The geniculocortical pathways of many mammals, including primates, are largely comprised of two neuron types. These are known as X and Y cells. Their functional significance in unclear, but the following highly speculative hypothesis is suggested. Y cells, because of their broad sensitivity to the crucial lower spatial frequencies as well as to the higher ones, are involved in a basic analysis of form vision. X cells, because of their fairly selective sensitivity to higher spatial frequencies, add to this certain details (i.e. enhanced acuity, etc.). If an animal develops with ametropia, Y cells should be adequately stimulated and as the lower spatial frequencies remain undistorted many Y cells would develop and reasonable form vision with milk amblyopia would result. Only X cells would be greatly affected by such an environment. However, if the animal develops with cataracts or is reared in an environment which abolishes all spatial frequencies, then neither X nor Y cells will develop. This would result in poor form vision and a deep amblyopia. Many of these phenomena have been observed in experimental studies of cats. The present relevance for clinical problems of this suggested functional dichotomy for X and Y cells is at best tenuous, and it is offered merely as a working hypothesis for future studies.