Although our perception of the world seems effortless and instantaneous, it actually involves considerable image processing, as we have noted in many of our previous columns. Curiously enough, much of the current scientific understanding of that process is based on the study of visual illusions.
Analysis and resolution of an image into distinct features begin at the earliest stages of visual processing. This was discovered in cats and monkeys by a number of techniques, the most straightforward of which was to use tiny needles—microelectrodes—to pick up electrical signals from cells in the retina and the areas of the brain associated with vision (of which there are nearly 30). By presenting various visual targets to monitored animals, investigators learned that cells in early-processing brain areas are each sensitive mainly to changes in just one visual parameter, not to others. For instance, in the primary visual cortex (V1, also called area 17), the main feature extracted is the orientation of edges. In the area known as V4 in the temporal lobes, cells react to color (or, strictly speaking, to wavelengths of light, with different cells responding to different wavelengths). Cells in the area called MT are mainly interested in direction of movement.
One characteristic of these cells that may seem surprising is that their activity when stimulated is not constant. A neuron that responds to red, for instance, will initially fire vigorously but taper off over time as it adapts, or “fatigues,” from steady exposure. Although part of this adaptation may result from depletion of neurotransmitters, it also likely reflects the evolutionary logic that the goal of the cell is to signal change rather than a steady state (that is, if nothing changes, there is literally nothing for the cell to get excited about).
How do we know that such cells also exist in humans? Simply put, we descended from apelike ancestors, and there is no reason why we would have lost those cells during evolution. But we can also infer the existence (and properties) of feature-detecting cells in humans from the results of psychological experiments in which the short-term viewing of one pattern very specifically alters the perception of a subsequently viewed pattern.
For example, if you watch a waterfall for a minute and then transfer your gaze to the grass on the ground below, the grass will seem to move uphill. This illusion occurs because the brain normally interprets motion in a scene from the ratio of activity among cells responding to different directions of movement. (Similarly, the wide range of hues you see on the screen of your television set is based on the relative activity of tiny dots reflecting just three colors—red, green and blue.) By gazing at the waterfall, you fatigue the cells for downward movement; when you then look at a stationary image, the higher baseline of activity in the upward-motion cells results in a ratio that is interpreted as the grass going up. The illusion implies that the human brain must have such feature-detecting cells because of the general dictum that “if you can fatigue it, it must be there.” (This is only a rule of thumb. One of us “adapted” to the dreadful climate and food in England, but there are no “weather cells” or “food-quality cells” in his brain.)
The waterfall effect (or motion aftereffect, as it is also known) was first noted by Aristotle. Unfortunately, as pointed out by 20th-century philosopher Bertrand Russell, Aristotle was a good observer but a poor experimenter, allowing his preconceived notions to influence his observations. He believed, erroneously, that the motion aftereffect was a form of visual inertia, a tendency to continue seeing things move in the same direction because of the inertia of some physical movement stimulated in the brain. He assumed, therefore, that the grass would seem to move downward as well—as if to continue to mimic the movement of the waterfall! If only he had spent a few minutes observing and comparing the apparent movements of the waterfall and the grass, he would not have made the mistake—but experiments were not his forte. (He also proclaimed that women have fewer teeth than men, never having bothered to count Mrs. Aristotle’s teeth.)
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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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