Babies reacted similarly when presented with pairs of small numbers (2 vs 3) and distant, large numbers (4 vs 12).
And researchers controlled for several, non-numerical variables that could have influenced babies—like the total surface area of the objects, the average area devoted to each object, the total luminance of each image, and the total area occupied by each group of objects.
So babies weren’t simply attending to the continuous extent of “stuff” in each image.
In fact, other experiments suggest that babies pay more attention to change in number than they do to changes in continuous extent or surface area (Cordes and Brannon 2008).
Nor is it all “in the eye.” If babies’ feats of numerical discrimination were confined to visible objects, we might suspect that the ability is domain specific—a specialization of the visual system rather than a more general conceptual ability. But babies aren’t one-trick ponies. In addition to distinguishing visual arrays, babies can also detect differences in the number of sounds and in the number of actions (Lipton and Spelke 2003; Wood and Spelke 2005).
Babies can even understand ordinality—the idea that numbers can be arranged in order of magnitude. When 11-month old babies were presented with sequences of numerosities, they could discriminate between sequences that increased and sequences that decreased (Brannon 2002).
Of course, this doesn’t mean that babies can distinguish any two numbers. There are limits. For instance:
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Moreover, when babies perform these feats they activate the same parts of the brain that are associated with mathematical thinking in adults.
Although we can get a precise measure of numerosity by counting, it’s possible to appreciate numerosity in a more approximate way.
For instance, if I ask you to glance in a crowded elevator and estimate how many people are there, you can probably make a pretty good guess.
Glance inside two elevators, and you’ll also be pretty good at guessing which one has more people in it.
All without counting.
It turns out that adult humans aren’t the only creatures who can pull this off. A variety of non-verbal creatures—including monkeys, rats, and human infants—can detect the approximate difference in magnitude between two sets (Dehaene 1999).
Show them two displays—one featuring 8 items and the other only 4 items—and they will respond differently depending on what they see.
And the difference is observable in the brain.
When 3-month old infants were shown a series of displays depicting different quantities of objects, the electrical activity of their brains changed in response (Izard et al 2008).
For example, babies were presented with a continuous stream of images, each depicting 4 objects. From image to image, the objects were arranged differently. But the total number of objects remained the same.
Infants subjected to this program showed signs of boredom.
But their brain activity (as measured by event-related potentials, or ERPs) would perk up if they were next presented with an image showing 8 objects.
What’s more, the part of the brain that was activated was the same region that lights up when adults process information about numbers (Izard et al 2008).
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