Neuron activity shows that the brain uses different systems for counting up to four, and for five or more.
For more than a century, researchers have known that people are generally very good at eyeballing quantities of four or fewer items. But performance at sizing up numbers drops markedly — becoming slower and more prone to error — in the face of larger numbers.
Now scientists have discovered why: the human brain uses one mechanism to assess four or fewer items and a different one for when there are five or more. The findings, obtained by recording the neuron activity of 17 human participants, settle a long-standing debate on how the brain estimates how many objects a person sees. The results were published in Nature Human Behaviour on 2 October.
Fun fact: there’s a name for the phenomenon of instantly recognizing the number of objects when it’s fewer than five. It’s called “subitizing.”
There’s a pretty interesting overview of what we know about math on the brain (or at least knew as of its writing) in the book “Where Mathematics Comes From” by Lakoff and Nuñez.