A left-sided visuospatial bias in birds

Humans primarily attend to objects in the left side of space, as shown in cancellation tasks routinely used during neuropsychological testing [1,2]. This asymmetry is thought to arise from a right hemispheric superiority in the control of spatial attentional resources [3], and is assumed to depend on the corpus callosum which mediates fast communication between two specialized hemispheres of the brain [4]. We tested two species of birds in a task that closely matches the cancellation task: the birds were required to explore an area in front of them and to sample grains. Birds displayed a clear bias into the left hemispace, as evident in the pecking activity or the order in which pecks were placed in the left or right hemispace. Birds thus exhibit a similar left-side bias to that of humans, but as birds have no corpus callosum, transcallosal interactions cannot be a critical prerequisite for spatial asymmetries. Lateralization of spatial attention is thus common to humans and birds, and may have evolved before their last common ancestor more than 250 million years ago [5].