Crossposted at Politicook.net.
We talked a bit about how visible light is generated and propagates last time, but this time I want to focus on the phenomenon of human vision. This a obviously a narrow sliver of a large topic, but is so significant that it should be discussed.
Vision in general is the interaction of EMR with sensitive cells in an eye. As I said about infrared, some reptiles "see" in the IR. In future, I will discuss the idea that many insects "see" in the ultraviolet.
But this time, the focus will be on human vision, the mechanisms of it, and some observations about our pootie and canine friends.
Humans have three types of visual receptors, rods, cones, and a third type that is not involved in images, but rather judges the intensity of light and adjusts the pupil, amongst other functions, such as circadian cycles. Since this is about vision, I will not discuss the third type except in this mention of them, and to note that they are only about 2% of the visually active cells in the retina.
Rods are the "general purpose" visual receptors, reactive to light of all visual wavelengths, but are slow to respond. 100 milliseconds is not unusual for a rod to respond to a stimulus, but that stimulus may be as small as a single photon of visible light. Rods are therefore not very useful for discerning quick movements, but are superb for low light level evaluation of the environment. Interestingly, rods are not very sensitive to frequencies below about 650 nm, so they respond better to blue than to red, but are otherwise pretty indiscriminate. That was the reason that police lights went from red to blue in the past several decades. Blue is just more visible in low light conditions than is red. With single photon sensitivity, rods are unexcelled in detecting gross structures in low light, and can detect slow movements there as well.
Cones, on the other hand, are very fast in response, but need more photons to respond. Typically, 100 or more are required for triggering, but the speed in which they do respond is from one to three orders of magnitude faster than rods. They are also much better suited for fine discrimination, but that is likely because they take in much more information before processing it, even though outnumbered by rods by around 20 to 1.
Cones are also distinguished by occurring in three types, sensitive to different wavelengths. They are designated as "S" for short, blue sensitive, "M" for medium, green sensitive, and "L" for long, red sensitive. Certain inherited forms of color blindness occur because one or another, or more, types are either not populated of have defective photoreceptive detection chemicals, or the biochemical infrastructure to drive their activity. Some of this is yet to be determined unequivocally. My eldest son suffers from lack of differentiation between blue and green, so it is personal to me.
Also of interest, blue, green, and red are termed the "primary additive" colors. Get up close and personal with your monitor (a magnifying glass helps), and you will see that the basic elements of each pixel are one of those colors, along with a white level. Direct stimulation from your screen to the retina gives the proper colors, because, and this important, the screen is EMITTING the light.
On the other hand, the "primary subtractive" colors are magenta, cyan, and yellow. This has to do with the interaction of reflected light off of, say, a sheet of paper with printing on it. Selected wavelengths are absorbed, and the ones that remain recreate the primary additive colors on the retina. It is much more complex than that, and also has to do with the way the brain functions, but the brain learns how to interpret signals pretty quickly. When I was in college, we did an experiment that involved wearing glasses that inverted everything, and when first putting them on everyone seemed upside down. After only half an hour or so, the brain turned them around and everyone was on the ground. After taking the glasses off, everyone was upside down for a bit until the brain compensated. If you get a chance, try that. It is fascinating.
The driving force for vision is our old biological friend, cyclic guanosine 3', 5' monophosphate (cGMP), a common energetic compound in cells. For those of you who remember biology, it is akin to adenosine triphosphate, but not as energetic. When a photon, or collection of them, impinge on a sensitive cell, the cells stops being active, so a negative signal results. In other words, in the dark, your eyes are working harder than in the light, and the ABSENCE of signal due to light is perceived as an image. Pretty cool, right? This is not unique to human vision. Many other systems use this idea. For instance, your smoke detector.
Smoke detectors have a tiny, tiny bit of a radionuclide that emits electrons, and the source and detector are placed in a direct line with each other. As long as the electron beam is clear, the positive signal keeps the alarm circuit "OFF". But when smoke (actually ionized materials) begins to steal away those electrons, the lack of signal activated the alarm. So this is not a queer thing, and is used all of the time everywhere.
After the cGMP is activated, nerve channels are opened up (sodium channels, mostly) and allow a potential difference to pass from the retina to the optic nerve, and that impulse continues to the brain. One reason that our eyes are so close to the brain (nearly touching) is that for survival, those signals have to be sent pretty quickly. If our eyes were in our toes it would not be as easy to avoid predators or adapt to our environment.
As soon as the impulse is sent, enzymes are lurking to break down the cascade, so that the image can be recharged. Otherwise, no motion would be sensible, only the initial image that triggered the initial cascade. This takes quite a bit of energy, so vision is fairly energy intensive. But I think that it is worth it.
Just a couple of loose ends to tie up before I go. Rods have one photosensitive chemical, and the three types of cones another. Whilst the ones from rods and cones are pretty different, the ones in the three types of cones are very closely related, differing mostly by the prosthetic groups on them (in chemistry, we call it the "chicken fat") that subtly affects the wavelength maximum.
Some theories of vision indicate that the signals sent to the brain are actually digitized, but that is controversial. Any insight (pun not intended) would be welcome. I will hang around a while for flames, questions, and comments. Warmest regards, Doc.