The discovery of canine achiasma and hemichiasma has been followed by the identification of human achiasma (four individuals, to date). See-saw nystagmus was present in all cases of achiasma (canine and human) and in one of two cases of canine hemichiasma studied. Human infants with see-saw nystagmus should be imaged for possible structural abnormalities of the optic chiasm.