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Differences between viewing light and dark explain old optical illusion

Astronomers and physicists starting with Galileo noticed centuries ago that when one looks at celestial objects — bright objects on a dark background — they appear to be too large. Now scientists have discovered the brain mechanisms underlying this effect. The findings are reported in the Proceedings of the National Academy of Sciences.

Galileo was puzzled by how the appearance of the planets changed depending on whether one looked at them with the naked eye versus a telescope. Viewed directly, planets seemed “expanded” and had “a radiant crown,” which made Venus look larger than Jupiter, despite the fact that Jupiter was actually wider. Galileo’s assumption was that this illusion was due to light scattered from aberrations and other factors in the eye. Hermann von Helmholtz, the renowned 19th-century German physician-physicist, realized that optical blur could not provide the answer, and suggested that a neural factor concerning how we see was needed to explain the illusion.

Now neuroscientist Jens Kremkow at the State University of New York College of Optometry and his colleagues have found this illusion is due to the responses of neurons in the visual pathway and may originate in the very first cells of the pathway — the photoreceptors, which transform light into electricity. Light and dark stimuli are processed separately by ON and OFF pathways in the retina, a thin layer of neural tissue in the back of the eye that processes signals from the receptors to send to the brain. The ON pathway responds to bright objects on dark backgrounds, like a star on the dark sky, while the OFF pathway responds to dark objects on bright backgrounds, like a crow on a bright sky. The scientists found that neurons in the OFF pathway increase their responses roughly linearly with stimulus contrast and accurately represent stimulus size. However, neurons in the ON pathway increase their responses in a non-linear, exaggerated way making the stimuli look bigger than they are. Since Venus is closer to the Sun and Earth than Jupiter, Venus is brighter than Jupiter and the nonlinear responses of ON neurons make Venus appear larger to the naked eye.

The researchers suggest that, on dark backgrounds, the visual pathway increases sensitivity to detect the presence of a faint stimulus as best as possible, which compromises the ability to report the size of bright objects accurately. On bright backgrounds, there is no need for high sensitivity and the brain can more accurately report an object’s size and shape. This effect may also help explain why it is easier to read writing involving black-on-white lettering rather than white-on-black lettering, a well-known phenomenon that was unexplained until now. It might even help explain why dark clothes have a slimming effect.

Categories: Neuroscience
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