Why do our eyes move during sleep?
We don't even know the function of one of the most famous sleep phenomena, rapid eye movements (REMs). It's been known for decades that during certain phases of sleep, the eyes show a pattern of rapid flickering movements, and that this REM sleep is when most (but not all) dreams occur.
But what are the eye movements?
In a new paper, French sleep researcher Isabelle Arnulf sets out the case for the "scanning hypothesis". The idea is that REMs represent the dreamer "looking at" things in the dream, just like waking eye movements - at least much of the time.
Some say that REMs are nothing to do with dreams, and it's just a coincidence that they tend to occur together. They may just be random, perhaps with the function of preventing the eyes from drying out during sleep, or maybe just a side-effect of sleeping brain activity with no function at all. A possible analogy: males usually get erections during REM sleep, even though most dreams have no sexual content.
There's lots of evidence that seems to support a deflationist view of dreams. In humans and other mammals, foetuses have lots of REMs, even though they've never seen anything. Lab animals with the visual areas of the brain removed also continue to display REMs, albeit not as many of them, and people who've been blind since birth have REMs.
Anaulf disagrees however, and discusses her work with the fascinating REM sleep behaviour disorder (RBD). RBD sufferers seemingly act out their dreams. Normally, we're paralyzed during REM by an inhibitory system which causes muscle relaxation during REM. The eyes are the exception, because they have a separate nerve pathway (which is also why some otherwise paralysed people can still communicate with their eyes). RBD can be a symptom of underlying neurological disease, such as Parkinson's, but it can also occur on its own.
Anaulf's team studied 56 patients with RBD over 1 or 2 nights (Leclair-Visonneau 2010). They found that the behaviours were correlated with the onset of rapid eye movements, although 80% of the eye movements were not accompanied by any actions. What's more, out of 19 distinct behaviours (ranging from running away from lions, to strangling someone), 60% were associated with REMs, and of these 90% were in the same direction as the actions.
This directional coherence between limb, head and eye movements during RBD suggests that, when present, REMs imitate the scanning of the dream scene. Because the REMs are similar in subjects with and without RBD, we suggest the extension of this concordance to normal REM sleep.
They suggest, however, that it may not be that the eye movements are the result of the dream content, but just correlated with it. It's not that something happens on the left in the dream, and then in response, the eyes move to the left; rather it's that whatever pattern of neuronal activity causes the dream, also causes the corresponding eye movements.
Either way it's an interesting idea, although it does rely on the assumption that RBD is a good model of normal sleep in this regard.
Arnulf I (2011). The 'scanning hypothesis' of rapid eye movements during REM sleep: a review of the evidence. Archives italiennes de biologie, 149 (4) PMID: 22205589