Many people take naps to refresh their brains during a draining afternoon. But new research from the University of Lancaster suggests that brief periods of sleep during the day can actually create false memories.
In a small experiment detailed in the journal Neuropsychologia, John Shaw and professor Padraic Monaghan discovered that healthy adults who had taken a 90-minute nap performed more poorly on a word recognition test compared with those who stayed awake for the same amount of time.
By monitoring their subjects’ brain activity using a device called a polysomnograph, the duo were able to pinpoint an unexpected potential cause of post-snooze memory impairment: a type of brainwave known as sleep spindles. Generated in the thalamus during a light, non-REM sleep cycle known as stage 2 sleep, sleep spindles are sudden bursts of neuron activity, originating in the thalamus, theorized to be involved in memory formation and consolidation.
Previous investigations have shown that when people are given a list of thematically linked words – for example: bed, snooze, slumber, pillow – then allowed to sleep, they will falsely identify more “lure” words on a recall test performed after waking than people who did not sleep. As in, when asked to select the words they saw out of a larger selection of words, people who have slept are more likely to claim they saw theme-fitting words that were not actually on the original list.
Studies have also indicated that the right hemisphere of the brain, associated with more abstract associations of words and concepts, is more prone to making these false memories.
Hoping to elucidate what brain mechanism causes this curious phenomenon, Shaw and Monaghan enrolled 32 adult volunteers, provided them a list of themed words, strapped polysomnographs on their heads, then sent 16 off for a nap in a blacked-out room and sent the other 16 to watch TV.
After 90 minutes, the participants’ memory was tested with a computer display that flashed words on either the far right or far left of the screen. If they recognized the word, they clicked a “yes” button, if they did not, they clicked “no”. This rapid-fire, one-sided design forces subjects to recognize the words with only one eye – and therefore only one hemisphere.
As anticipated, the participants who had napped identified far more “lure” words than the non-nappers, and the rate of faulty recognition was higher for words presented to the right hemisphere (RH).
Yet counter-intuitively, the brainwave data revealed that subjects experienced more spindles in their right hemisphere than they did their left hemisphere (LH) during naps. Moreover, the frequency of sleep spindles in the RH was correlated to the number of false RH memories, leading the authors to speculate “that RH sleep spindles enhanced false memories in the RH.”
Other studies have demonstrated an association between increased spindle activity during stage 2 sleep and improved word and movement-related task recall. However, Shaw explained to Live Science that past work has focused on the positive relationship between spindles and true memories. This is the first to examine the link between spindles and false memories.
Although this study was limited by its small size, the authors believe their results imply that the RH is somehow more susceptible to mistakes that occur during the spindle-mediated memory consolidation process.
They hope to recreate their experiment in larger groups and with volunteers who have had a whole night’s sleep.
[H/T: Live Science]