Decades of research suggests that humans share with other animals a way to mentally keep track of numbers and quantities - a number sense. These mental representations underlie animals' ability to make decisions (e.g., where to forage... the farther patch with larger fruit? or the closer patch with more numerous, but smaller fruit?) and thus guide our behavior. In humans, we even know which brain areas are responsible for these abilities. [pdf]
Developmental scientists study the origins of these abilities in infants because understanding their development can give us important insights into how we might intervene in atypical development in order to promote positive outcomes.
In our lab, we conduct basic research exploring infants' number abilities. Some of our studies reveal quite impressive abilities in young infants. In some early work, 6- and 7-month-olds were able to tell when a puppet made a different number of sounds [pdf], and they were also able to tell when a single sound had changed in duration [pdf]. Interestingly, babies' ability to compare numbers and their ability to compare durations (time!) was so similar, we concluded that they may be using the very same brain processes to represent both number and time.
In subsequent studies, we asked whether infants can combine number and duration to compute ratios. In a series of experiments, former graduate student Yi Mou showed that by 6 months of age, babies are not only sensitive to ratio information in visual displays (e.g., red:blue dots), but they can detect which of two visual displays matches in ratio the number of rising:falling tones they hear.
In a fun study conducted at the MU Child Development Lab, we allowed preschoolers' to "forage" from two reward locations. Children were optimal foragers, dividing themselves between each location according to the amount of reward available there. This behavior is seen in other animals, and is considered "ideal" because it maximizes reward at the group level.
Numbers get their meaning from their ordering. Numbers that come earlier in the count list are smaller than numbers that come later, and later numbers are bigger than earlier numbers. Knowing that babies are sensitive to number in their environment, what might they do with that information? In a series of studies, we show that babies use their number sense to choose the larger of two quantities of food! [pdf]
But they still have lots to learn. One early limitation is that until almost 2 years of age they seem unable to compare small sets (i.e., 1, 2, 3) to large sets (4 or more). Our studies suggest that babies in fact use different cognitive mechanisms to keep track of small sets and large sets, and that until about 22 months of age, these two brain systems are unable to communicate, making it hard for babies to tell whether 4 crackers is more than 2 crackers, or the other way around! [pdf]
To test whether infants might be influenced to represent small and large numbers within the same brain system, we ran a series of studies in which we "primed" infants to use their "large number" system for small numbers too. Simply showing infants a bunch of 10 Cheerios on a plate (before hiding the comparison amounts) seemed to enable infants to encode the small and large sets similarly, allowing them to make comparisons across this "set size boundary" several months earlier than is typical.
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