Functional cerebral asymmetries, once thought to be exclusively human, are now accepted to be a widespread principle of brain organization in vertebrates [1]. The prevalence of lateralization makes it likely that it has some major advantage. Until now, however, conclusive evidence has been lacking. To analyze the relation between the extent of cerebral asymmetry and the degree of performance in visual foraging, we studied grain-grit discrimination success in pigeons, a species with a left hemisphere dominance for visual object processing [2,3]. The birds performed the task under left-eye, right-eye or binocular seeing conditions. In most animals, right-eye seeing was superior to left-eye seeing performance, and binocular performance was higher than each monocular level. The absolute difference between left- and right-eye levels was defined as a measure for the degree of visual asymmetry. Animals with higher asymmetries were more successful in discriminating grain from grit under binocular conditions. This shows that an increase in visual asymmetry enhances success in visually guided foraging. Possibly, asymmetries of the pigeon's visual system increase the computational speed of object recognition processes by concentrating them into one hemisphere while preventing the other side of the brain from initiating conflicting search sequences of its own.

In the present study, pigeons were trained under binocular conditions in a conditional visual discrimination in which they were faced with two identical patterns arranged one above the other. In half of these stimulus pairs the animals had to peck the upper pattern, in the other half the lower one. Although only six pairs of stimuli were used, only four out of eight birds reached learning criterion. These animals needed up to 6 months of training with 3050 to 6650 trails. Then, the experiment proceeded under identical conditions using eye caps restricting vision alternatively to the left or the right eye. These monocular tests revealed that three out of four birds virtually had no knowledge of the task contingencies using their left eye (right hemisphere). Again, several thousand trials were needed to train the birds to criterion with their left eye, while they were simultaneously discriminating at a very high level with their right. These results show that memories on task contingencies are stored unihemispherically in the visually dominant left side despite extensive training with both eyes open. Additionally, it can be concluded that the subsequent read-out by the 'naive' hemisphere can be largely restricted, resulting in a 'natural split-brain' like situation in birds. It is speculated that the absence of a corpus callosum in birds restricts interhemispheric transfer of information. #