Altered mental number line in amblyopia--reduced pseudoneglect corresponds to a decreased bias in number estimation

Amblyopic subjects show a wide range of changes within the visual system, starting from deficits in simple perceptual processing up to changes of higher functions of the visual dorsal pathway. Recent studies suggest that subjects with amblyopia also demonstrate alterations in visuo-spatial attention. In contrast to normal sighted subjects, who demonstrate a leftward bias ("pseudoneglect") during physical line-bisection, amblyopic subjects show a rightward bias ("minineglect"). Numerous studies have provided considerable evidence that humans represent numbers on a mental number line oriented from left to right, partly analogous to a physical line. In the present study, we investigated whether number representation is also changed in amblyopic subjects. Participants were asked to vocally name the numerical midpoint of two acoustically presented numbers without calculating. Supporting other studies, two control groups of normal sighted participants (each with n=14, group 1 matched for age and gender, group 2 matched for eyedness and education) underestimated the mean number, analogous to a leftward bias ("pseudoneglect"). Interestingly, amblyopic subjects (n=14) estimate the mean numbers less biased. These results support the assumption, that amblyopia also influences higher cognitive processes like number processing. Surprisingly, amblyopia seems to induce a more balanced number processing in contrast to healthy participants.