In addition to allowing us to see, the mammalian eye also detects light for a number of non-visual phenomena. A prime example of this is the timing of the sleep/wake cycle, which is synchronized by the effects of light on the circadian pacemaker in the hypothalamus.
In a study published online on December 13th in Current Biology, researchers have identified two totally blind humans whose non-visual responses to light remain intact, suggesting that visual and non-visual responses to light are functionally distinct. Indeed, this separation was suggested by earlier studies in mice that demonstrated that circadian rhythms and other non-visual responses remain sensitive to light in the absence of rods and cones, the two photoreceptor types that are responsible for vision.
It turns out that mammals have an additional light-sensitive photoreceptor in the retinal ganglion cell layer (pRGCs) that is directly sensitive to light and is primarily responsible for mediating these responses. These cells are most sensitive to short-wavelength light with a peak sensitivity at ~480 nm, in the visible blue light range. While these studies and others in sighted subjects suggested that this non-rod, non-cone photoreceptor might play an important role in human photoreception, this had yet to demonstrated unequivocally until now.
To address whether the cells identified in rodents and primates also exist in humans, Zaidi and colleagues first had to find patients who lacked functional rods and cones, but retained pRGCsa formidable task, given that fewer than 5% of totally blind people are thought to retain this response.
This group of researchers was able to identify two such rare patients, allowing them to perform a series of complementary experiments to address whether non-visual responses are possible in the absence of rods and cones and to determine the most effective wavelength, or color, of light that induced a response. In the first patient
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