Astigmatism of the mammalian cornea: evolutionary and perceptive significance.

Astigmatism enables spatial, linear and directional discrimination. These faculties are demonstrated by astigmatic photographic experiments. Comparative anatomic deductions lead to the assumption that the eyes of early mammals were astigmatic. Thereby these animals, lacking binocularity and accommodation, could achieve visual spatial information. This assumption is supported by the fact that features of astigmatic refraction, specially straight linearity, have been adopted by various intra-ocular structures, and by the neuronal structuralisation of receptive fields in the visual cortex of mammals.