Brain Region That Distinguishes Amounts Mapped

Meaning, the neurons that work to make this “numerosity” evaluation are laid out in a shape that allows those most closely connected to communicate and interact over the shortest possible distance.

This layout, referred to as a topographic map, is characteristic of all primary senses and scientists have long believed that numerosity, while not a primary sense (but perceived similarly to one), might also be characterized by such a map. However, they have not been able to find it, a fact, which has caused some doubt in the field as to whether a map for numerosity exists. Now, however, Utrecht University’s (The Netherlands) Dr. Benjamin Harvey, working with his colleagues, has found signals that illustrate the hypothesized numerosity map is tangible.

Numerosity is different from symbolic numbers. “We use symbolic numbers to represent numerosity and other aspects of magnitude, but the symbol itself is only a representation,” Harvey said. He went on to explain that numerosity selectivity in the brain is derived from visual processing of image features, where symbolic number selectivity results from recognizing the shapes of written words, numerals, and linguistic sounds that represent numbers. “This latter task relies on very different parts of the brain that specialize in written and spoken language.”

Determining whether the brain’s processing of numerosity and symbolic numbers is linked, as one may think, is only one area that will be better informed by Dr. Harvey’s new map. To reveal it, he and his colleagues asked eight adult study participants to look at patterns of dots that varied in number over time, all the while analyzing the neural response characteristics in a numerosity-linked part of their brain using high-field functional magnetic resonance imaging (fMRI). Use of this advanced neuroimaging modality allowed them to scan the subjects for a lot less hours per sitting than would have been required with a less powerful scanning technology.

With the fMRI data that resulted, Dr. Harvey and his team employed population receptive field modeling, with a goal of measuring neural response as straightforwardly and quantitatively as possible. “This was the key to our success,” Dr. Harvey said. It allowed the researchers to model the human fMRI response properties they observed following results if recordings from macaque neurons, in which numerosity research had been conducted more comprehensively.

The investigators’ research revealed a topographic arrangement of numerosity in the human brain; the small amounts of dots the participants saw were encoded by neurons in one part of the brain, and the larger quantities, in another region. This finding demonstrates that topography can emerge not just for lower-level cognitive functions, like the primary senses, but also for higher-level cognitive functions. “We are very excited that association cortex can produce emergent topographic structures,” Dr. Harvey said.

Because scientists know a lot about topographic maps and have the tools to probe them, this research may help them better analyze the neural computation that is the basis of number processing. “We believe this will lead to a much more complete understanding of humans’ unique numerical and mathematical skills,” Dr. Harvey said.

Dr. Harvey and colleagues were amazed that they were able to obtain the findings they did, noting that the task was complicated. They also found the variations between their subjects interesting. “Every individual brain is a complex and very different system,” Dr. Harvey explained. “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. On the other hand, the extent of individual differences … is also striking.” Harvey explained that understanding the consequences of these differences for their subjects’ perception or task performance would require further study.

The study’s findings were published September 6, 2013, in Science.

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