Tuesday, May 05, 2009

No Redness in the Retina


I've been listening to a lecture series on cognitive science- regarding 'qualia': for example- the ineffable, inexplicable, instantaneous experience of "redness".
We (most of us) have 3 functioning color receptors in our retinas... one that responds maximally to violet-blue light at 420nm, one that responds best to green light at 534nm and one that responds most to yellow-green light at 564nm (ignore the colors of the lines above). It's interesting to note that none of these color receptors responds maximally to red light - presumably the brain interprets excitation of the 564nm receptor in the absence of any 534nm excitation as being "red". I find this fascinating. When you think about it- there really isn't a "blue" receptor either! Excitation of the 420nm receptor can't tell your brain if it's looking at violet, blue, purple or cyan! It's only when the output of all the receptors are integrated that the "true colors" (whatever they are) can begin to be perceived.

Two receptors are coded on the X-chromosome and can be defective- resulting in 'color-blindness'. What's more interesting is that up to 10% of women carry 2 copies of one of them- making them "tetra-chromatic". In other words, they can theoretically perceive colors that the rest of us cannot imagine.- this would then redefine 'normal' vision as a form of color blindness! (It's possible, however, that their brains cannot deal with the higher retinal bitrate) . The problem with tetra-chromatic mothers, however, is that their sons have a high chance of being color blind.

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