This difference allows the brain to generate a three-dimensional (3D) image of our environment. How is this visual comparison achieved? Almost a century ago it was discovered that the axons of the ganglion cells located in the lateral or medial half of each retina separate at the optic chiasm. Those originating in the medial region continue to the brain in the opposite optic nerve, while those originating in the lateral region of the retina remain in the optic nerve on the same side and remain there until the brain. As a result, the visual centres on each side of the brain are binocular: they receive information from both eyes. It is the comparison of the two images that allows the brain to see in 3D. In certain pathologies, such as albinism, the proportion of retinal axons projecting to the opposite side is abnormally high, which disturbs the vision of relief (In the picture: The central part of the visual field is seen by both eyes (binocular). In each eye, the retina on the lateral side (green for the left eye) is connected to visual centres of the brain on the same side, while the medial retina (purple) projects to the opposite side).
Comparative study of the visual system of many mammalian species has revealed that the ratio of retinal neurons connected to the opposite side of the brain is highly variable: it is about 50% in primates, 85% in a ferret, and 97% in a mouse, so that the binocular vision of primates is more developed than that of rodents, and more generally it is more developed in species that are predators, which would correlate with a better ability to catch prey.
Further comparative anatomy work has shown that this bilateral organisation of visual projections is also found in birds, reptiles and amphibians, which together with mammals make up the tetrapods (vertebrates with four limbs).30 years ago, studies suggested that this was not the case in fish, and that their eyes projected only into the half of the brain on the opposite side. Therefore, it was thought that the development of bilateral visual projections was one of the major evolutionary events that, together with the appearance of limbs and lungs, allowed the emergence of fish from the water (320 million years ago).
Work carried out at the Institut de la Vision by the teams of Filippo Del Bene and Alain Chédotal, and published in the American journal Science (DOI: 10.1126/science.abe7790), has completely challenged this model.