Leftward bias in number space is modulated by magical ideation

Productive symptoms of schizophrenia and positive-symptom schizotypy have both been related to signs of right-sided hemispatial inattention ("pseudoneglect"). We here set out to explore, in healthy subjects, the relationship between one form of mild schizotypy ("magical ideation"; MI) and asymmetries in number space, which is a bias toward relatively small numbers, reportedly represented to the left of larger numbers.

Forty right-handed participants filled in the MI scale and performed a number-line bisection (NLB) task and a randomization task (the Mental Dice Task, MDT, requiring randomization of the digits from 1 to 6).

We found pseudoneglect in number space, that is, more errors toward small numbers in the NLB task and an overproduction of small digits in the MDT. Individual participants' MI scores were correlated to the size of pseudoneglect in both numerical tasks.

Explicit (NLB) and implicit (MDT) assessments of the exploration of number space may be relevant to studies of the mechanisms underlying the formation of delusional and schizotypal beliefs. We propose that, in healthy subjects, a trait-like imbalance in hemispheric cooperation may not only produce asymmetries in physical and representational space, but also predisposes to develop magical ideas. Specifically, an over-proportional influence of the right hemisphere semantic system (preferentially coding oblique and remote associations) leads to the assumption of connections between randomly associated events.