How many times have you come across a contest asking you to guess how many marbles, paper clips, or some other objects, are inside of a large jar? Estimating gets easier the smaller the number of marbles or paper clips even with a single glance, almost like identifying a smell with a single whiff. Scientists say their discovery of the specific parts of the brain responsible for so-called “numerosity” act almost like a sixth sense.
“When we see a small number of items visually, we don’t need to count them,” Ben Harvey, of Utrecht University in the Netherlands, and lead author of the study, told NPR. “We just know how many there are straight away.”
Previous research on monkeys found that certain areas of their brains became active when given the task of placing images in order according to the number of objects they contained. Their brain cells are organized in a way that suggests numbers were represented topographically, in almost the same ways that other senses are mapped in the brain. But until now, these areas of the brain that sense numbers weren’t explored.
Numerosity is different from numerical symbols, Harvey says. “We use symbolic numbers to represent numerosity and other aspects of magnitude, but the symbol itself is only a representation.”
Harvey and his team examined eight adult participants’ brains using high-field functional magnetic resonance imaging (fMRI), a powerful brain-imaging technology. The participants were told to look at different patterns of dots that changed over time. They were able to model their human responses to those of the monkeys. Responses occurred in the parietal cortex, a part of the brain just above and behind the ears that has been associated with numerical thinking. They found that small quantities of dots were encoded by neurons closer to the midline of the brain, while neurons furthest from it responded to larger quantities, creating a map of how people estimate numbers.
“Every individual brain is a complex and very different system,” Harvey said in a statement. “I was very surprised then that the map we report is in such a consistent location between our subjects, and that numerosity preferences always increased in the same direction along the cortex.”
Although accuracy worsened as the quantity of dots increased — Harvey says most people can accurately sense five dots — the patches in the brain responsible for these senses work similarly to the way the brain maps other senses, such as sight and touch. “There are maps on the brain that represent the surface of the skin — or the surface of the retina,” Harvey told NPR. “These all reflect an organ. We found the first map for a cognitive function.”
Source: Harvey B, Klein B, Petridou N, et al. Topographic Representation of Numerosity in the Human Parietal Cortex. Science. 2013.