Sinistrals' upper hand: evidence for handedness differences in the representation of body space.

A difference in the perception of extrapersonal space has been shown to exist between dextrals and sinistrals. On the classical line bisection task, this difference is evident in a greater left bias for dextrals compared to sinistrals. Different modalities and regions of space can be affected. However, it has not yet been investigated whether a systematic bias also exists for the perception of personal or body space. We investigated this by using three tasks which assess different aspects of personal space in an implicit and explicit way. These tasks were performed by strongly right-handed (dextrals), strongly left-handed (sinistrals) and mixed-handed participants. First, a task of pointing to three areas of one's own body without the use of visual information showed dextrals to have an asymmetric estimation of their body. In right hemispace, dextrals' pointing was at a greater distance from the midsagittal plane compared to pointing in left hemispace. No such asymmetry was present for sinistrals, while mixed-handers' performance was intermediate to that of strong right- and strong left-handers. Second, a task of recovering circular patches from their body surface whilst blindfolded also showed superior performance of sinistrals compared to dextrals. On these two tasks, there was also a moderate relationship between handedness scores and performance measures. Third, a computer-based task of adjusting scaled body-outline-halves showed no handedness differences. Overall, these findings suggest handedness differences in the implicit but not explicit representation of one's own body space. Possible mechanisms underlying the handedness differences shown for the implicit tasks are a stronger lateralization or a greater activation imbalance for dextrals and/or greater access to right hemispheric functions, such as an "up-to-date body" representation, by sinistrals. In contrast, explicit measures of how body space is represented may not be affected due to their relying on a different processing pathway.