VISUAL LATERALIZATION IN BIRDS - FROM NEUROTROPHINS TO COGNITION

Birds which are tested monocularly in visual discrimination tasks gene rally show higher performance levels with the right eye seeing. Due to the virtual complete decussation of the optic nerves, a right eye sup eriority is probably related to a left hemisphere dominance. If visual processes between the hemispheres differ, each halfbrain might be dif ferently prone to be deceived by visual illusions. Indeed pigeons test ed with the herringbone illusions are deceived to a stronger extent wi th the right eye. These functional asymmetries are accompanied by anat omical left-right differences in the ascending thalamo-and tectofugal visual pathways in chicks and pigeons, respectively. The neuroanatomic al and behavioral assymmetries result from an asymmetrical posture bef ore hatching in which the embryo keeps his head turned to the right, s uch that the right eye is stimulated by light shining through the shel l. The lateralization of adult animals are induced by this prehatching asymmetric photic stimulation since dark incubation abolishes behavio ral and anatomical asymmetries. Ir is conceivable that the asymmetrica l embryonal light stimulation increases the release of neurotrophins i n the developing avian visual system in an activity dependent matter. Neurotrophins play an important role in neuronal survival and morpholo gy and thus might represent a molecular switch bridging the gap from e mbryonal light stimulation to asymmetries of visual cognition in adult s.