Scientists estimate that 3 to 6 percent of the population may be unable to count objects quickly. By isolating the brain's counting region, they are figuring out just how people calculate the number of items present.

The problem in identifying the precise region is that counting typically involves language, and the language areas also come online when the brain enumerates. To keep them offline during experi-ments, postdoctoral researcher Fulvia Castelli of the California Institute of Technology used colors. That was when she found that the intraparietal sulcus--a long sliver of tissue in the back of the brain--tabulates how many and not how much. Volunteers were shown a series of blue and green flashes of light filling rectangles on a video chessboard. When the colors appeared in isolated squares the sulcus was activated, but when the colors were strung together in a row it was not.

This article was originally published with the title Quick, How Many?.

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