What feels parallel strongly depends on hand orientation

Parallel in the outside world is not necessarily perceived as parallel. Previous studies have shown that what is felt as parallel can deviate significantly from what is physically parallel. In a new set-up, the influence of hand/arm orientation is investigated in detail by systematically varying the angle between the two hands, while the participants have to make a test bar parallel to a reference bar. Large positive deviations were found of about 32 % of the angle between the hands. The deviations were always in the direction of the rotation of the right hand with respect to the left hand. These findings are consistent with the hypothesis that the haptic perception of spatial relations is biased in the direction of the egocentric reference frame connected to the hand.