Our understanding of human biology has taken large strides in the last century. However, progress in our understanding of dreams has been really slow. The biological function of dreams is a grey area; the only sure thing is that most human beings dream regularly.
The phase of sleep associated with vivid dreaming is called REM (rapid eye movement) sleep. People who have woken up in this phase of sleep often report that they were dreaming. The rapid eye movements are a puzzle to researchers because they are difficult to measure.
A recent report (Science Vol. 377, 2022) addresses the question of whether eye movements are related to whatever is going on in the dream. Could the movements carry information about a dream that could be analysed and interpreted?
Interpreting dreams
But first, some background on the interpretation of dreams. The early twentieth century was dominated by the theories of Sigmund Freud, focused on the symbolic meaning of images recollected from dreams. The discovery of REM sleep, in 1952, led to a shift away from psychoanalysis.
The brain was found to be as active in REM sleep as in the fully awake state. Yet the body was inactive, asleep. REM sleep was found in all mammals and birds. Michel Jouvet showed that inducing damage to the brain stem in a cat freed it from bodily immobility in the dream state. This cat would noisily fight with other cats, stopping when it woke up.
The recording of brainwaves (EEG) provided fresh insights. These recordings showed very little difference between REM sleep and the awake state. More surprisingly, the neuroscientist Matthew Wilson recorded brain activity in a rat while it was exploring a maze and, not much later, obtained identical brainwaves when the same rat was in REM sleep — was it solving the maze in the dream?
Database of dreams
Another way of studying dreams was to compile vast databases of dreams. Analysing 50,000 dreams led the compiler, Calvin Hall, to conclude that most dreams did not resemble surrealist paintings, and were fairly predictable. Children may smile while dreaming, as children were more likely to dream of animals, but adult dreams were not very pleasant and were often filled with moments of anxiety.
We are anxious about important things, things that need to be resolved. In a theory proposed by Francis Crick and Graeme Mitchison, dreaming served as a housekeeping function, a nightly sorting of that particular day’s happenings. While sorting, a few important events (possible sources of anxiety) were stored away as memories, the rest treated as clutter.
Could there be a real-time output from dreams that could be recorded? The results are conflicting. Some studies indicated that either the direction or the frequency of eye movement matched the recollected mental process in the dream. An electrooculograph (EOG) was used to record eye movements in sleeping human volunteers, it recorded whether eye movement was chiefly vertical (up-down) or horizontal (left-right). If the volunteer reported that he was looking upward in his dream, his recorded rapid eye movements were up-down.
Other studies attributed REM to random activity in the brain.
Practicing strategies
When awake, eye movements are necessary for survival. A mouse in an open field frequently moves its eyes upward, scanning the sky for danger from birds. A human pedestrian will perform left-right scans, looking out for oncoming traffic. In both these instances, the eyes move in the same direction as the head. The brain keeps track of which way your head is pointed using nerve cells called Head Direction (HD) cells. In mice, using electrodes inserted in a HD cell, it has been shown that these cells are active when the head is moving.
Senza and Scanziani recorded both REM as well as HD cell activity in sleeping mice. Remarkably, they showed that in its REM sleep, their mouse performed up-down eye movements similar to daytime scans of the sky. The HD cells too indicated the corresponding movement, although the head itself did not move, the mouse being asleep. It appears that the dream was about escaping from a predatory bird.
Can these studies be used to benefit humans? Persons who have experienced sudden, intense trauma suffer from PTSD (post-traumatic stress disorder). A soldier shocked by a hand grenade exploding just behind him, although otherwise unhurt, can suffer from recurring nightmares and anxiety for several years. What does he ‘see’ every night? A better understanding would lead to better rehabilitation strategies.
(The article was written in collaboration with Sushil Chandani who works in molecular modelling. sushilchandani@gmail.com)
