Hockenbury Discovering Psychology 5th txtbk

350 CHAPTER 8 Motivation and EmotionFigure 8.10 FearCircuits in the Brain Whenyou’re faced with a potentiallythreateningstimulus—like a snakedangling from a stick—information arrives inthe thalamus, and is relayedsimultaneouslyalong two pathways.Crude, archetypal informationrapidly travels thedirect route to the amygdala,triggering an almostinstantaneous fear response.More detailed informationis sent along thepathway to the visual cortex,where the stimulus isinterpreted. If the cortexdetermines that a threatexists, the information isrelayed to the amygdalaalong the longer, slowerpathway. The amygdalaVisual cortexAmygdalaThalamusSympatheticnervous systemtriggers other brain structures, such as thehypothalamus, which activate the sympatheticnervous system and the endocrine system’srelease of stress hormones.Source: Adapted from LeDoux (1994a, b).EndocrinesystemLeDoux (1996) describes the direct thalamus→amygdalapathway as a “shortcut” from the thalamus to the amygdala.This is a “quick-and-dirty” route that transmitscrude, almost archetypal, information about the stimulusdirectly to the amygdala. This rapid transmission allowsthe brain to start to respond to the possible danger representedby a writhing, curved snake dangling from a stickbefore you have time to consciously think about the stimulus.The amygdala activates and triggers the brain’s alarmsystem.What happens next? The amygdala sends informationalong neural pathways that project to other brainregions that make up the rest of thebrain’s fear circuit. One pathwayleads to an area of the hypothalamus,then on to themedulla at the base ofthe brain. In combination,the hypothalamus andmedulla trigger arousal of thesympathetic nervous system. Anotherpathway projects from theamygdala to a different hypothalamusarea that, in concert with the pituitary gland, triggers the release of stress hormones(LeDoux, 1995, 2000).The result? You respond instantly to the threat—“SNAKE!”—by leaping backward,heartbeat and breathing accelerating. But even as you respond to the threat,information is speeding along the other neural pathway that reaches the amygdalaby traveling through the cortex. The thalamus sends information to the visual cortex,which creates a detailed and more accurate representation of the visual stimulus.You can also reevaluate the signal that prompted the initial instinctive response.Now you realize that the “snake” is actually just a rubber toy. The cortex sends the“false alarm” message to the amygdala. But note that information traveling thethalamus→ cortex→ amygdala route takes about twice as long to reach the amygdalaas the information traveling along the direct thalamus→amygdala route. Thus, thealarm reaction is already in full swing before signals from the cortex reach theamygdala.LeDoux believes these dual alarm pathways serve several adaptive functions. Thedirect thalamus→amygdala pathway rapidly triggers an emotional response to threatsthat, through evolution, we are biologically prepared to fear, such as snakes, snarlinganimals, or rapidly moving, looming objects. In contrast, the indirect pathway allowsmore complex stimuli to be evaluated in the cortex before triggering the amygdala’salarm system. So, for example, the gradually dawning awareness that your job is injeopardy as your boss starts talking about the need to reduce staff in your departmentprobably has to travel the thalamus→cortex→amygdala pathway before you begin tofeel the cold sweat break out on your palms.In situations of potential danger, it is clearly advantageous to be able to respondquickly. According to LeDoux (1995, 2000), the direct thalamus→amygdala connectionrepresents an adaptive response that has been hard-wired by evolution intothe human brain. From the point of view of survival, as LeDoux (1996) remarks,“The time saved by the amygdala in acting on the thalamic interpretation, ratherthan waiting for the cortical input, may be the difference between life and death.”In support of this evolutionary explanation, Swedish psychologist Arne Öhmanand his colleagues (2001b; Schupp & others, 2004) have found that people detectand react more quickly to angry or threatening faces than they do to friendly or neutralfaces. Presumably, this reflects the faster processing of threatening stimuli via thedirect thalamus→amygdala route (Lundqvist & Öhman, 2005).