Bruno G. Breitmeyer, is a Professor of Psychology at the University of Houston. In his new book, Blindspots: The Many Ways We Cannot See, we move from cataracts and color blindness through blindsight, acquired dyslexia, and visual agnosias. Breitmeyer uses what we’ve learned about the limits of our sight to illustrate the limits of our ability to mentally visualize and our ability to reason, covering everything from logical fallacies to how our motives and emotions relentlessly color the way we see the world. In the excerpt below we learn about one kind of blindness, caused by Vitamin A deficiency.
Even if we are blessed with caring parents and a stimulating environment during infancy and childhood, uncontrollable environmental factors can cause or promote visual deficits in an otherwise healthy visual system. Imagine the following. It is March of 1797. You are a settler living in the subarctic wilds near Hudson Bay in present day Quebec, Canada. Your crop of winter squash was nipped in the bud by the early onset of the harshest winter you have yet experienced. It lingers into spring, by which time you have depleted your larder’s store of dried halibut and Arctic cod. For some time your dietary mainstay has been nothing but the wild rice and the dwindling rations of dry beans you purchased late last fall. Although high in carbohydrates, protein, and several of the B vitamins, these foods do not support your dietary needs for Vitamin A. You have noticed some minor visual problems during your daytime activities; however, at night, your vision has deteriorated dramatically to the point where you do not dare to leave the immediate vicinity of your dwelling. You are wondering whether or not you are suffering from some kind malady that will leave your vision permanently impaired. By July, after you have feasted on plenty of fish harvested from the bay and on ripe berries gathered in the wild, you are relieved to find your vision has returned to normal. Although you could not have known it, you were suffering from avitaminotic nyctalopia, a type of night blindness, during the prior winter. It was caused by a prolonged (but fortunately temporary) vitamin A deficiency in your diet.
Vitamin A is known to play an important role in the regeneration of the rod photopigment, rhodopsin. Rhodopsin molecules are “bleached” when they react with photons of light, rendering them inert to further stimulation by light. If, due to a chronic dietary Vitamin A deficiency, bleached rhodopsin molecules are not restored to their prior unbleached photoreactive state, nocturnal vision eventually deteriorates to dysfuctional levels. Epidemics of such night blindness have been known to occur in regions where the availability of plants or animal food sources containing vitamin A was drastically reduced. For example, it have been common in parts of Southeast Asia where rice is the staple food. Fortunately, avitaminotic nyctalopia can be readily treated by increasing dietary vitamin A.
However, dietary night blindness can occur even when food sources rich in vitamin A are plentiful. Imagine the next sad scenario. You have been a closet alcoholic for several years. Besides the spider angianomas that give the tip of your nose a ruddy and friendly glow, you recently have noticed an increase of fatigue, an intolerable itch has been building up in your torso and arms, and you no longer wear your favorite pair of boots due to the swelling of your ankles and lower legs. In addition, you have noticed that you have difficulty driving at night – even when you are sober. Troubled, you go to your doctor and are diagnosed with cirrhosis of the liver. What you did not know is that vitamin A deficiencies can result from this disease, as well as from other liver diseases such as hepatitis, and that night blindness can be a symptom of it. Like avitaminotic night blindness, it too can be treated. However, since the effects of cirrhosis are not reversible, you must treat your night blindness with (massive) daily doses of vitamin A to compensate for the permanent loss of normal liver function, recalling with regret a wise saying you read years ago: Rum makes for a great servant, but a bad master.