It takes two to tango: hemispheric integration in pigeons requires both hemispheres to solve a transitive inference task

Cerebral lateralization, the dominance of one brain side for a specific function, is a core feature of the vertebrate brain. Lateralized processing requires complex intra- and interhemispheric interactions mediating exchange, integration or suppression of information. The underlying functional organization of cooperative or independent processing is only basically understood and may differ between vertebrate species depending on the organization of commissural systems and overlap of sensory input. We explored intrahemispheric integration capacities in pigeons, Columba livia; although their visual system is primarily crossed and lateralized, it can still integrate interhemispheric information. Pigeons were trained in overlapping colour discriminations in which each hemisphere learned only half the information that represented a linear hierarchy. Therefore, interhemispheric memory about the relational values of the premise stimuli pairs had to be transferred and combined to master a transitive inference task. Pigeons displayed transitive responding under binocular but not under monocular seeing conditions. Hemispheric-specific strategies in accessing the associative values of transfer stimuli resulted in potential conflict with intrahemispheric memory and led to unihemispheric impairment in performance. The response pattern might represent a consequence of neuronal mechanisms avoiding interocular conflicts, and it also indicates that interhemispheric communication in pigeons is an active process that integrates intra- and interhemispheric information in a context-dependent and hemispheric-specific manner.