October 1, 2004 | David F. Coppedge

Your Eyes Have Automatic Light Meters

Every pupil knows that pupils constrict in bright light and dilate in dim light, but how?  Physiologists had assumed the retina signalled the iris muscles, but now it appears there is an independent mechanism in the iris itself, at least in birds, and probably in mammals, too.  A report in EurekAlert summarizes a finding from Washington School of Medicine published in Science:1 “Working with embryonic chicken eyes, Washington University ophthalmology researchers found that cryptochrome allows the pupil to react independently from light-sensitive photoreceptor cells at the back of the eye.”
    Cryptochrome is a protein distinct from the opsin family of proteins usually involved in light response.  This molecule is apparently acting like a light meter on the camera.  A light meter doesn’t take a picture but helps the camera receive the proper amount of light.  Experiments suggested “it is as if the light meter of the eye is controlling the pupil without vision being involved.  In the mouse, the meter is located in the retina and primarily uses melanopsin to do its work with cryptochrome proteins amplifying the signal.  In the chick, it is as if the light meter is contained in the pupil itself.”  The team is trying to determine if this mechanism works in human eyes also.  They make no mention of evolution, other than indirectly to suggest, “These data characterize a non-opsin photoreception mechanism in a vertebrate eye and suggest a conserved [i.e., unevolved] photoreceptive role for cryptochromes in vertebrates.”


1Daniel Tu et al., “Nonvisual Photoreception in the Chick Iris,” Science, Vol 306, Issue 5693, 129-131, 1 October 2004, [DOI: 10.1126/science.1101484].

The less we take things in the body for granted, the more we see exquisite mechanisms working together to achieve complex functions that could rightly be described as ultra-high-tech.  Humans only designed auto-exposure cameras in relatively recent times, after a lot of intelligent design.  Who designed opsins and crytochromes, and all the signalling and response mechanisms that cause them to make muscles, larger by orders of magnitude, respond rapidly to shifting quantities of light?  The human iris is far more complex than any Nikon aperture.  Yet it is only one of several automatic mechanisms on our stereo camcorders that provides a dynamic range of 10 million to one and transfers data at a gigabyte per second.  When a feeble little chick hatches out of the egg and sees the world for the first time, its automatic light meters are already working.
    For more on the complexity of the eye, see this description by Dr. Howard Glickman

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