Besides forgetting their spouses' birthdays or where they left their car keys, some amnesiacs confabulate--that is, they invent stories that they believe are true. In this month's issue of Nature Neuroscience, researchers provide evidence that the fantasies come from old memories dredged up by associations with contemporary situations. The findings suggest that an area of the brain implicated in emotion and rewarding behavior is important for constructing a sense of present reality.
Confabulators can tell some pretty strange tales: Take the case of a former dentist who would leave the hospital telling the nurse that he had patients waiting. The stories are almost always rooted in past experiences, although confabulators confuse when the events happened. A woman may want to drop what she's doing and feed her baby, even though her "baby" is 35 years old.
Wondering whether confabulators are failing to suppress old memories, a team led by Armin Schnider at University Hospital in Geneva, Switzerland, showed a series of pictures, some turning up more than once, to 12 nonconfabulating amnesiacs, six confabulating amnesiacs, and 10 control subjects. In three consecutive experiments, participants were asked to identify which pictures were repeated in each series. The team stacked each deck with pictures shown in a previous experiment but appearing only once in the current trial.
Both groups of amnesiacs fared worse than controls at picking out the repeated pictures, but only the confabulators fell for the red herrings from earlier experiments. "They cannot suppress information which they have seen before but which is not relevant now," says Schnider. The confabulating patients all had damage to areas of the brain that make up the anterior limbic system, which set them apart from the other subjects. Schnider suggests that this region exerts an influence on the mind and "allows you to base your thinking and actions on current reality."
"It's beautiful data," says neuroscientist Tim Shallice of University College London. This is the first time anyone has proposed a suppression mechanism to explain confabulations, he says, but the idea fits nicely with some previous findings.