The Amygdala

The Amygdala: An Interference Between Memory and Emotion

The Amygdala plays a major role in the analysis of affective information and the expression of emotional output. This literature has been developed to emphasize the role of the amygdala as an interface between memory and emotion.

The initial evidence for this role of the amygdala came from the amnesic patient H.M. Opens in new window His surgery, which included bilateral removal of his amygdala, left him with a decreased ability to access information about his internal states.

In a systematic study of his responsiveness to pain and hunger (Hebben et al., 1958), H.M. differed from other amnesic patients in whom the amygdala was intact, and from neurologically intact individuals, as he failed to identify pain stimuli as “painful” no matter how intense they were. He also failed to show changes in his ratings of hunger before and after meals. Indeed, on one occasion, he rated his hunger as 50 on a scale of 0–100 both before and after a full dinner.

Afterward, he was engaged in conversation with the experimenters and then given another full dinner. He did not remember the earlier dinner, ate the second dinner at his usual pace, and when done, still rated his hunger as 50.

Since those initials studies, we have learned that the amygdala plays two distinct and critical roles in the interaction of emotion and memory.

1. First, it mediates the learning and expression of emotional responses to stimuli whose emotional significance is not automatic but has been learned via association.
2. Second, it allows emotional experience to modulate certain aspects of long-term memory. Each of these roles is discussed in turn.

Role #1

The amygdala is critically involved in emotional memory and the learning of emotional responses. Perhaps the best-studied example of emotional memory involves the brain system that mediates Pavlovian fear conditioning (e.g., LeDoux, 2000), in which a stimulus comes to invoke fear because it is paired with an aversive event.

For example, rats are placed in a chamber in which they are presented multiple times with a 10-second pure tone that is terminated with a brief electric shock through the floor of the cage. They come to exhibit conditioned fear to the subsequent presentation of just the tone, because it was paired with the shock. This fear is expressed by changes in automatic responses, such as arterial blood pressure; in motor responses, such as stereotypic crouching or freezing behavior; and in suppression of the urge to drink sweetened water, which rats usually like.

Animals with selective lesions in the lateral amygdala show dramatically reduced conditioned autonomic and motor responses to the tone. Intact animals also exhibit contextual fear conditioning, meaning that their fear response is selective to the context, or environment, in which conditioning occurs.

When intact rats are again placed in the conditioning chamber after initial exposure, they begin to freeze even before the tone is presented. Their reactions have been conditioned both to the tone and to the environmental context in which tones and shock have been paired. If placed in a different environment, they do not freeze unless a tone is presented. Amygdala lesions block this contextual fear conditioning, just as they block Pavlovian fear conditioning.

Converging evidence for the role of the amygdala in emotional learning is provided by studies in humans (Phelps & LeDoux, 2005). For example, fear conditioning is associated with increased activation of the amygdala activation as measured by fMRI, and the larger the amygdala activation, the stronger is the conditioned fear response (Phelps et al., 2004).

Damage to the amygdala precludes individuals from exhibiting a conditioned fear response. Nonetheless, if their hippocampus is intact, they can report the particulars of the conditioning paradigm, such as that a specific tone was paired with a shock. In contrast, individuals with hippocampal damage will show the appropriate physiological response to fear conditioning, although they cannot recall the particulars of the conditioning paradigm (Bechara et al., 1995). Stimulus reward learning is also disrupted in humans with amygdala damage (Johnsrude et al., 2000).

Role #2

The second contribution of the amygdala to memory involves the modulation of memory by emotional experiences.

Strong evidence for this comes from a paradigm developed by Cahill and colleagues for studying human memory (1999). Their test involves presentation of a single series of slides and two alternatives narratives, one of which is emotionally charged (e.g., a story about a mother and son involved in a traumatic accident), and one of which is not (e.g., a story about a safety drill).

In subsequent delayed-memory testing, neurologically intact people showed a selective enhancement of recall for the emotional component of the tragic story but not the analogous portion of the neural story.

Bilateral damage to the amygdala in a patient with Urbach-Wiethe syndrome selectively wiped out the enhancement of memory for the emotional part of the tragic story, but did not affect memory for the neutral components of the story. This damage also did not prevent such individuals from appreciating the emotional content of the tragic story. Moreover, when viewing emotionally intense stimuli, the greater the degree of activation of the amygdala is for a particular item, the better the subsequent recall of the item (Canli et al., 2000).

The amygdala is thought to exert such an influence over memory through its interaction with many structures involved in memory processing, such as the hippocampus and striatum, and other regions, such as frontal cortex.

The amygdala is also thought to respond to emotional situations that are highly arousing. When it becomes active, it in turn influences portions of the memory circuitry in the brain. Thus, at least a portion of the amygdala’s action on memory can be thought of as modulating activity within the memory circuitry as a function of arousal. Such a mechanism, as you might imagine, is quite adaptive. Highly arousing situations are often those that can be important for survival as when one encounters a serious threat. Having a mechanism that enhances memory for such situations is obviously advantageous.