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A.
The amygdala is the linchpin of human emotion. A deep structure of the brain, it is located in front of the hippocampus on the underside of the temporal lobes. Its shape is reflected in its name, which is derived from the Greek amugdale, or almond. The vital responsibility of the amygdala is to receive information from the environment, evaluate its emotional significance, and organize a fitWhat is the ting response. amygdala, and It is well situated for this work. From the what are its functions? hypothalamus, it receives information about the body’s reactions to the environment (for example, heart rate, blood pressure, digestive processes). It responds to the brain’s conscious evaluation of noteworthy events by communicating with the cerebral cortex. The amygdala coordinates these complementary viewpoints, integrating conscious experience with physical sensations that are not under conscious control. And through its connections to the hippocampus, a center of memory consolidation, the amygdala plays a role in registering emotional memories. Because virtually all brain circuits governing emotion are connected to the amygdala, neuroscientists have studied it closely. With functional and structural (three-dimensional) magnetic resonance imaging and positron emission tomography, they can watch the amygdala and related structures in action under controlled conditions. Here are some examples of what they’ve found: Amygdala activity is greater when subjects view pictures with emotional as opposed to neutral content, and they remember those emotionally charged pictures better. The left amygdala of patients with borderline personality disorder becomes more active than the same structure in control subjects when they are shown facial expressions. A similar increase in amygdala activity occurs during some phases of PTSD. The amygdala may be smaller and less active than average in people with schizophrenia (who have trouble “reading” emotions). People with social anxiety show more amygdala activity than others when they look at pictures of angry faces. People
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with genetically determined irregularities in the transport of the chemical messenger serotonin, who are thought to be more vulnerable to depression, also have hyperactive amygdalae. Researchers can also learn about the amygdala when it is absent. Experimental animals whose amygdalae have been removed are unusually tame. This observation confirms what we know from other experiments — that the amygdala is especially devoted to registering and managing fear. One of its main functions is to mobilize us in an emergency, sometimes before we are fully conscious of the threat. The activity of the amygdala may also explain how fear becomes contagious. Sometimes survival depends on our recognizing that others are afraid, and that may be impossible without a functioning amygdala. In a recent report in the journal Nature, researchers discuss the case of a woman with severe amygdala damage from a chronic illness. She does not recognize fear in facial expressions because she is not attentive to cues from the eyes. The researchers were able to help her recognize a fearful expression by directing her to look at eyes, but even with training she could not do it spontaneously. Apparently the amygdala doesn’t just passively absorb information but prompts the visual system to search for the information and make it available for evaluation. The dark side may not be the amygdala’s only concern. Recent studies in which brain scans were taken while viewers watched “Seinfeld” and “The Simpsons” have explored how the amygdala reacts to humor. Other research has shown how it may help us recognize scary music in a movie soundtrack. It would be lovely to learn that the amygdala not only helps keep us safe but makes life more interesting and enjoyable.
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Michael Craig Miller, M.D. Editor in Chief Because of the volume of correspondence we receive, we can’t answer every letter or message. Nor can we provide personal medical advice.
MAY 2005
