The Human Amygdala

Data on the amygdala in humans have come primarily from lesion studies and from functional imaging studies [e.g., positron emission tomography (PET) or functional magnetic resonance imaging] that image brain activity in neurologically normal individuals. These studies have provided evidence that the amygdala responds to emotionally salient stimuli in the visual, auditory, olfactory, and gustatory modalities. Lesion studies involve patients who have had amygdala damage because of encephalitis (such as Herpes simplex encephalitis) or other rare diseases or who have had neurosurgical resection of the amygdala on one side of the brain to ameliorate epilepsy.

Human Brain Medial View

Figure 3 Some of the human brain areas important for emotion. (Left) Medial view of left hemisphere showing orbitofrontal cortex and, deep within the temporal lobe, amygdala, as well as hypothalamus and periaqueductal gray. (Right) Lateral view of the right hemisphere showing orbitofrontal cortex and, buried in temporal cortex, amygdala and also somatosensory cortex and, buried under overlying cortex, insula. As shown schematically in Fig. 1, amygdala and orbitofrontal cortex receive perceptual information and project to effector structures including, principally, hypothalamus and periaqueductal gray. These latter structures effect automatic changes in body state, including blood pressure and heart rate etc. The representation of body state (feeling) depends critically on somatosensory areas, including somatosensory cortex and insula. (See color insert in Volume 1).

Figure 3 Some of the human brain areas important for emotion. (Left) Medial view of left hemisphere showing orbitofrontal cortex and, deep within the temporal lobe, amygdala, as well as hypothalamus and periaqueductal gray. (Right) Lateral view of the right hemisphere showing orbitofrontal cortex and, buried in temporal cortex, amygdala and also somatosensory cortex and, buried under overlying cortex, insula. As shown schematically in Fig. 1, amygdala and orbitofrontal cortex receive perceptual information and project to effector structures including, principally, hypothalamus and periaqueductal gray. These latter structures effect automatic changes in body state, including blood pressure and heart rate etc. The representation of body state (feeling) depends critically on somatosensory areas, including somatosensory cortex and insula. (See color insert in Volume 1).

As in animals, the human amygdala appears to be important for fear conditioning—for associating conditioned sensory stimuli with an aversive unconditioned stimulus. A variety of neuropsychological tasks have been used in humans to investigate in more detail both the recognition of emotion from stimuli (such as the recognition of emotions from viewing facial expressions of other people) and the experience of emotion triggered by emotional stimuli or emotional memories. Studies of the amygdala's role in emotion recognition have primarily used photographs of emotional facial expression (such as those developed by Paul Ekman). One subject with selective bilateral amygdala damage has been studied extensively with regard to her recognition of emotion in these photographs of facial expressions. This subject, SM046, has been shown in several different tasks to be specifically and severely impaired in regard to faces of fear. When rating the intensity of emotions in facial expressions, SM046 consistently failed to rate the emotions surprise, fear, and anger as very intense. She was particularly impaired in rating the intensity of fear, on several occasions failing to recognize any fear whatsoever in prototypical facial expressions of fear.

SM046's spontaneous naming of the emotions shown in faces in a labeling experiment using identical stimuli was impaired relative to normal controls: She virtually never used the label "fear," typically mislabeling such faces as surprised, angry, or disgusted. Thus, subject SM046's impairment in recognizing emotional facial expressions is disproportionately severe with respect to fear. However, she also has lesser impairments in recognition of highly arousing emotions that are similar to fear, such as anger. This is consistent with a more general impairment in recognition of negative emotions observed in other subjects with bilateral amygdala damage, and it leads to the question of how specific the amygdala's role is in recognition of certain emotions. Interestingly, SM046 is also impaired in her ratings of the degree of arousal present in facial expressions of emotion. When asked to place photographs of emotional facial expressions on a grid with valence (positive/negative) and arousal (low/high) as orthogonal axes, SM046 was normal in her valence ratings but abnormal in the level of arousal that she assigned to negative facial expressions. Thus, it is not the case that bilateral amygdala damage impairs all knowledge regarding fear; rather, it impairs the knowledge that fear is highly arousing.

A likely interpretation of the previous results from SM046, in conjunction with results from other subjects with bilateral amygdala damage, is that the amygdala is part of a more general neural system for recognizing highly arousing, unpleasant emotions—in other words, emotions that signal potential harm to the organism—and rapidly triggering physiological states related to these stimuli. Such physiological states involve both specific sets of behavioral responses and the modulation of cognitive processes, including those involved in knowledge retrieval necessary for normal performance on the previously mentioned tasks. In animals, the amygdala may trigger predominantly behavioral reactions; in humans, it may trigger both behavior and conscious knowledge that the stimulus predicts something "bad." How it is that conceptual knowledge about the arousal component of emotions comes to depend on the amygdala, in addition to emotional arousal itself depending on the amygdala, is a key issue for future research.

Functional imaging studies in normal individuals have corroborated the lesion studies implicating the amygdala in recognition of signals of unpleasant and arousing emotions. Visual, auditory, olfactory, and gustatory stimuli all appear to engage the amygdala when signaling unpleasant and arousing emotions. These studies have examined the encoding and recognition of emotional stimuli, as well as emotional experience and emotional response, but it has been exceedingly difficult to disentangle all these different components. Although there is now clear evidence of amygdala activation during encoding of emotional material, it is less clear whether the amygdala is also activated during retrieval. Two findings suggest that the amygdala's role may be specific to linking external sensory stimuli to emotion and not for triggering emotional responses that are internally driven. First, subjects with bilateral amygdala lesions can volition-ally make facial expressions of fear. Second, when normal subjects induced a subjective experience offear in themselves while undergoing a PET scan, no activation of the amygdala was observed.

Further insight has come from studies that used stimuli that could not be consciously perceived. Amygdala activation was observed when subjects viewed facial expressions of fear that were presented so briefly they could not be consciously recognized, showing that the amygdala plays a role in noncon-scious processing of emotional stimuli. In summary, an important function of the amygdala may be to trigger responses and to allocate processing resources to stimuli that may be of special importance or threat to the organism, and ecological considerations as well as the data summarized here all appear to argue for such a role especially in regard to rapid responses that need not involve conscious awareness.

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Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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Responses

  • Paciano Romani
    Can the amygdala be damaged on one side of the brain?
    3 months ago

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