Migraine sufferers have long complained about how their headaches worsen with bright light, and in case you ever doubted their complaints, Rami Burstein and other researchers from Harvard Medical School and the Moran Eye Center at the University of Utah recently made a giant step in understanding the light-to-headache mechanism in Nature Neuroscience. They found neurons in the rat thalamus sensitive to both light and to the dura (the membrane surrounding the brain).
More intriguing than the “how” of light and headaches is the “why”. Why should light be linked with pain mechanisms at all? Why should the retina be in the business of eliciting pain in your brain?
Upon reflection, however, we all know of occasions where looking hurts. The most obvious case is when we look at the sun. And another obvious case is when someone shines a flashlight in our eyes in the dark. In each case we are likely to respond, “Ouch!” From these real-world links between light and pain can we discern what the link may be for?
The example of the sun may coax us into suggesting that it is the retina-scorching amount of light that hurts. However, the fact that the same kind of discomfort occurs when someone shines a flashlight in our eyes shows it is not the intrinsic amount of light that is the source of the pain. A flashlight can be so dim that we can hardly see it in daytime, and yet hurt when shone in our eyes at night. The flashlight’s beam is not scorching anything, although the pain it elicits is every bit as real.
Instead, I suggest that these light/pain phenomena are similar to pain in other domains of our life. The general role of pain is not merely to tell us that something has been damaged, but to motivate us to modify our behavior toward safer or smarter action (and to so without our having to consciously think about it). For example, subtle pain signals are constantly causing me to shift my weight as I sit here and type this, leading to healthier blood flow in my lower extremities. Our eye fixations are like fingertips, reaching out and touching things in the world; just as fingertips need a pain sense to help optimally guide their behavior, so do our eye fixations.
In our normal viewing experiences there are very often wild fluctuations in brightness in our visual field, often due to the sun or to reflections of the sun. We are typically not interested in looking at objects having this full breadth of brightnesses, but, instead, at a range of “interesting objects” at a narrower range of brightnesses. To help us best see the objects of current interest, our visual system adapts to the brightness levels among them. If we were to fixate on a part of the scene that is much brighter than these interesting objects (perhaps a spot of glare), then our eyes would begin to adapt to the new brightness level, and when we look back at the objects of interest, we will be unable to see them well.
“Eye pain” of this kind may be the principal unconscious mechanism that keeps us fixating in a smart fashion within our visual field; it is what keeps our eyes performing at their best given our interests at the time. Although this kind of mechanism is unconscious, it by no means needs to be stupid. Instead, it may be able to infer where the brightest parts of the scene are on the basis of global cues in the scene.
For example, look at the earlier photograph of the glaring sun. It feels somewhat discomforting to look at this photograph, and our eyes want to steer clear from the sun. Yet the brightest spot at the center of the sun in the photograph is no brighter than the white elsewhere on this web page which causes us no discomfort to look at. Our brain seems to be able to recognize the sun-glare-like cues in the photograph, and elicits the glare-avoidance pain mechanisms for it but not for the white elsewhere on screen.
In light of these ideas for the role of light in pain, could it be that migraine-like headaches are the normal kind of pain elicited for these light/pain mechanisms, and that the trouble for migraine sufferers is the overactivation of these usually-functional mechanisms?
This first appeared on February 26, 2010, as a feature at ScientificBlogging.com.