VMany aspects of sleep are poorly understood, but authors are study published in the journal Science have now given new insights into a special sleep phase. This is characterized by rapid eye movements, the “rapid eye movements” (REM). People often report particularly vivid dreams when awakened from REM sleep. Since the first studies of this sleep phase around 70 years ago, it has been assumed that the eye movements may be related to the dream content – and perhaps even allow conclusions to be drawn about it. But study results were inconsistent: some researchers suspected that the movements were triggered by random brain activity, for example in the brainstem.
Neuroscientists Yuta Senzai and Massimo Scanziani from the University of California in San Francisco now report that the movements of the eyes were apparently related to imagined rotations of the head. To do this, they used electrodes on six mice to measure the activity of neurons, which fire depending on the direction of the head – some when the head is turned to the right, others when it is turned to the left. And this also applies to apparently imagined movements during sleep, while the head is physically still. At the same time, they used small cameras to record the rotation of the eyes – some mice sleep with their eyelids not completely closed.
Navigation of the sleeping brain
Here, Senzai and Scanziani found a clear connection: If the neuronal activity corresponded to a head turn to the right, there was also a right eye turn in REM sleep. This is similar when awake: When turning their heads, mammals, like humans, scan their surroundings with many rapid eye movements, so-called saccades. The researchers report similar movements in sleeping mice; in a sequence of movements, the first also correlates with the magnitude of the turn. “Our results show that the rapid eye movements reflect the internal representation of head direction in the sleeping brain,” the authors write. This suggests that changes in the virtual direction of the head are part of an “orchestrated representation” of how the sleeping brain navigates – and may correspond to imagined movements in dreams.
Neuroscientists Chris De Zeeuw and Cathrin Canto of the Netherlands Institute for Brain Research go even further in an accompanying comment: If the eye movements correspond to sleep contents, their analysis “could open a window for reading and potentially manipulating thoughts in dreams,” they write in ” sci-fi”. Colleagues, on the other hand, see no evidence that there is any connection to the dreams: it is clear that eye and head movements are tightly “wired” when awake – it is obvious that this is also the case during sleep without reference to dream content, said the neuroscientist Mark Blumberg from the University of Iowa the US magazine “The Atlantic”. This is how people who were born blind move their eyes in their sleep who have no visual dreams. It is possible that the body would train its coordination during sleep, says Blumberg.