The limits of the human ability to estimate large quantities have puzzled many generations of scientists. In an 1871 Nature article, economist and logician William Stanley Jevons described his investigations into his own counting skills and concluded “that the number five is beyond the limit of perfect discrimination, by some persons at least.”
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Some researchers have argued that the brain uses a single estimation system, one that is simply less precise for higher numbers. Others hypothesize that the performance discrepancy arises from there being two separate neuronal systems to quantify objects. But experiments have failed to determine which model is correct.
Then, a team of researchers had a rare opportunity to record the activity of individual neurons inside the brains of people who were awake. All were being treated for seizures at the University Hospital Bonn in Germany, and had microelectrodes inserted in their brains in preparation for surgery.
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“The higher the preferred number, the less selective these neurons were,” says co-author Andreas Nieder, an animal physiologist at the University of Tubingen in Germany. For example, neurons specific to three would only fire in response to that number, whereas neurons that prefer eight would respond to eight but also to seven and nine. As a result, people made more mistakes when trying to quantify a larger number of objects.
