Spatial biases in number line bisection tasks are due to a cognitive illusion of length.

Placing arrow heads (Judd Illusion) or numbers of different magnitude at the end of a line biases perception of the centre of the line. For the Judd Illusion, it is known that this bias depends on the method used: a deliberate (more perceptually based) marking of the centre with a pen is more subject to the illusion than are fast (more action-based) ballistic pointing movements made towards the centre. It has been suggested that the number bias also reflects a cognitive illusion of length. To test this assumption, we used two different response methods in line bisection tasks while lines were flanked by arrow heads or numbers of different magnitudes. For both conditions, we found that the more action-based response method showed less bias. Since the pattern of biases induced by flanking numbers and arrow heads are similar, we confirm that the spatial bias produced by numerical magnitude reflects a cognitive illusion of length.