MORPHOLOGICAL ASYMMETRIES OF THE TECTUM OPTICUM IN THE PIGEON

Pigeons are visually lateralized with a dominance of the right eye. Du e to the virtually complete decussation of the optic nerves in birds, a right eye superiority probably depends on a left brain hemisphere do minance. The aim of the present study was to analyze whether morpholog ical asymmetries in the cross-sectional area of perikarya can be found within the retina and the optic tectum. With an image-analyzing syste m the cross-sectional areas of the somata of retinal ganglion cells an d rectal neurons were measured in the left and the right side under bl ind conditions. The results reveal significant morphological left-righ t differences, with cells in superficial layers 2-12 being larger on t he left side while neurons in laminae 13-15 have larger somata in the right tectum. No retinal asymmetries could be revealed. Since pigeon e mbryos keep their head turned to the right within the egg, such that t he right eye is stimulated by light shining through the shell, it is p ossible that the morphological asymmetries at the rectal level are ind uced by left-right differences in prehatching photic stimulation. This embryonic sensory asymmetry might lead to a higher activity level of right eye ganglion cells and to a larger amount of released neurotroph ins in the left tectum. This in turn could exert the morphological eff ects on soma sizes in the superficial retinorecipient layers.