The principle of motion adaptation isn’t all that different from the one illustrated by the color aftereffect. Stare at the fixation spot in a between the two vertically aligned squares—the top one red, the bottom one green. After a minute, look at the blank gray screen in b. You should see a ghostly bluish-green square where the red used to fall in your visual field and a reddish square where the green used to be. The effect is especially powerful if you blink your eyes.
This color-adaptation aftereffect occurs mainly in the retina. The eye has three receptor pigments–for red, green and blue—each of which is optimally (but not exclusively) excited by one wavelength. Light that contains all wavelengths and thereby stimulates all three receptors equally yields a ratio that the brain interprets as white. If your red color receptors become fatigued from staring at a red square, then when you look at a field of white or light gray, the ratio of activation shifts in favor of greenish blue, which is what you see.
Orientation adaptation, discovered by Colin Blakemore, then at the University of Cambridge, is another striking example of this phenomenon, except that (like the waterfall effect) it occurs in the brain, not the eye. Stare at the anticlockwise-tilted lines in c for a minute (while moving fixation within the central disk) and then transfer your gaze to the vertical lines in d. You will be startled to find the vertical lines tilted in the opposite direction, clockwise. This perception allows the inference that orientation-specific cells do exist in the human brain: the adaptation to tilt “tilts” the balance of activity among the orientation-specific neurons, favoring those that are attuned to the opposite, clockwise direction.
Even more exciting was Celeste McCollough’s discovery during the early 1960s, while on sabbatical from Oberlin College, of “double duty” cells in humans. Her experiments showed that in addition to cells that respond specifically to a color or an orientation, there are cells that respond only to a line that is both tilted and colored appropriately (that is, a cell for “red line tilted 45 degrees clockwise” or for “green line tilted 10 degrees anticlockwise,” and so on).
Look at e (horizontal black and red bars) for 10 seconds, moving your eyes around the central fixation (don’t keep staring just at the fixation) and then at f (vertical green and black bars) for 10 seconds. Alternate between them about 10 times each. By doing so, you tire all the color receptors in your retina about equally. If you then look at white paper, you see white—no colors. But an astonishing thing happens if you look at g and h, which consist of black and white horizontal or vertical bars. (Move your eyes back and forth betweeen them.) The white horizontal lines now look tinged green and the vertical ones red! The effect is even more striking if you look at the patchwork quilt (i).
Why does this happen? The McCollough effect suggests that subsequent to the retinal processing, some cells in the brain’s visual pathway extract two features along independent dimensions simultaneously. For simplicity, assume there are just four types of these cells: red-vertical, green-vertical, red-horizontal and green-horizontal. Because e fatigues only the red-horizontal cells, you are left with nonfatigued green-horizontal cells, which are then relatively active when you look at white horizontal stripes. Consequently, the white horizontal stripes look greenish; f has the reverse effect on the cells: because green-vertical cells have been selectively adapted, white vertical stripes now appear reddish. But none of these aftereffects occurs when you look at blank white paper because your eye movements ensure that all color receptors are equally stimulated on the retina, whereas cortical cells that have an orientation specificity are not stimulated.
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Reply | Report Abuse | Link to thisVery interesting experiments indeed. It would help a lot to have the corresponding images though... Where are they?
Reply | Report Abuse | Link to thisHow do you know how many teeth Mrs Aristotle had? Aristotle does not differ from today's scientists in accepting data from other researchers, in not having enough time to check everything by his own experiment, nor in concentrating his efforts in only one of theory vs experiment.
Reply | Report Abuse | Link to thisBut Aristotle did have a stronger sense of logic and creativity and insight than most of the people in science today. He is remarkable in having produced work that passed later researchers' empirical and theoretical trials for millennia. It will be interesting to see what ideas from today come to be so far reaching in scientific thought for millennia to come.
the lack of pictures probably raises the average customer's frustration level. management probably hopes this will raise the occurrence of paying customers.
Reply | Report Abuse | Link to this"Curiously enough, much of the current scientific understanding of that process is based on the study of visual illusions."
Reply | Report Abuse | Link to thisWhy should this be "curious" sight and hearing are certainly the most 'available' of the 6 senses,(the sixth being consciousness).
When anyone thinks he has identified a mistake made by one of the philosophers, we are often quite interested in examining the text. Sometimes a mistake is just a mistake, I imagine, but such cases are the least interesting. On careful consideration, some apparent mistakes come to seem deliberate and revealing. Others dissolve upon reinterpretation. I'm delighted you had recourse to Aristotle's texts when writing your article. When you write about him in the future, please include mention of the name of the work and cite the passage. Some of your readers will love you all the more.
Reply | Report Abuse | Link to thisScientific American's error...
Reply | Report Abuse | Link to this"The eye has three receptor pigmentsfor red, green and blueeach of which is optimally (but not exclusively) excited by one wavelength."
Sorry, but the color receptors in the retina do not correspond to the classic additive primary colors of red, green and blue. The three distinct cone cell types (S, M and L) responsible for color perception respond to chromatic light with peaks in violet, green and yellow-orange. For reference:
http://en.wikipedia.org/wiki/File:Cone-fundamentals-with-srgb-spectrum.png
Will you Please clear up the question between (l). blue, green and red Vs (2). violet, green and yellow-orange? ?
Reply | Report Abuse | Link to thisIt seems quite remarkable to me, in the implicatons, and the apparent color combinations of the second.
Waiting.
Hypodontia (having fewer teeth) occurs in about a quarter to a third of Caucasians. One fifth of them are men; four fifths are women. Hyperdontia (having extra teeth) is in contrast very rare among Caucasians. Is it not the case, then, that Caucasian women have, on average, fewer teeth than Caucasian men? I guess that the authors didn't bother to count, which leads me to wonder what other facts they fabricate in the guise of science.
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