Those of us old enough to remember Paul McCartney as a young man will also remember the fascination we all had with colour televisions. Colour gave even programs like My Mother the Car added sparkle, and as for Star Trek…well!
Objectively speaking, though, colours are just certain wavelengths of light. Light, in turn, is merely that very narrow band of electromagnetic radiation to which our eyes are sensitive. The phenomenon of colour is created in our brain from a fairly limited set of signals provided by the cells in our eyes.
The human retina contains two kinds of specialized cells, rods and cones. The 125 million rods give us vision in dim light, but only in shades of gray (which is why it’s hard to see colour by moonlight). The seven million cones give us vision in bright light, and come in three types, receptive to red (long), green (medium) and blue (short) wavelengths, respectively.
Light-sensitive pigments in the cones undergo a constant bleaching and regeneration as they are exposed to the wavelengths they are attuned to. This chemical change results in a nerve signal. The brain takes in the information from all those millions of cells and constructs the image which we see in full, vibrant colour.
Combining red and green light produces yellow, combining blue and red gives you magenta, and combining blue and green gives you cyan. Add them all equally and you get white.
How we construct yellow from red and green light was a subject of controversy for many years (Why yellow? Because whereas magenta is obviously red-blue and cyan is obviously blue-green, yellow is not obviously red-green.) One theory had it that the composition of yellow took place in the eye, with nerves from the red and green cones feeding inputs to special cells. Another, the central synthesis theory, held that the yellow was created in the brain. The fact that shining green light in one eye and red light in the other makes us see yellow seemed strong evidence for the latter, but the matter was really settled just a few years ago, in 1997, thanks to a clever experiment by Cornell University psychologist Romi Nijhawan.
Because signals take about 50 milliseconds to travel from our eyes to our brains, moving objects wouldn’t appear in the right place in our visual map of our surroundings if the brain didn’t correct for the discrepancy. The brain extrapolates, from the object’s rate and direction of motion, where it should appear, and shows it to us there, even though it hasn’t yet received the actual signals that show it in that location. This works dandy if the object is moving in a smooth, regular fashion–but it means our brains can be tricked.
Suppose a car moves past you on a stormy night. It’s so dark that all you can see is one tail light. But then, in a flash of lightning, you see the car’s body–and the tail light appears to be in the middle instead of at the rear. That’s because your brain extrapolated the location of the tail light based on its apparent motion, but didn’t have time to make that extrapolation in the brief instant the car was visible. The tail light appears where the brain has computed it to be, while the car appears where it was 50 milliseconds ago.
Nijhawan recreated that phenomenon. His volunteers watched a moving green bar on which a strobe would briefly flash a thin red line. In one set of tests, they saw the bar only as it passed by a slit in a barrier between them and the bar. In that situation, the flash of red strobe light appeared to as a thin yellow line on the bar itself. But when the barrier wasn’t there, the test subjects saw a moving green bar–and then, when the red strobe flashed, they saw a red line lagging behind the green bar. Their visual systems couldn’t map the red strobe into its appropriate place in the moving green bar, and thus could not compose the red and green into yellow. And that, in turn, proved once and for all that the composition of yellow takes place in the brain, not the eyes, because it was created solely through movement–and human retinas can’t sense motion on their own.
The fact our brain works so hard to create a full-colour image of our environment is evidence of the importance of color to our species.
Although personally I always thought Paul McCartney looked better in black and white.
