Scientists Find New Explanation for Perception Paradox
First proposed only 15 years ago, stochastic resonance now is thought to be important in many sciences, from engineering to biology. Some researchers believe that it may play a major role in the onset of ice ages -- glacial periods that repeat about every 100,000 years. In the animal kingdom, stochastic resonance figures prominently in life-and-death struggles. For example, it enhances the sensitivity of cells in a crayfish's tail, so that the crustacean can escape from approaching predators.
Recently, for the first time, a neuroscientist now at Johns Hopkins University and two other researchers have used a computer model to demonstrate the same phenomenon at work in the human brain, where the combination of several extraneous stimuli might actually enhance perception instead of interfering with it. The general scenario, also proposed by other scientists, goes something like this: a person is unable to pick up a sound that is too quiet for the range of human hearing. But if there are some extraneous noises at the same time, those noises might boost the overall sound so that the person is now able to hear the sound that originally was too quiet.
Common sense suggests that noise does not lead to better perception, but rather, to interference. The new computer model, however, supports previous theories to the contrary. The model was devised by neuroscientists Ernst Niebur, from Johns Hopkins; Martin Stemmler, a graduate student at the California Institute of Technology; and Marius Usher, a researcher from the University of Kent at Canterbury.
They created their model to solve a paradox of perception. The paradox arises from the following observations:
A person is asked to look at a line on a computer screen, and the line is continually dimmed, eventually becoming too faint to be seen. But if the dim line is surrounded by two brighter lines, one on either side, the person can see the middle line better and past the point where it would have become invisible if presented by itself.
So far, so good; these experiments seem to indicate that lines surrounding some area of the visual field enhance the activity of the neurons in this area. However, a paradox arises from animal experiments conducted by other researchers. They showed animals similar stimuli, a central line surrounded by others, and used an electrode to measure the activity of the neurons responding to the central stimulus directly. Surprisingly, they found just the opposite of what one would expect from the experiment on humans: the surrounding lines made the activity of the central neurons smaller rather than bigger.
Why then do people see the dim line better when it is surrounded by brighter lines? Niebur and his colleagues suggest that stochastic resonance is responsible.
If the center line is too dim to be seen, the outer lines add extraneous "noise" to the activity of the center neurons, and this noise boosts the overall stimulus (which would be too weak by its own) to within the range of perception. That behavior conforms to the rules of stochastic resonance: extraneous noise strengthens a weak signal. But once the center line is well within the range of perception,the beneficial influence of the noise becomes less and less important.
It was the first time scientists had applied stochastic resonance to perception in the brain of higher animals and humans. A paper about the work was published on Sept. 29, 1995, in the journal Science. At that time, Niebur, now at Johns Hopkins' Zanvyl Krieger Mind/Brain Institute, was a research fellow at Caltech, and Usher worked for Carnegie-Mellon University in Pittsburgh.
They will have to wait to see whether their model is correct. At present, there are no experimental data with which to test the model; no scientists have conducted specific experiments with animals to see how changing the brightness of the center and outer lines would alter the brain-cell response to impulses, Niebur said.
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