Morphological asymmetries of the tectum opticum in the pigeon.

Pigeons are visually lateralized with a dominance of the right eye. Due to the virtually complete decussation of the optic nerves in birds, a right eye superiority probably depends on a left brain hemisphere dominance. The aim of the present study was to analyze whether morphological asymmetries in the cross-sectional area of perikarya can be found within the retina and the optic tectum. With an image-analyzing system the cross-sectional areas of the somata of retinal ganglion cells and tectal neurons were measured in the left and the right side under blind conditions. The results reveal significant morphological left-right differences, with cells in superficial layers 2-12 being larger on the left side while neurons in laminae 13-15 have larger somata in the right tectum. No retinal asymmetries could be revealed. Since pigeon embryos keep their head turned to the right within the egg, such that the right eye is stimulated by light shining through the shell, it is possible that the morphological asymmetries at the tectal level are induced 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 neurotrophins in the left tectum. This in turn could exert the morphological effects on soma sizes in the superficial retinorecipient layers.