A Little to the Left, a Lot to the Right

Infants were familiarized with a long line paired with eighteen-syllable sequences (top), then tested with six-syllable sequences paired with a short line appearing on the left and right sides of the screen in subsequent trials.

Maria Dolores de Hevia, Ludovica Veggiotti, Arlette Streri, Cory D. Bonn

To cultures that read from left to right, numbers are usually represented as increasing in that same direction, such as the indicators on a ruler. Scientists have long wondered whether this directional mapping of number to space is innate or culturally taught. A new study of Parisian newborns has shown that infants as young as seven hours old tend to associate small quantities with left and large quantities with right.

A team of researchers, led by Maria Dolores de Hevia, at Paris Descartes University and the CNRS (National Center for Scientific Research) Psychology of Perception Laboratory, tested a total of eighty newborns, ranging in age from seven hours to three days, with various audiovisual cues indicating quantity and spatial location. In one experimental condition, a group of newborns heard several short sequences of six spoken syllables while viewing a short line projected in the center of a screen. Then, they were shown a longer line, once projected to the left and once projected to the right, while they listened to longer sequences of eighteen spoken syllables. The infants looked longer when the longer line was on the right than when it was on the left, suggesting that they associated the longer sequences and image with the right side of the screen. When another group of infants experienced the reverse condition—first seeing a long line in the center while hearing the long sequences, and then seeing left and right shorter lines while hearing the short sequences—they looked longer while listening to the short sequences when the short line was on the left, showing that they associated shorter with left.

The research team ran additional conditions to test whether this spatial mapping was driven by numeric information (the number of spoken syllables) or general differences in quantity (differences in line length or total sound time). When numeric information was removed, either by only showing the lines without any auditory information or by playing single long and short tones without discrete, countable syllables, the differences in left/right looking times disappeared. Thus, the researchers concluded that differences in number, not visual line length or total sound length, were behind the left to right mapping.

Maria Dolores de Hevia says she chose to study newborns because they “open the door to the understanding of the origins of cognition. With this study, we now know that our brain might be hardwired” to spontaneously organize numbers. (Current Biology)