Corinne Bareham and her team asked 26 healthy volunteers (17 women; all right-handers) to relax in a comfortable reclining chair, to close their eyes, and listen to a series of tones. The tones occurred either on the left or right side of space, some further from the centre than others.
After each tone, the participants pressed a button to indicate whether they thought it had originated on the left or right side of space. While this was going on, the researchers recorded the participants' surface brain activity using EEG (electroencephalography). This provided an objective marker of their sleepiness.
The task may appear easy, but when the participants were sleepy, they mislocated nearly 25 per cent of left-sided tones to the right. This compares to an error rate of under 14 per cent when they were alert. "A participant was 17 times more likely to show a right-ward shift with drowsiness ... than a leftward shift, or no shift," the researchers said. In contrast, the participants were slightly more accurate at locating right-hand tones when sleepy compared with when alert.
The finding that tiredness triggers a shift in attention to the right-side of space is not new - researchers have shown this before. However, past demonstrations of the phenomenon have used visual stimuli. This study is novel because of its use of auditory tones and because of the highly accurate measurements of participants' alertness.
Research on this topic has clinical relevance. The drowsiness-induced attentional shift towards the right side of space is similar to a phenomenon known as "spatial neglect" that's observed in patients who have suffered right-hemisphere brain damage. People with left-sided brain damage show the opposite pattern - they tend to ignore the right-hand side of space. However, right-hemisphere brain damage leads to more prolonged and profound spatial neglect than left-sided damage, and this new study offers a clue as to why.
One explanation for spatial neglect following left or right-sided brain damage is that the two hemispheres of the brain are usually in competition, so that when one is damaged, balance is lost, and attention is skewed towards the same side of space as the brain damage. However, people with right-sided brain damage suffer twice, because damage to the right hemisphere is known to induce sleepiness, which - as this study shows - also leads to a skewing of attention to the right side of space.
In the researchers' words, patients with right-hemisphere damage are "doubly compromised" - by the loss of hemispheric balance, and by the effects of drowsiness. The good news is that this insight offers an avenue for treating patients with right-sided brain damage. "The results here confirm that the maintenance of alertness should be ...[an] important therapeutic target," the researchers said.
Bareham, C., Manly, T., Pustovaya, O., Scott, S., & Bekinschtein, T. (2014). Losing the left side of the world: Rightward shift in human spatial attention with sleep onset Scientific Reports, 4 DOI: 10.1038/srep05092
Hemispheric bias shifts with tiredness
Space is compressed by a fast turn of your head
Novelty seekers are biased to the right
Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.