Dissociation of egocentric and object-centric processing in mental rotation of hand: Effect of viewpoints of the visual stimulus and the viewers' own hands

Abstract

 

We explored the possible dissociation of egocentric or object-centric processes in the mental rotation of hand, which may invoke a mental transformation of either the stimulus or viewer's own body parts. The stimulus consisted of back or palm view of human hands created by a 3D graphics software. Each picture of the hand was presented at an orientation rotated in medial or lateral direction from the upright orientation. Each participant completed (1) a left or right hand judgment task (LR task) when either a left or right hand picture was presented and (2) a same or different judgment task (SD task) when pictures of two hands were presented. During testing, participants seated in front of a computer screen with their hands positioned on their laps with either the back or the palm facing up. The results of the LR task showed that the performance was better (hands were recognized more quickly and accurately) for rotations in medial direction (in-rotation) than rotations in lateral direction (out-rotation) (we termed this “out-rotation” effect). When the viewpoints of the stimulus and the “viewpoints” (imagining participants looking down) of the participants' hands were congruent, the performance was better than that for the condition with incongruent viewpoints. All these effects were found only in the LR task but not in the SD task. These results suggest that the processing of mental rotation in the LR task is limited by the biomechanical constraints of the corresponding physical rotation. Participants use different spatial transformation mechanisms in the LR (egocentric) and the SD (object-centric) task. It appears that the type of material and the gesture of the observers' own body parts and paradigms of mental rotation all determine the reference frame adopted and the out-rotation effect might serve as indicator for the dissociation of egocentric and object-centric mental rotation.

 

Footnotes

 National Key Discipline of Basic Psychology in Southwest University (Grants NSKD05001) and the Natural Sciences and Engineering Research Council of Canada.