May 27, 2005 When it comes to the world laid before us, our mind's eye has a bias. For reasons that are not entirely clear, during some tasks humans have a tendency to devote more visual attention to the left side of the visual world than the right side, a phenomenon known as pseudoneglect. Researchers now report that pseudoneglect is not restricted to humans but is shared by birds, suggesting not only that brain structures thought to play a requisite role in pseudoneglect may not actually be essential for this phenomenon, but also that pseudoneglect may reflect evolutionary adaptations that allow animals to devote attention to multiple aspects of their environment.
The findings are reported in the May 24 issue of Current Biology by Bettina Diekamp (now at Johns Hopkins University) and colleagues at Ruhr University, Bochum, Germany; the University of Padova, Italy; and The University of Trieste, Italy.
It has been known for some time that human patients who have suffered injury to the brain's right hemisphere can experience a much more severe bias in their spatial attention--spatial hemineglect--in which the entire left side of the visual world seems nonexistent as the brain performs spatial tasks. In a classic example, a patient asked to draw a daisy can only manage to put petals on the right side of her drawing.
The more subtle leftward bias in attention present in healthy humans likely has to do with asymmetries in the wiring of the brain's attention in the two hemispheres; the new finding in birds offers some insight into how and why this might be.
In the new work, researchers tested two bird species, the domestic chick and the pigeon, for their performance on a task in which they were allowed to freely peck at grains of food that were spread evenly in an area before them. Though the birds' bodies were positioned at the midline of the search area, both chicks and pigeons showed a considerable leftward bias in pecking. The experiment is similar in concept to those that reveal pseudoneglect in humans--so-called cancellation tasks in which subjects are asked to "cancel-out" evenly distributed visual targets on a sheet of paper placed before them.
The finding that birds also exhibit spatial pseudoneglect is somewhat surprising, given that birds lack a corpus collosum, the structure in human brains that is thought to facilitate rapid communication between the two hemispheres. In the past, such communication via the corpus collosum has been thought to form the basis for asymmetries in human spatial attention, but the new observations suggest that this view warrants reconsideration.
It isn't clear why humans or birds should benefit from biased spatial attention, but past work has suggested that brain organization underlying attention asymmetries may offer benefits in spatial learning and in performing simultaneous spatial tasks, such as looking for food while being vigilant for predators.
The researchers include Bettina Diekamp and Onur Güntürkün of the Ruhr-Universität Bochum, Germany; Lucia Regolin of University of Padova, Italy; and Giorgio Vallortigara of the University of Trieste, Italy.
Bettina Diekamp, Lucia Regolin, Onur Güntürkün and Giorgio Vallortigara: "A left-sided visuospatial bias in birds." Current Biology, Vol. 15, R372–R373, May 24, 2005. http://www.current-biology.com
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