Unravelling the mysteries of sleep: how the brain ‘sees’ dreams
We’ve known for some time that our eyes move around during the dreaming phase of sleep, much like when we’re awake and looking at a visual scene. The phase of sleep is called rapid eye movement sleep, or REM sleep.
New research, published today in the journal Nature Communications, shows brain activity during the dreaming phase of sleep is remarkably similar to brain activity when we’re awake and processing new visual images, suggesting the brain “sees” dreams.
While researchers have suspected this may be the case, it’s the first time investigators have been able to record brain activity from within the brain.
A quick history of dream research
Dreams and their purpose have been one of the enduring mysteries of sleep. Early dream theorists, such as Sigmund Freud, argued that the function of dreaming was to preserve sleep by expressing unfulfilled desires or wishes in the unconscious state.
More recently, researchers have investigated the function and processes of sleep and dreams by measuring the physiological signals that characterise this state of consciousness.
Just over 60 years ago, American sleep researcher Eugene Aserinsky stumbled across rapid eye movements during sleep almost accidentally, during an overnight sleep study recording of his eight-year-old son. His seminal 1953 paper reported “rapid, jerky and binocularly symmetrical” eye movements during periods of sleep.
These eye movements were also associated with increased brain activity, thus discounting the idea that sleep is a completely passive phenomenon. During REM sleep, our brains are active and behave similarly to wakefulness or light sleep. But muscle activity is suppressed so we can’t physically carry out our dreams.
In a pioneering 1957 paper, American researchers William Dement and Nathaniel Kleitman examined the relationship between eye movements and dream content. They woke participants during REM sleep and asked them to describe their dream. The researchers then looked at how their dream description related to the type of eye movements they were experiencing at the time (vertical, horizontal, or a mix of both).
Participants who were woken after a series of vertical movements reported “climbing up a ladder”, and “standing at the bottom of the cliff operating a hoist and looking up at climbers”, whereas one participant who was woken after horizontal eye movements reported dreaming about “two people throwing tomatoes at each other”. In contrast, those who had mixed eye movements tended to be watching people close to them with no description of distance or vertical vision.
Since this study, the evidence for this association between the REMs and dream content is not consistent. Individuals who have been blind from birth, for instance, have REMs but no visual dream content.
But in support of Dement’s finding, a recent study in patients with REM behaviour disorder (where people act out their dreams due to a lack of muscle paralysis), found a strong association between goal-oriented limb and head action and eye gaze direction during REM sleep.
Brain activity during sleep
In everyday life, when we see things, our eyes and brain behave in characteristic ways to gather and process the information in our visual field and give it meaning. But the function of eye movements during sleep and dreaming are relatively unknown. Today’s Nature Communication paper provides some insights.
Usually, brain activity is measured non-invasively from the scalp. But the investigators, from Tel Aviv University, recorded the activity of the brain, from within the brain, in patients with epilepsy.
Patients whose epilepsy cannot be controlled with medication have electrodes surgically placed within the brain as a clinical means to map their epileptic activity, and assess suitability for surgery as a treatment. These electrodes were implanted in the medial temporal lobe – a region that is associated with visual awareness.
Researchers compared brain activity of these patients across three settings: REM sleep brain activity, wakeful eye movements in darkness (no visual processing) and wakeful fixed-gaze visual processing (no eye movements). They wanted to test whether brain behaviour during sleep was more closely related to physical movement, or the processing of visual information.
Results showed that during rapid eye movements in sleep, the brain activity was more closely related to the brain activity during visual processing during wakefulness (without movement) than physical movements of the eyes in darkness where no visual processing was taking place.
These results suggest that the rapid eye movements that occur in sleep are linked to visual processing rather than just physical activation or movement. So, the participants may have actually been looking at a dream image, rather than these eye movements simply reflecting motor discharge in the brain.
While much remains unknown, this detailed processing of our dream images suggests that rapid eye movements may actually modulate our brain activity during sleep. We know that sleep is needed for rest and rejuvenation, but it’s likely to have other important functions as well.
In line with the earliest of theories about why we dream, are we processing content that has been consciously or unconsciously avoided during wakefulness, but somehow “needs” to be dealt with at least during sleep to maintain our psychological well-being?
Are the eye movements a simple byproduct of the visual processing that occurs of the images we dream?
Is there a psychological basis to why we need to process these images during sleep, and does this lend to better psychological outcomes in a similar way to sleep aiding physical functioning?
These and many questions drive the ongoing research into why we sleep, and what its precise benefits are.