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Our Brains Have a Map for Numbers

Jan 14, 2014 |By Emilie Reas

“Come on. Get out of the express checkout lane! That’s way more than twelve items, lady.”

Without having to count, you can make a good guess at how many purchases the shopper in front of you is making. She may think she’s pulling a fast one, but thanks to the brain’s refined sense for quantity, she’s not fooling anyone. This ability to perceive numerosity – or number of items – does more than help prevent express lane fraud; it also builds the foundation for our arithmetic skills, the economic system and our concept of value.

Until recently, it’s remained a puzzle how the brain allows us to so quickly and accurately judge quantity. Neuroscientists believe that neural representations of most high-level cognitive concepts – for example, those involved in memory, language or decision-making – are distributed, in a relatively disorganized manner, throughout the brain. In contrast, highly organized, specialized brain regions have been identified that represent most lower-level sensory information, such as sights, sounds, or physical touch. Such areas resemble maps, in that sensory information is arranged in a logical, systematic spatial layout. Notably, this type of neural topography has only previously been observed for the basic senses, but never for a high-level cognitive function.

Researchers from the Netherlands may have discovered an exception to this rule, as reported in their recently published Science paper: a small brain area which represents numerosity along a continuous “map.” Just as we organize numbers along a mental “number line,” with one at the left, increasing in magnitude to the right, so is quantity mapped onto space in the brain. One side of this brain region responds to small numbers, the adjacent region to larger numbers, and so on, with numeric representations increasing to the far end.

To examine how the brain responds when perceiving quantities, the researchers conducted functional magnetic resonance imaging of the brain while participants viewed different numbers of dots on a screen. They included multiple versions of the task, keeping key features — like dot size, circumference and density — constant, to be certain that any effects were indeed attributable to dot quantity, rather than dot shape or size. The participants weren’t asked to judge the number of dots, to ensure that brain activity related to perceiving quantity, rather than counting. The researchers then looked for brain activity that systematically varied with the number of dots the participants viewed.

The scientists identified a region, a few centimeters wide, in the right superior parietal lobe (in the upper back part of the brain) that mapped numerosity. One edge of this patch (closer to the middle of the brain) responded maximally to small quantities, and the opposite edge (closer to the outside of the brain) responded to the largest quantities. The location and layout of this map was remarkably consistent across all eight individuals’ brains. Earlier studies reported that this same brain area in humans, and single neurons in an analogous part of the monkey brain, responded to numerosity. However, these studies had not detected this systematically organized map.

The researchers more closely examined how activity in this neural map related to the numbers and types of dots the participants viewed. They found that the parietal cortex map represented relative, not absolute, quantities. For instance, a given region might respond to two dots in one task condition, but to three in another; but across tasks, it always responded to small numbers of dots. Furthermore, the amount of cortex devoted to a given quantity varied, such that disproportionately more area represented small quantities, and less area represented large quantities. The map was more selective for smaller than larger numerosities. This system makes intuitive sense, as it corresponds with our subjective experience. It’s much easier to distinguish between one or two cookies left in the jar, than between eleven and twelve cookies. In light of these findings, this finer discrimination for smaller quantities might arise from their overrepresentation in the brain.

This isn’t the first time neuroscientists have observed maps in the brain. In fact, it’s well established that sensory and motor information, including representations of our visual surroundings, bodily space or sound frequency, is also topographically organized to subserve vision, touch, taste, smell and movement. For example, a homunculus, or “little man,” is mapped onto the brain’s motor and somatosensory cortices, such that different regions of this cortical map support movement and sensation in different body parts. The brain areas devoted to feeling the face and lips are adjacent, and the area responsible for toe movement lies next to that involved in ankle movement. This new study reveals that such maps aren’t limited to sensory and motor functions, but also exist for an abstract feature - numerosity.