Deciding whether memories are true or false is a matter of considerable legal controversy. What is beyond controversy, however, is how easily false memories can be formed. Consider, for example, these words: candy, cake, sugar, taste. Keep those words in mind for a few minutes and then ask yourself: Was sweet one of the words on the list? The chances are pretty good that you’ll recall--falsely--that yes, indeed, sweet was one of the words on the list. What, then, is the difference between remembering cake and remembering sweet? In trying to answer that question, a group of Harvard researchers last August captured the first glimpse of the brain as it retrieves both true and false memories.
First, several lists of semantically related words (for example, candy, cake, sugar, taste) were read aloud to 12 adult subjects. Ten minutes later, the subjects were shown words from each of the lists and asked if they had been among the words the subjects had heard. As they were thinking, their brain activity was monitored by pet scans. (This technique detects areas where increased blood flow--and presumably increased mental activity--is occurring.) Then the subjects were shown another list, containing only words similar--but not identical--to the words on the original lists (for example, sweet, frosting, sticky). Any recognition of words on this list would constitute a false memory. Again the researchers monitored brain activity.
The subjects were slightly better at recognizing words they had indeed heard than at recognizing that they had not heard other words. What intrigued the researchers, though, were the patterns of brain activity during the two tests. Both tests showed activation in the medial temporal lobe, a region deep within the brain that has been shown to be involved in forming memories of recent events. But there was also a telling difference. In the first test, when subjects were asked to recall words they had actually heard, an additional region--high in the temporal lobe and close to the surface of the brain--was active.
This region is thought to process sounds, says Daniel Schacter, a cognitive neuroscientist at Harvard, and its activity may reflect the auditory memory of how the words sounded when they were read aloud. In contrast, a memory of sensory detail wouldn’t exist for words on the false word list, because the subjects hadn’t been read those words. Past studies have shown, not too surprisingly, that true memories tend to be supported by more physical and sensory details than false memories. While subjects were pondering whether they’d heard a false word before, Schacter notes, their brains sometimes showed activity in the frontal cerebral cortex--the decision-making center--as if they were frantically searching for sensory evidence.
The wild-goose chase probably originates in the medial temporal lobe, which seems to be active whenever we try to recall something, and which may, Schacter suspects, be the source of the human predisposition to false memories. Its job may be to store or retrieve associations--among the various sensations of a single experience, say, or among words that usually go together--and then tell us where in the higher regions of the brain to look for the associated bits. That allows us to reconstruct a package of remembered information when presented with one piece. It may also prompt us to look for and even find memories that aren’t really there, because they are similar to ones that are. When someone is saying yes to false items like sweet, says Schacter, they’re probably thinking that really feels familiar--there were words like sugar on the list. And ultimately the memory for the gist carries the day.
Pet scans are not detailed enough to reveal the differences in activity between a brain that is falsely remembering a word and one that is correctly recognizing that the word is new. And they are certainly not likely to serve as some kind of false-memory detector for therapists or criminal justice officials--who tend to be interested in events that happened more than a few minutes ago. We don’t know if we would even see this activity if we asked our subjects to remember the words several days later, Schacter says. The best way to think about this study is as a first foot in the door for building an understanding of the biology of illusory memories.