Beyond the SNARC effect: distance-number mapping occurs in the peripersonal space

Estimating distance of objects relative to one's body is important for interaction with the environment. Given that distance is an interval of magnitude describing space, distance and the commonly used estimations of magnitude, i.e., numbers, may share a common representation system (the ATOM theory, Walsh in Trends Cogn Sci 7(11):483-488, 2003). The current study systematically examined the association between distance and number representations on both the sagittal and transverse axes on the transverse plane in the peripersonal space. Participants in Experiment 1 judged the parity of digits by pressing one of two buttons (both were in front of participants): One was near the body and the other away from it. We found that near responses were faster when paired with smaller numbers and far responses with larger numbers. When one button was set in front and the other in back in Experiment 2, no mapping was found. In Experiment 3, when both buttons were on the right side aligned with the transverse axis, small-near and large-far mapping were found. However, no such effect was found on the left side. These results suggest that numbers are mapped onto the whole transverse plane of the peripersonal space, not only a left-right oriented number line.