Number processing induces spatial performance biases

Background: Response speed in parity judgments is faster with the left hand for small numbers (e.g., 1 or 2) and faster with the right hand for larger numbers (e.g., 8 or 9). This effect suggests that number processing can induce systematic spatial biases in bisection tasks. Method: Neurologically healthy participants bisected visually presented stimuli with a pencil. Stimuli were long strings of uniform digits (experiment 1) or lines with single digit flankers (experiment 2). Results: Bisection performance was biased to the left of center for strings made of digits 1 or 2 and to the right of center for strings made of digits 8 or 9. Line bisection was biased toward the flanker representing the larger magnitude, regardless of its position. Conclusions: These results extend previous findings and support the notion of an automatic association of number magnitudes with spatial response codes. The effect may be useful for an assessment of semantic number processing in special populations.