Binocular vision and ipsilateral retinal projections in relation to eye and forelimb coordination.

It is commonly proposed that the number of fibers that do not cross in the optic chiasm (OC) is proportional to the size of the binocular visual field, and that the major advantage of binocular vision is acute depth perception. I present an alternative, an 'eye-forelimb' (EF) hypothesis, suggesting that alterations in the OC influence the length of neural pathways that transmit visual information to motor nuclei and somatosensory areas involved in forelimb coordination. Evolutionary processes resulting in increased ipsilateral retinal projections (IRP) are of adaptive value in animals that regularly use the forelimbs in a frontal position, while evolutionary change towards reduced IRP is of value for animals that mainly use the forelimbs in lateral positions. Primates and cats, to a large extent, use visually guided forelimb maneuvers, and both groups have high proportions of IRP. The fact that vertebrates' IRP arise exclusively from the temporal retina supports the hypothesis, since IRP from the nasal retina would increase the length of neural pathways involved in forelimb coordination. The EF hypothesis offers new perspectives on why a high proportion of IRP among early limbless vertebrates became reduced during the evolution of laterally situated limbs, and why reptiles that lost their limbs (snakes) evolved more IRP. Anatomical, neurophysiological, phylogenetic, ontogenetic and ecological data suggest that mutations changing the proportions of ipsilateral visual connections in the OC may have selective value for EF coordination.