THE patterns of brain waves that occur during sleep can predict the likelihood that dreams will be successfully recalled upon waking up, according to a new study published in the Journal of Neuroscience. The research provides the first evidence of a ‘signature’ pattern of brain activity associated with dream recall. It also provides further insight into the brain mechanisms underlying dreaming, and into the relationship between our dreams and our memories.
Cristina Marzano of the Sleep Psychophysiology Laboratory at the University of Rome and her colleagues recruited 65 students, selected on the basis of their sleeping habits. All of them had a regular sleep ‘routine’, going to bed at around the same time, and sleeping for an average of seven-and-a-half hours, every night. For the study, the participants slept for two consecutive nights in a sound-proof, temperature-controlled room in the lab. They were left to sleep uninterrupted on the first night, so that they would get accustomed to the new surroundings.
On the second night, the researchers used electroencephalography (EEG) to measure the participants’ brain waves while they slept, and woke them up during specific types of sleep. Sleep occurs in five distinct stages, each characterized by a distinct pattern of brain waves, and most dreaming occurs in the rapid eye movement (REM) stage. Some of the participants were woken up during REM sleep, as determined by their brain waves, while others were woken up during stage 2 sleep. Soon after being woken up, they were asked filled out a ‘sleep and dream diary’, giving details of whether or not they dreamt, how many dreams they had and, if they could remember, the contents of any dreams they had.
The researchers found that they could predict successful dream recall from the brain activity patterns recorded just before awakening. Of the participants woken up during REM sleep, those who exhibited more low frequency theta waves in the frontal lobes were more likely to remember their dreams than those who did not. Dream recall after waking from stage 2 sleep was associated with a reduction in higher frequency alpha waves recorded from the right temporal lobe. The predictions were most accurate when based on the recordings of activity that occurred about five minutes before awakening.
We know that the brain processes newly acquired information while we sleep, and although the function of dreaming is unknown, some researchers believe that it plays an important role in memory consolidation. Our dreams may be a manifestation of the brain activity associated with ‘replaying’ the day’s events, so dream recall can be thought of as a form of episodic, or autobiographical, memory.
Indeed, previous studies have shown that brain waves in the frontal and temporal lobes can predict encoding and subsequent recall of episodic memories, and that they can even be used to distinguish between true and false memories. We also know that oscillatory activity in the frontal and temporal lobes plays an important role in memory formation, and that interactions between these two regions are necessary for long-term memory storage.
The new study shows that dreaming and dream recall appear to involve the same brain wave patterns as encoding and retrieval of episodic memories. This suggests that the brain mechanisms underlying memory encoding and retrieval during sleep are the same as those during waking hours.
Marzano, C., et al. (2011). Recalling and Forgetting Dreams: Theta and Alpha Oscillations during Sleep Predict Subsequent Dream Recall. J. Neurosci. 31: 6674-6683 DOI: 10.1523/JNEUROSCI.0412-11.2011.