A reader asks:

Would light therapy be effective for a person who is totally blind? Some people with blindness seem to have mood swings with the seasons. Perhaps they react differently — to the warmth of the sun rather than the sight of sunshine.


The sight of sunshine, in fact, has little or nothing to do with the mood-regulating effect of light. Almost certainly, the effect is mediated by nerve fibers that travel from the retina of the eye into the brain area that controls the body’s circadian rhythms — the suprachiastmatic nuclei of the hypothalamus. Other neurons from the retina independently project to visual sensory areas of the brain. One can be totally blind to visual sensation while still processing light signals in the suprachiastmatic nuclei, and in such cases we would expect the seasonal effect (and response to light therapy) to be similar to that of sighted people. There have been studies showing that light can affect melatonin production by the pineal gland — which is also mediated by the suprachiastmatic nuclei — in people who lack all visual sensory perception. If that effect were similar in blind people with bilateral ocular enucleation (removal of the eyeballs because of swelling, pain or other pathology), this interpretation would have to be modified. . . .