Hemispheric dominance is commonly discussed either as a matter of preferred usage, e.g., choosing which hand to use when responding, or a differential in skill level, such as the relative ability to process and execute language. However, for humans and nonhuman primates in which hemispheric dominance has been studied, it is generally assumed that both hemispheres have equal access to the information that has been stored, so that a decision of how to respond to a given stimulus does not require a system for controlling which hemisphere will respond.
In this article, Adam & Güntürkün point out that the neural connections in the brains of birds does not assure that each hemisphere has equal access to stored information. The eyes of pigeons and many other bird species are positioned on the sides of the head, and the optic nerve of each eye projects almost completely to the opposite hemisphere. Therefore, similar to split-brain patients, it is relatively easy to provide one hemisphere with information that might not be available to the other hemisphere. Compared to normal humans, the two hemispheres of the pigeon are relatively independent in what each can be taught.
The present study establishes that the two hemispheres of the pigeon can, in fact, store different memories. Further, if the memories in each hemisphere are at odds, one hemisphere can manifest dominance over the decision process. The authors describe this process as "metacontrol," and submit their results as evidence that the process is found in animals other than humans and nonhuman primates. Further, their study challenges any suggestion that the corpus callosum is essential for metacontrol, for this structure is not part of the design of the pigeon brain.