Neuroscientists Kerry Jordan and Elizabeth Brannon had previously shown that rhesus monkeys have a natural ability to match the number of voices they hear to the number of individuals they expect to see. When presented with a soundtrack of "coo" sounds, the monkeys chose to look at a picture containing the same number of fellow monkey faces. If the monkeys heard two coos, for example, they preferred to look at a picture of two monkeys rather than three and vice versa. The researchers expected the same to be true of human babies.
But other studies with infants had delivered ambiguous results. Babies trained to expect to see two objects when presented with two tones stared longer at results that violated this convention, such as two tones and then three objects. And although babies correctly matched up drumbeats and household objects in one study, efforts to duplicate the result failed. Jordan and Brannon argue that each of these studies was flawed, either because training might have skewed the response, the tasks were too difficult or the objects were irrelevant to a baby.
The two researchers found in their own study that babies spent more time looking at videos showing the same number of unfamiliar human female faces as those represented in a simultaneous soundtrack of "look" sounds. "As a result of our experiments, we conclude that the babies are showing an internal representation of 'two-ness' or 'three-ness' that is separate from the [sounds and sights] and, thus, reflects an abstract internal process," Brannon says.
The 20 seven-month-old infants in the study spent an average of nearly 22 seconds looking at the numerically appropriate video compared to just more than 14 seconds looking at the numerically wrong video. This represented 59.2 percent of their total time looking--nearly exactly the same percentage of time that the 20 rhesus monkeys spent looking at the video with the right number of monkeys. "This spontaneous matching of [numbers] across [sight and sound] supports the contention that human infants, human adults and nonhuman primates share at least one common nonverbal numerical representational system," the researchers conclude. Their report on these findings appears in this week's Proceedings of the National Academies of Science.