in humans correspond to the F5 region in monkeys (Rizolatti & Arbib, 1998). One of them is called Broca’s area, and, as described later, it is an important component of our ability to speak. It makes sense that Broca’s area contains a mirror mechanism, because language is a skill that we learn partly by imitation. The new fMRI ﬁndings suggest that Broca’s area may also be important for many other skills that involve imitation. One recent study found that this area “lights up” when a guitar student learns chords by watching a professional guitarist (Buccino et al., 2004). Other fMRI research has found that mirror systems in other parts of the brain become active when a person sees someone experiencing emotion. For example, the brain area that is activated when you experience disgust (from the smell of rotten eggs, for example) is also activated if you see a video in which someone else reacts to a smell with disgust (Wicker et al., 2003). So fMRI can be uniquely useful, say its defenders. Without it, research on mirror neurons in humans could not have taken place. And because of it, we have evidence that the experience of empathy comes about because seeing the actions and emotions of others activates the same brain regions that would be active if we were doing or feeling the same things ourselves. Some fMRI studies have also found that malfunctioning mirror mechanisms are associated with the impairments in language development, in imitative skills, and in empathy seen in children diagnosed with autistic disorder (Dapretto et al., 2006; Miller, 2005b; see the chapter on psychological disorders).