Color blindness as part of the retinal damage caused by diabetes. Vitamin A deficiency may also cause color blindness. Some subtle forms of color blindness may be associated with chronic solvent-induced encephalopathy (CSE), caused by long-time exposure to solvent vapors. Red–green color blindness can be caused by ethambutol, a drug used in the treatment of tuberculosis. The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cone cells, each containing a different pigment, which are activated when the pigments absorb light.
Retina is the only part of the CNS that can be visualized non-invasively. The vertebrate retina is "inverted" in the sense that the light sensing cells are in back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones. In contrast, in the cephalopod retina the photoreceptors are in front, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot. The overlying neural fibre layer does not degrade vision in the inverted retina. The neurons are transparent and the accompanying glial cells