Hemispheric bias shifts with tiredness

Normally we have a slight bias to the left-hand side of space. So if we’re asked to mark the centre of a line, for example, we tend to overestimate the length of the left-hand portion. Now Tom Manly (pictured) at the MRC Cognition and Brain Sciences Unit in Cambridge and colleagues have shown that as we grow tired this bias is shifted over to the right-hand side. The finding could have implications for the treatment uni-lateral spatial neglect – an inability to attend to things on one side of space that often affects people after they’ve had a stroke.

Manly’s team tested 10 healthy volunteers who worked shifts at Addenbrooke’s Hospital. They were presented with horizontal lines with a vertical divider and were asked to say which half of the line was longer. When they’d just come on shift they showed the usual bias of overestimating the left-hand portion, but at the end of a shift they showed the opposite bias. In a second study, six volunteers from the authors’ own MRC research unit performed a similar task for 60 minutes. As the task wore on, their spatial bias shifted from the left to the right. The findings complement previous work showing uni-lateral neglect following stroke can be worse when patients are tired.

“Our findings suggest that the dramatic effects on conscious awareness resulting from damage to [fronto-parietal] networks in patients and the subsequent modulation [of their neglect] with changes in alertness, may be echoed in a much milder form in healthy participants”, the authors said.
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Manly, T. Dobler, V.B., Dodds, C.M. & George, M.A. (2005). Rightward shift in spatial awareness with declining alertness. Neuropsychologia, 43, 1721-1728.

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