From: Dickey, Michael F (michael_f_dickey@groton.pfizer.com)
Date: Thu May 09 2002 - 09:01:08 MDT
> -----Original Message-----
> From: naddy@mips.inka.de [mailto:naddy@mips.inka.de]
> Sent: Wednesday, May 08, 2002 12:00 PM
> To: extropians@extropy.org
> Subject: Re: Dark Angel - An Extropian TV Show?
>
> Some corner of my memory says that optimizations for night vision
> or color vision, respectively, run actually counter to each other,
> so I expect some serious engineering difficulty there.
"As I remember from having to dissect a cow eye in a color theory class,
color is detected using cones and grayscale is perceived through the rods.
I know that cones require more available light to function, thus the rods
are responsible for our night vision. If we could somehow (genetically?)
increase the sensitivity of our cones we could improve both night and color
vision at the same time."
As I understand it, the cones in a retina are tuned to pick up the frequency
of a photon more so then intensity of the light. A rod, for example, is a
stack of folded receptors formed into a cylinder. between the folds is a
protein which, at its base energy, is folded in a particular manner and does
not fit into any of the receptors in the rod. However, if a photon of
sufficient energy hits it the protein unfolds and re-folds in a different
manner with a high energy level, this new folded form now fits into the
receptor of the rod and triggers a nerve to conduct a polarity reversal down
the length of the nerve. The protein, having lost its energy in the form of
the polarity reversal traveling down the nerve, un-folds and refolds into
the lower energy level position where it does not fit into a receptor. This
folding / unfolding can occur about 60 times per second. As a photon
travels through the various stacks in the rod, the longer the rod the more
likely it is to hit a protein and send a signal down the nerve. Some
animals cheat and make the rod appear longer by having a reflective coating
at the base of it, which means the photon has to travel through the stack,
reflect and travel back through the entire stack again, making it twice as
likely to hit a protein. Not all of the photons do though, and some escape
back out of the eye. This is why some animals eyes, such as cats, shine
when you point a light on them at night. Cats have extraordinarily good
night vision, as do many other animals. This is mainly because they have
far fewer cones and are much less tuned to pick up the frequency of light
but are much more likely to pick up any particular photon because they
contain more rods. Rods appeared before cones in evolutionary history, and
were probably formed as a variation on cones, when certain proteins respond
to particular frequencies more frequently than other frequencies of light,
and then fit into particular receptors more frequently, the nerve signals
started to become associated with particular frequencies. Human eyes have a
greater concentration of cones toward to fovea and a greater concentration
of rods as you move away from the center of the eye.
You could increase the chances of a rod picking up a photon by making it
longer and / or adding a reflective surface at the back. I think the same
would apply to cones as well, and we might be able to determine colors
better in low light.
As an aside, you often hear that many animals see in black and white, or a
little less accurately, shades of gray. In reality, colors we perceive are
arbitrary labels that are associated with particular frequencies of light,
animals that have few or no cones may have brains that associate 'colors'
with intensities of light. So they may very well see in color, but they
don't actually see color.
Michael
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