Less than four at one blow

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The Power of Numerical Discrimination, Nature, 9 February 1871), and ever since, there are scientists who have wondered about this threshold of 4. If Jevons wondered about it, too, he didn’t have access to the tools of modern neuroscience to help him find an explanation. But a recent study does have a possible explanation (Distinct neuronal representation of small and large numbers in the human medial temporal lobe, Esther F.

Kutter et al., Nature Human Behaviour, 2 October 2023). The idea was to monitor how the brain behaves when faced with such an estimation task. To do this, naturally we cannot simply implant electrodes into the brains of living humans.

Instead, these scientists found a set of people who already have such implants—epileptics who have the electrodes for medical reasons—and were willing to be studied. These patients were asked to do simple mental arithmetic while their brains were monitored. What they found is, to me, simply startling.

For each number, individual neurons lit up, or became active. That is, a “three"-tuned neuron fires up only when offered the number three; a “six"-tuned one, only the number six. Such “number neurons" had been identified in animals before, but never in humans.

But after she analysed 801 of these neuron firings, Kutter was able to tease out two different patterns. As you might guess, one was for small numbers, the other for large ones. The neurons tuned for four and below were nearly error-free.

That is, they lit up only for their preferred numbers and not for others. In contrast, the neurons tuned to steadily higher numbers than four were steadily less precise in their firing. Sometimes they didn’t fire when expected; sometimes they fired when fed a slightly different