have employed EMG to measure muscle movements in the face that are thought to reflect emotional and/or attitudinal states, and self-reports of depth of message processing (Cacioppo & Petty, 1981). The underlying rationale is that the activity of specific muscles is correlated with verbal self-reports and behavioral measures, and thus the EMG can provide additional evidence of cognitive and affective processes (Blascovich, 2000; Blascovich & Seery, 2007). For instance, Cacioppo and Petty (1981) describe how increased movement (and electrical activity) of facial muscles associated with speech parallel other measures of deeper processing of persuasive messages. As we’ll discuss in Chapter 7, how carefully we examine the arguments in a message impacts how much we are persuaded by them. It is fascinating that our facial muscles can reflect whether or not we are thinking deeply or superficially! Facial EMG can also detect the positivity or negativity of emotional reactions and the intensity of those reactions among message recipients during message exposure (Cacioppo, Petty, Losch, & Kim, 1986). Social psychologists have utilized facial EMG in research on the mere exposure effect (see Chapter 11) (Harmon-Jones & Allen, 2001), stereotyping (see Chapter 10) (Vanman, Saltz, Nathan, & Warren, 2004), and other topics in social psychology (Bartholow & Dickter, 2007; Hess, 2009).
Electroencephalography (EEG) and Magnetoencephalography (MEG)
An EEG measures electrical activity generated by the brain, whereas an MEG targets the magnetic signals associated with electrical output. You have most likely read about EEG in your introduction to psychology course in the context of measuring brain wave patterns as a way to identify and track the stages of sleep. EEGs are also used to record abnormal electrical activity associated with epilepsy. Regardless of its purpose, an EEG is conducted by placing electrodes at numerous places on the scalp in order to detect brain wave patterns. Social psychologists are specifically interested in how the brain responds to particular social stimuli and therefore focus on changes in the electrical activity, called event-related potentials (ERPs), during and immediately after stimulus presentation. These stimuli can be visual, auditory, olfactory, or tactile (Cacioppo, Lorig, Nusbaum, & Berntson, 2004).
A patient undergoes an electroencephalography (EEG). EEG examines the electric activity of the brain using electrodes placed at key places on the skull.
C3336 Klaus Rose Deutsch Presse Agentur/Newscom.
Given that ERPs are fairly weak and can easily be drowned out by baseline electrical activity, researchers enhance signal detection by gathering data over many trials and averaging the data from multiple electrodes. MEGs typically utilize form-fitting caps with multiple electrodes that, like EEGs, examine how the brain responds to various stimuli. Both technologies are useful for examining these responses over time. For instance, researchers interested in person perception have measured differences in ERPs with an EEG immediately after a person’s exposure to body movements (Puce & Perrett, 2005) or to faces from blacks and whites (Ito, 2011). The magnitude of the ERP should increase when a person is exposed to stimuli she perceives as very different from one another (such as when viewing a black and then a white face). Similarly, ERPs associated with the categorization of faces have been studied using MEG (Bartholow & Dickter, 2007; Liu, Harris, & Kanwisher, 2002). Figure 2.7 shows how the amplitude of the ERP response at particular brain sites is greater after exposure to human faces than to other objects (Liu et al., 2002). This difference in activation provides further evidence that the brain processes social and nonsocial information in different ways.
Figure 2.7 Strength of ERP Response to People Versus Other Objects
Source: Liu, J., Harris, A., & Kanwisher, N. (2002). Stages of processing in face perception: An MEG study. Nature Neuroscience, 5(9), 911.
Event-Related Potentials (ERPs): Changes in electrical activity in the brain that reflect how it responds to particular stimuli
Functional Magnetic Resonance Imaging (fMRI)
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 &
