eh

The way the human eye perceives colors of visible light is easy enough to understand, but what about black? I understand that we see black in the absence of light. But the question is, why black instead of a real color?

I mean, if our brains perceived blue in the absence of light, we couldn't tell the difference between blue light and darkness, but we would know that the brain is simpy displaying a default color in the mind. But I'm wondering about black, specifically because it is NOT a color. With all light absorbed, what is this default black state?

Does anyone know what the hell I'm talking about? Maybe it's just a technical question on why the brain acts the way it does in regards to the eye.

tronvillain

Well, basic vision is really just the distinction between light and dark - perception of certain wavelengths of light as colour comes later, and representing one of those colours with the representation for dark wouldn't work all that well. It's just the way we've evolved: we see black, white, and various colours.

The Lone Ranger

There are three types of color-detecting cells in the retina of the eye -- (they're called "cones.") When we see color, it's due to which one(s) of these cones are stimulated by the wavelength(s) of light that are falling on your retina.

If no light is falling on some portion of your retina, then none of the cones (or rods) there are sending signals to the brain, and you see nothing there -- i.e. "black."

"Black" is simply lack of signal, if you will. Thinking that "seeing" black is weird is kind of like thinking that it's weird to "hear" nothing when you're in a completely silent place.

As was pointed out earlier, it would be confusing and inefficient if the brain's "default" when no cones were active was some actual color, like blue.

Cheers,

Michael

[ May 14, 2002: Message edited by: The Lone Ranger ]</p>

Ron Singh

Close "The Lone Ranger".

The human eye has about 12 million rod cells and 6 million cone cells. They are named that for their respective shapes. The ratio of cones to rods is relative to whether the organism dwells in the day or night. Rods help you see only in black and white, but are more responsive to light. Nocturnal animals have greater porportions of rods. Cones distinguish colors in the daytime, optimally. The center of the visual field in the retina is called the fovea. There are no rods there, and is what people refer to as the "blindspot". You see an object best when looking straight at it, because cones are located in their densest form at the fovea. There, cones are found at 150,000 cones per square mm.

Theses rod cells and these cone cells resemble squid to some extend. They are very tubelike. I will expand on the Rod cell, since it better pertains to the discussion regarding the lack of light. The Rod cell has disc like proteins that detect light. The protein for Rod cells is "Rhodopsin". When Rhodopsin detects light, its complementary Retinal protein becomes altered. It in turn activates another protein called "transducin". This warps another messanger, a nucleotide called "cyclic guanosine monophosphate (cGMP)". Wehn it is dark, cGMP is bound to sodium ion channels in the rod cell plasma membranes. At this time, it is depolarized and release inhibitory neorotransmitters. When light enters the Rod cells however, the cGMP is altered to GMP, which releases the GMP from the sodium channels. This actually depolarizes the rod cells, and in turn decreases the signals to the brain. You were close "Lone Ranger", but actually it is the lack of signaling to the optic nerve that signals the presence of light. This is why sometimes you walk into a dark area from a very bright area, it is so difficult to detect faint light.
When light bounces off of objects, the color is the wavelegnth of the light. I believe that a wavelegnth of 720 nanometers is some sort of color, I forget. However, chlorophyll absorbes this type of light. The reflected color or wavelegnth is the color or wavelegnth that was not absorbed by the object, that was reflected, and which is in this case green. When you look at a black object, no color is reflected. It as atheists may put it, has a "lack of color". When you look in a pitch-dark room, there is no reflected light. The result is the same, and there is a "lack of light".

Hopefully this helps

~Your friendly neighborhood fifteen year old Sikh.

tronvillain

Most of it probably does, but this doesn't:
Anyone referring to the fovea as "the blindspot" is wrong. The blindspot occurs where the optic nerve joins the retina, about fifteen degrees towards the nose from the fovea, and since the neurons have to cross the retina it has no rods or cones. That's why it's called the blindspot.

Ron Singh

You're right, I'm wrong. Thanks for correcting me Tronvillain. It's been a while..

~Your friendly neighborhood fifteen year old Sikh.

The Lone Ranger

Yep, I was writing off the top of my head and got the signalling process backwards. My bad.

For what it's worth, the "visible spectrum" (that is, that portion of the electromagnetic spectrum which can be detected by the human eye) ranges from about 400 nanometers (which we see as blue/violet/purple) to about 800 nm (which we see as red). The specific color we see depends upon how strongly the wavelength in question is responded to by the three "cone" types.

Cheers,

Michael

Major Billy

An interesting anecdote, Dr. Oliver Sacks studied people who lived on a pacific island where a great many were <a href="http://abcnews.go.com/sections/science/DailyNews/colorblindness_gene000626.html" target="_blank">color-blind</a>. He observed a young child look at something that was black in color, and say, "That is red."

(Those with this particular form of total color-blindness cannot see red light at all)

ohwilleke

Yes, my wife has precisely this form of color blindness (achromatopsia, which is the near absence of cones in the eye), and distinguishing black and red is very difficult. One interesting side effect of this is that she distinguishes things differently. Most of us identify objects based on color, and faces based on facial features. She picks up on patterns much more than I do (for example, thinking of two dramatically different colored wool sweaters as similar, but an almost identically colored cotton sweater as very different), and picks up much more on whether someone is light or dark haired more than the particulars of someone's face (since poor vision also comes with this condition).

Thanks for the article by the way. This is an orphan condition (about 6,000 sufferers in the U.S.) which it is hard to find information on. New information is always appreciated.

[ May 17, 2002: Message edited by: ohwilleke ]</p>