A disassociation between physical and mental number bisection in developmental dyscalculia

Non-deficit participants present a leftward bias (pseudoneglect) in a physical line bisection task. This is also true for the mental number line bisection task. Physical and number line bisection were examined in university students diagnosed as suffering from developmental dyscalculia (DD) (IQ and readings abilities in the normal range and no indication for Attention-Deficit Disorder) and matched (hand, age, and sex) controls. DD participants presented an abnormal pattern of performance in these two tasks. In the physical number bisection task, no consistent bias was observed. This result implies a difficulty in visual attention. In contrast, in the mental number line bisection task, the DD group presented larger pseudoneglect than the control group. This result implies that the representation of numbers on the mental number line is different in the DD group and in the control group. It is suggested that those with DD have a more prominent tendency toward a logarithmic representation of numbers. The dissociation between physical and numerical bisections in those with DD implies that the source of the pseudoneglect in the two tasks is not identical.