False Memories

Memories of our personal experiences are extremely vulnerable because memory often is not reconstructive but constructive. When retrieving an episodic memory, we try to reproduce the event as closely as possible, constructing the most plausible approximation. Consequently, we often incorporate aspects of the event that are close to the original but not exactly it, and we can even insert information that never occurred at all. This vulnerability can even go so far as to produce elaborate situations that never actually happened, though the person might swear that they did.

Neuroimaging studies have begun to examine differences in brain activation, comparing retrieval of a true past event from false memory for an event that did not occur. Most of these studies have used a task in which participants are shown lists of words to study. After a long retention period, the participants are asked to perform a recognition task, indicating which words on a second list were present on the first list. Some of the words on the second list that were not present on the first list, called foils, are semantically related to the words on the first list. For example, the words "pajama, bed, night'' might have appeared on the first list, but the word "sleep" might appear at the time of the recognition test. Semantically related foils often were falsely recognized as having appeared on the originally studied list. Furthermore, participants often rated their confidence that the words had been on the original list as highly as they rated their confidence that actual words had appeared. Thus, it appeared as if the participants had created false memories of seman-tically related foil words.

Neuroimaging studies have compared activations due to falsely recognized words to activations due to correctly recognized words. One difference that emerges is activation in the frontal cortex during true recognition. This is consistent with other retrieval studies, as reviewed earlier. Another feature of activation is that words on the recognition test that had actually been presented sometimes caused activation in the primary sensory cortex of the modality in which they had appeared. For example, a word presented aurally, when tested at the time of recognition, might show activation in the superior temporal cortex. By contrast, words that did not appear showed no such sensory cortex activation. Thus, a neural signature apparently exists that permits one to distinguish actually presented words from semantically related foils, even if one does not access this signature in the recognition judgments.

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