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The techniques discussed so far—GSR, EMG, EEG, MEG—are all considered noninvasive because they do not have any effects on the body or brain but merely record electrical activity in a passive manner. In contrast, functional magnetic resonance imaging (fMRI) is categorized as invasive because it temporarily changes the brain. fMRI is a technique for examining the soft structures or tissues of the brain that Xrays cannot capture because they pass right through. The fMRI was a considerable advance over the MRI, because the MRI provides only static images of the brain, akin to a still shot of a running dog. Although static images are useful, tracking changes in the brain over time allows us to better examine brain processes, which is more like watching a video recording of the dog in motion. Ogawa, Lee, Kay, and Tank (1990) were the first to observe that the MRI could be used to examine dynamic processes.

Briefly, a relatively strong magnetic field—one thousand times stronger than the Earth’s—is uniformly applied to the brain, and this field forces randomly oriented hydrogen atoms to change their spatial orientation and become aligned with one another. Next, a radio pulse is sent into the brain that pushes the hydrogen atoms into a position that is at a 90-degree angle from that new alignment. The shift in position of the atoms produces a tiny fluctuation in their magnetic properties. As the brain performs various tasks and functions, oxygenated blood is dispersed to active areas, and this changes the oxygen content of the blood. The magnetic properties of oxygenated and deoxygenated blood diverge slightly, and this difference is what the fMRI actually detects. This is referred to as the blood oxygen level dependent (BOLD) response: Heightened activity of the neurons leads to an increase in both blood flow and the ratio of oxygenated to deoxygenated hemoglobin (which carries the oxygen in the blood) (Cacioppo & Berntson, 2005). Why is this ratio important? The reason is that it shows how the brain changes over time, such as when a person is reading a passage or engaged in social interaction. The fMRI tracks changes in these magnetic properties of the blood to create a series of three-dimensional images that reflect dynamic brain activity.

There are several distinct advantages of fMRI versus other physiological approaches for social psychologists (Cacioppo et al., 2004; Wager & Lindquist, 2011). First, as previously stated, it provides dynamic measurement and thus allows for the observation of changes in the brain over time. Second, it has high temporal resolution, which means that it can capture changes that occur over very short time periods, such as a few seconds. Third, fMRI has very good spatial resolution, and consequently, researchers are able to pinpoint specifically where in the brain the focal activity is located. It is important to note that, unlike the EEG, which directly measures neuronal electrical activity, fMRI measures a consequence of neuronal activity (i.e., magnetic properties associated with BOLD changes) and not the activity itself. Thus it is a little bit like tracking how a tennis ball moves after being hit without recording the action of the racket hitting the ball.

Brain researcher in the control room of a functional magnetic resonance imaging (fMRI) scanner.

Philippe Psaila / Science Source.

fMRI has helped social psychologists understand the physiological basis for a wide variety of social behaviors. Particularly interesting examples are activation of various brain regions in interactive games with other people (Rilling, 2011), differentiating between person and object knowledge (Mitchell et al., 2002), the dehumanization of undesirable others (Harris & Fiske, 2009), and social pain associated with rejection by others (DeWall et al., 2012). Figure 2.8 includes fMRI images obtained in an investigation of brain activity during exposure to social versus nonsocial objects. Later on we will highlight additional fMRI studies that have identified neural correlates to key social behaviors.

Figure 2.8 Social Versus Nonsocial Brain Activation

Source: Mitchell, J. P., Heatherton, T. F., & Macrae, C. N. Distinct neural systems subserve person and object knowledge. Proceedings of the National Academy of Sciences of the United States of America, 99(23), 15238-15243. Copyright © 2002 National Academy of Sciences, U.S.A.. Reprinted with permission.