Unihemispheric memory in pigeons-knowledge, the left hemisphere is reluctant to share.

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.