Trying to suppress memories of past events leads people to forget unrelated experiences from periods surrounding the time of suppression, reports a study published online in Nature Communications. These findings could help explain memory lapses following traumatic events.
Justin Hulbert and colleagues elucidate the mechanism through which current events may be forgotten by studying memory retrieval in 381 participants across seven experiments. The participants were asked to memorize word-pair associations (such as ‘leap-ballet’). After the first word in the pair was displayed on a computer screen, they were instructed either to think of or suppress the thought of the second word. Occasionally, during the trials, pictures of improbable events were displayed (for instance, an image of a peacock in a parking lot). Subsequently, memory retrieval was tested by displaying the background of a given picture on its own and asking participants to recollect the object associated with it.
The authors found that the instruction to suppress the memory of words also made it harder to remember details about objects presented shortly before or after reminders of the to-be-suppressed words. They also used magnetic resonance imaging to scan brain activity of participants during suppression and showed that the impairment of memory formation was directly correlated with the degree of reduced activity in the hippocampus - a brain region known to be essential for the formation of new memories - as well as with the degree of activation in the lateral prefrontal cortex.
These results identify cognitively-triggered amnesia that begins with the voluntary suppression of an unwanted memory as a new mechanism for forgetting. This mechanism may help explain the memory deficits observed in patients suffering from post-traumatic stress disorder or other acute trauma.