Tots Who Tote: Babies show neural signs of budding number sense

A 3-month-old baby can’t help you with your taxes, but nonetheless possesses a brain-based grasp of numbers, a new study indicates.

Previous studies suggested that, by 4-1/2 months, infants detect changes in the number of items in a set. Some researchers regard this feat as evidence for an early number sense. Others attribute it to babies’ tendency to notice novel events or to other nonnumerical cues. The new study is the first to connect infant number sense with brain activity.

By 3 months of age, infants already display distinct neural pathways for distinguishing the number of objects that they see as well as for identifying objects, say neuroscientist Véronique Izard of Harvard University and her colleagues. These pathways correspond to a pair of neural networks also found in adults.

A “where-and-how” network primarily monitors an object’s location, size, and potential uses, while a “what” network handles its identification.

“Our findings show that infants as young as 3 months are able to process numbers and to identify objects thanks to specific neural systems,” Izard says. Her team presents its findings in the February PLoS Biology.

Izard’s group studied 36 infants, all 3 months old. Each youngster wore a cap with sensors that recorded electrical activity from the brain’s surface.

While being held by a parent, babies watched a series of images on a computer screen. Most screens depicted the same object, such as a cartoon character, shown in arrays of the same number. Occasionally, an image showed either a new object or a different number of previously seen objects.

The infants’ brain surfaces displayed distinctive electrical responses within a fraction of a second after number and object changes.

The researchers then used a computer model to estimate neural sources of the infants’ electrical responses.

As has been reported for adults and children, brain areas along the top of the left temporal lobe responded vigorously in the infants’ brains as they observed object changes, accessing the “what” network.

Number changes elicit responses primarily from the “what-and-how” network, which functions in the lower part of the brain. But in the infants, this activity was focused in right-brain areas. Other studies indicate that left-brain contributors to this network appear during childhood and strengthen by adulthood.

The new study also counters thinking that infant awareness of changes in objects and numbers merely reflects a tendency for novel events to grab a baby’s attention, Izard maintains, because the networks triggered in the study lie largely outside of brain regions involved in attention.

Intriguingly, infants showed comparable neural responses to changes in both small (four or fewer) and large numbers of items. Psychologists have theorized that small-number detection relies on a system for tracking a few individual objects, whereas large-number detection depends on a system for estimating the magnitude of a group of items.

“These are interesting findings,” comments psychologist Susan C. Levine of the University of Chicago. A common neural pathway for small and large numbers in infants suggests that the magnitude-estimation system operates for all numbers, although a separate small-number system must also exist, Levine proposes.