When we’re drowsy, and on the point of falling asleep, our awareness of the outside world tends to dim. But a fascinating new paper reports that, for most people, it’s the left side of the world that dims the most.
The study comes from neuroscientists Corinna Bareham and colleagues from the Cognition and Brain Sciences Unit in Cambridge, UK.
Volunteers (n=26) were asked to complete a very simple task: they had to indicate whether various sounds were coming from the left, or the right. These sounds were played through stereo headphones.
The task was easy enough, but it was also rather boring, and the volunteers were asked to keep their eyes closed throughout. As such, many of them became sleepy… or even nodded off.
In most psychological experiments, it’s bad news if the volunteers fall asleep. But in this case, sleepiness was the whole object of the study.
The results showed that participants were equally good at locating sounds coming from the left and the right when they were alert.
But when they became drowsy (as defined on the basis of brain activity which was concurrently recorded with EEG), a strange thing happened: “a remarkable unidirectional tendency to mislocate left-sided stimuli to the right”.
Sleepy participants selectively made more errors, hearing sounds from the left as though they were coming from the right – but not vice versa. These charts show the rates of errors (so higher means worse performance)
Bareham et al discuss these curious findings:
The results show that reduced alertness, indexed by EEG or behaviour, in healthy participants is associated with a markedly asymmetric increase in error rates in lateralising left- compared with right-located auditory stimuli…
Lateralised spatial awareness has been conceptualised as a competition between the cerebral hemispheres; the left hemisphere pushing attention to the right and vice versa. Such competition produces a finely balanced system vulnerable to perturbation.
They point out that people who have suffered a stroke affecting the right hemisphere of the brain may experience ‘spatial hemineglect’ - a striking tendency to ignore the left hand side of the world. Perhaps, the authors suggest, drowsiness induces a milder form of the same state.
It’s a very interesting finding. But I wonder if this lateralized effect of sleep has any manifestations in everyday experience?
For example, many people experience hypnogogic hallucinations, including hearing voices, when falling asleep. I quite often hear my own name, and sometimes other words and phrases. I wonder, do these voices tend to sound like they come from the right?
As far as I can remember, my hypnogogic voices are definately lateralized. They sound like they’re being spoken right next to one of my ears. But unfortunately, I can’t remember which ear it is… maybe someone could do a rigorous study of this.
Bareham CA, Manly T, Pustovaya OV, Scott SK, & Bekinschtein TA (2014). Losing the left side of the world: Rightward shift in human spatial attention with sleep onset. Scientific reports, 4 PMID: 24867667