We all know how difficult it is to think when, perhaps because of tiredness or anxiety, our minds are 'racing'. As soon as we try to concentrate on one thing, another, irrelevant, thought will intrude to throw our reasoning off course.

In a healthy brain, the worst that this mental 'busyness' can cause is a headache. In a brain that has problems with memory, this kind of distraction mechanism - in other words, the activation of too many memory traces at the same time - can undermine cogent reasoning. Research published in the July issue of Nature Neuroscience suggests that such distraction could be at the root of some confusing and distressing information storage and retrieval problems.

Armin Schnider and Radek Ptak of the University Hospital, Bern, Switzerland have been studying a small subset of amnesiacs who suffer from what is technically known as 'confabulation'. These patients imagine, and believe in, totally fabricated experiences - some plausible, others less so ("I am a space pirate"). How these fantasies are generated and given internal credence is the subject of much controversy.

Schnider and Ptak repeatedly showed two series of pictures - one abstract and one meaningful - to groups of confabulators, non-confabulating amnesiacs and ordinary, healthy people at various time intervals. The series were made up of several different pictures and some copies. At each showing, the duo asked their subjects to identify the copies in the current round, disregarding their recollections of the series from showings five minutes or half an hour previously.

Confabulators were the only group to have great problems with this task of ignoring the past (even that from thirty minutes before) and concentrating only on the present. Although they were well able to single out genuine copies, they also had very high 'false-positive' rates, finding copies where there were none. This indicates that they were giving memory and incoming information equivalent status. To confabulators, then, "memories seem," say the researchers, "as real and pertinent as current reality."

Thus, Schnider and Ptak propose that the part of the brain responsible for suppressing information irrelevant to the task in hand is at the front of the brain, in the foremost (or anterior) part of the limbic system (the region believed to be involved in generating and controlling emotions such as joy and rage). This area is always damaged in confabulators, but is often spared in non-confabulating amnesiacs.

Altogether, these findings - and similar results in animals - hint that, ironically, some types of amnesia may be the result of too much recall rather than too little.