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Blind humans lacking rods and cones retain normal responses to non-visual effects of light
Date:12/13/2007

, the effect of light on melatonin secretion was examined. Melatonin is a hormone produced at night that influences arousal and is secreted in a cyclic fashion. Just like sighted individuals, the blind patient exhibited acute suppression of melatonin in response to light and was most sensitive to blue-light exposure.

Furthermore, blue light also shifted the timing of the circadian pacemaker and improved alertness, as measured by subjective scales, auditory reaction time, and changes in brain activity. While a few rods and/or cones may remain, Zaidi and colleagues have strong evidence to show that they contribute little, if at all, to these effects. Thus the authors were able to show that the effects were maximal in response to wavelengths of light that the retinal ganglion cells respond best to, and not the wavelength that the visual system detects best.

In the second patient, a different a set of tests was administered to assess the effects of light. First, the pupil-constriction response to various wavelengths and intensities of light was examined. Consistent with the major role of the pRGCs in mediating this response, pupillary contriction was stimulated most by blue light (~480 nm), the wavelength that pRGCs are most stimulated by.

Given that the non-visual responses to light appeared to be intact in this patient, the researchers were prompted to ask whether some minimal awareness of light might still be retained despite the inability to detect any response to light by conventional measures and the patients inability to see light. Remarkably, the patient was able to tell that the blue light, but not any other color, was switched on, demonstrating that the pRGCs also contribute to our ability to see light.

These results have a number of important implications for human vision and vision-related diseases. First, they suggest humans possess light-sensitive cells, apart from rods and cones, that are important for non-visual lig
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Contact: Cathleen Genova
cgenova@cell.com
617-397-2802
Cell Press
Source:Eurekalert

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