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David Burr, John Ross; A visual sense of number. Journal of Vision 2008;8(6):691. doi: 10.1167/8.6.691.
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Evidence exists for a non-verbal capacity to apprehend number, in humans (including infants), and in other primates. We investigated numerosity perception in adult humans, using the psychophysical techniques of adaptation. Adapting to large numbers of dots increased apparent numerosity (by a factor of 2–3), and adapting to small numbers increased it. The magnitude of adaptation depended primarily on the numerosity of the adapter, not on size, orientation or contrast of test or adapter, and occurred with very low adapter contrasts. Varying pixel density had no effect on adaptation, showing that it depended solely on numerosity, not related visual properties like texture density. We propose that just as we have a direct visual sense of the reddishness of half a dozen ripe cherries so we do of their sixishness. In other words there are distinct qualia for numerosity, as there are for colour, brightness and contrast, not reducible to spatial frequency or density of texture.
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