Hockenbury Discovering Psychology 5th txtbk

508 CHAPTER 12 Stress, Health, and CopingFOCUS ON NEUROSCIENCEThe Mysterious Placebo EffectThe placebo effect is perhaps one of the most dramatic examplesof how the mind influences the body. A placebo is an inactivesubstance with no known effects, like a sugar pill or aninjection of sterile water. Placebos are often used in biomedicalresearch to help gauge the effectiveness of an actual medicationor treatment. But after being given a placebo, many researchparticipants, including those suffering from pain or diseases,experience benefits from the placebo treatment. How can thisbe explained?In Chapter 2, we noted that one possible way that placebosmight reduce pain is by activating the brain’s own naturalpainkillers—the endorphins. (The endorphins are structurallysimilar to opiate painkillers, like morphine.) One reason forbelieving this is that a drug called naloxone, which blocks thebrain’s endorphin response, also blocks the painkilling effects ofplacebos (Fields & Levine, 1984). Might placebos reduce pain byactivating the brain’s natural opioid network?A brain-imaging study by Swedish neuroscientist PredragPetrovic and his colleagues (2002) tackled this question. In thestudy, painfully hot metal was placed on the back of each volunteer’shand. Each volunteer was then given an injection of eitheran actual opioid painkiller or a saline solution placebo. About30 seconds later, positron emission tomography (PET) was usedto scan the participants’ brain activity.Both the volunteers who received the painkilling drug and thevolunteers who received the placebo treatment reported that theinjection provided pain relief. In the two PET scans shown here,you can see that the genuine painkilling drug (left) and theplacebo (right) activated the same brain area, called the anteriorcingulate cortex (marked by the cross). The anterior cingulateReceived opiate painkillerReceived placebocortex is known to contain many opioid receptors. Interestingly,the level of brain activity was directly correlated with the participants’subjective perception of pain relief. The PET scan on theright shows the brain activity of those participants who hadstrong placebo responses.Many questions remain about exactly how placebos work, butthe PET scan study by Petrovic and his colleagues (2002) vividlysubstantiates the biological reality of the placebo effect. In a recentstudy, Jon-Kar Zubieta and his colleagues (2005) showedthat a placebo treatment activated opioid receptors in severalbrain regions associated with pain. Further, the greater the activation,the higher the level of pain individual volunteers wereable to tolerate. As these studies show, cognitive expectations,learned associations, and emotional responses can have a profoundeffect on the perception of pain. Other studies haveshown that placebos produce measurable effects on other typesof brain processes, including those of people experiencingParkinson’s disease or major depression (Fuente-Fernández &others, 2001; Leuchter & others, 2002).Figure 12.5 Stress and the CommonCold Are you more likely to catch a coldif you’re under a great deal of stress? In aclassic series of studies, Sheldon Cohen andhis colleagues (1991, 1993) measured levelsof psychological stress in healthy volunteers,then exposed them to a cold virus.While quarantined in apartments for aweek, the participants were monitored forsigns of respiratory infection. The results?As shown in the graph, the researchersfound an almost perfect relationshipbetween the level of stress and the rate ofinfection. The higher the volunteers’psychological stress level, the higher therate of respiratory infection.What are the practical implications of reduced immune system functioning?One consistent finding is that psychological stress can increase the length of timeit takes for a wound to heal. In one study, dental students volunteered to receivetwo small puncture wounds on the roofs of their mouths (Marucha & others,1998). To compare the impact of stress on wound healing, the students receivedthe first wound when they were on summer vacation and the second woundthree days before their first major exam during the fall term. The results? Thewounds inflicted before the major test healed an average of 40 percent morePercentage of participantswith respiratory infections504540353025Low Average HighLevel of psychological stressslowly—an extra three days—than thewounds inflicted on the same volunteersduring summer vacation. Otherstudies have shown similar findings(Glaser & Kiecolt-Glaser, 2005).What about the relationship betweenstress and infection? In a series of carefullycontrolled studies, psychologistSheldon Cohen and his colleagues(1991, 1993, 1998, 2006) demonstratedthat people who are experiencing highlevels of stress are more susceptible to infectionby a cold virus than people whoare not under stress (see Figure 12.5).