Interference between number processing and line bisection: a methodology

It is usually thought that numbers are represented on a “mental number line” on which the progression of smaller to larger numbers occurs from left to right suggesting a spatial representation of numbers. Reciprocally, number processing has been shown to influence space representation [Fischer, M. (2001). Number processing induces spatial performance biases. Neurology, 822-826]. Specifically, it was found that when bisecting a string of numbers made up of smaller digits (1 or 2), the bisection was shifted to the left compared to bisecting a larger digit-string (8 or 9). In order to avoid any effect of physical difference in stimuli Here, we provide a simple methodology to investigate this relationship using a task involving the bisection of French number words (e.g.“DEUX”, “NEUF”). Results showed that subjects bisected strings more towards the left for smaller number words (“DEUX”, 2) than for larger number words (“NEUF”, 9), confirming that automatic number processing can influence visuo-motor aspects of behaviour. Similar results have been obtained with mirror versus canonical presentation, indicating that the reading direction is not the crucial factor of this number-induced bias. In addition, our sample of subjects showed a larger effect for number word strings than for Arabic number strings. It is therefore concluded that the implicit activation of the left/right side depends on the magnitude of the number being embedded within the stimulus lines, irrespective of its actual presentation format.