Decade of the Brain
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Discovering Our Selves: The Science of Emotion
Executive Summary

Panel: The Science of Memory and Emotion

"Memory: The Fragile Power"

Daniel L. Schacter, Ph.D., Professor of Psychology at Harvard University, is the author of Searching for Memory: The Brain, the Mind, and the Past.

I often refer to memory as a "fragile power." Memory is fragile because we are subject to forgetting and memory is not always as accurate as we would like to believe. Memory is powerful because most of the time it serves us well, forming the foundation of our knowledge of the world and of ourselves. In the case of emotionally experiences, memory is a source of tremendous power in our lives.

One of the key lessons we have learned is that memory is not unitary: there is no one area or structure in the brain that we would identify as memory. Moreover, there are multiple forms of memory. We make a major distinction between explicit memory and implicit memory. (Some researchers refer to these two forms of memory as declarative and non-declarative.)

Explicit memory involves the conscious, intentional recollection of previous experiences, what we tend to think of as memory in our everyday lives. It may involve reliving or reexperiencing past events. Implicit memory refers to nonconscious, unintentional influences of past experiences on current behavior and performance.

We have learned that these two kinds of memory have different bases in the brain, as well as having different properties, from the study of patients with amnesic syndromes. A landmark study dating from the 1950s by Brenda Milner and her colleagues involved a patient, H.M., who had his hippocampus and much of the surrounding temporal neocortex removed in an operation to relieve an intractable epilepsy.

H.M. taught us that removal or damage to these structures results in a severe impairment of explicit memory. H.M. and patients like him cannot remember what they have done in the recent past; they cannot learn new facts. They cannot remember from one hour to the next, for example, that they have just been introduced to someone. The same patients, however, can show normal implicit memory. And they can acquire new implicit memories, such as learning to ride a bicycle, even if they cannot remember being taught.

If the hippocampus and related medial temporal lobe structures are crucial to explicit memory, which structures are involved in implicit memory? Through neuroimaging techniques such as PET scanning that allow us to track regional variations in blood flow in the subject's brain--which we assume are indicative of neuronal activity--we have found that during explicit memory retrieval, especially when remembering is easy, there is clear activation in the hippocampal region. When remembering is more difficult, there is also considerable activity in the front of the brain, in the frontal lobes. During implicit memory retrieval, we observed decreased blood flow in regions of visual cortex in the back of the brain.

We know that memory is not a perfect record of what has occurred. Rather, we construct memories out of an interaction between prior knowledge and current incoming information.

These memories are usually accurate, but not always. We have learned, for example, that people will "remember" having heard a word before--and be quite certain that it was among a previous list of words they heard--when in fact it is only closely related to many of the words on the list. In this case, the listener is retaining not a literal recording of the list but what the mind has constructed. When we make PET or MRI images of people during a true-and-false memory task, we find that generally the same areas are activated whether the memory is true or false. This may be part of the reason why a false memory seems so real.

What is the role of emotion in relation to memory? We have heard that the amygdala plays an important role in emotional memory, and that patients with damage to the hippocampus and related structures will be virtually unable to remember any recent events. By contrast, patients with selective damage to the amygdala do not have that across-the-board deficit in explicit memory. Rather, they have a more narrowly focused deficit in remembering some of the emotional aspects of their recent experiences. So even though emotionally powerful and traumatic experiences are usually the ones we remember most vividly, damage to the amygdala or drugs that interfere with the operation of the amygdala will prevent that extra boost in remembering. People who have taken drugs that interfere with operation of the amygdala or who have lesions on the amygdala do not become amnesic, but they show a different pattern of memory.


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