Birds represent a particularly suitable model for cognitive neurosciences to study the prenatal factors at play in the establishment and modulation
of brain lateralization. In pre-hatching stages, genetic factors determine the asymmetrical position of the embryo that in turn allows asymmetric
environmental stimulation. In such tight gene–environment dialogue, genes promote the direction of asymmetries and environmental experience modulates the strength, or presence, of anatomical asymmetries and related cognitive specializations. Embryos of birds are easily accessible at all stages from fertilization to hatching due to development in eggs, and specific environmental effects (e.g., light-exposure) have been observed at the level of visual pathways and of related behaviors mediated by visual analysis.
Thanks to the nearly complete decussation of optic nerve fibers at the chiasma, the noninvasive temporary occlusion of one eye allows direct post-hatching investigation of single contralateral hemispheric processing to understand the effects of asymmetric prenatal manipulation. This chapter outlines the two main visual pathways of birds, their developmental timeline and the effects of light stimulation on the establishment and modulation of lateral biases. The detailed method of applying embryonic photostimulation in the domestic chick is provided and the relevance of the influence of environmental light exposure on the development of cerebral asymmetries in other species is discussed.
