1. Dissociation of Egocentric and Object-Centric Processing in Mental Rotation Tao, W.D. 1, Yan, J.J. 1, Wang, P. 1, Zhou, L. 1, and Sun, H-J. 2 1 Southwest University, ChongQing, China 2 McMaster University, Hamilton, Ontario, Canada, sunhong@mcmaster.ca Introduction Experiment 1 Method Experiment 1: Result and Discussion Experiment 2 Method Experiment 2: Result and Discussion General Conclusion Mental rotation can take either egocentric or object-centric forms. We explored a possible dissociation of these two processing mechanisms by using a special stimulus (hand), which may invoke a mental transformation of either the viewer’s own body (hand) or the visual display. Stimulus: Back view of human hands which were created by a 3D graphics software, was presented at 45°, 90°, 135°in medial (In-Rotation) or lateral (Out- Rotation) direction from the upright orientation in the picture plane. LR Task: A left or right hand was presented on the screen, participant were asked to indicated the presented hand was left or right one by pressing keyboard. Design: 2 (Hand: left or right)× 2 (Directions of rotation: medial or lateral ) × 3 (Degree of rotation: 45°, 90°, 135°) SD Task: Pictures of two hands were presented simultaneously on screen, the left side of screen was always in an upright hand and the right side was in several different orientations of hand. Participants were asked to indicate whether the presented pictures were the same or different by pressing keyboard. Design: 2 (Match State: match or no-match)×2 (Hand: left or right)× 2 (Directions of rotation: medial or lateral )× (Degree of rotation: 45°, 90°, 135°) Participants: N=23 (11 male, 12 female). Stimulus: the same to Exp 1 except that both the back and palm picture of the hand were presented at 25°, 55°, 85°, 115°, 145° LR task and SD task: the same to Exp1 except both back and palm views were presented while the back of participants’ hand facing up. Participants responded by microphone Participants: N=16 (7 male, 9 female). The results of Exp1 and Exp2 suggest Ss use different spatial transformation mechanisms in LR (egocentric) and SD (object-centric) task. It appears that both the material of the body parts and paradigms of mental rotation determine the reference frame participants adopted and the Out-Rotation effect might serve as indicator for the dissociation of egocentric and object- centric mental rotation. A B Rotate medially laterally Fixation “ ＋ ” (200ms) Blank screen (500-1000ms) Target stimulus (until respond) Response Delay before next trial (800ms) Trial Procedure Material A was used in Exp1 while both A and B were used in Exp2. Respond through keyboard in Exp1. Respond by microphone in Exp2. Effect of Direction of Orientation (we termed it “our-rotation effect”) LR task, In-Rotation (rotated medially) > Out-rotation SD task, no difference Explanation: LR task: limited by the biomechanical constraints of the corresponding physical rotation especially in Out-Rotation SD task: no such constrains Conclusion LR task: egocentric frame of reference SD task: object-centric frame of reference. The same patterns of result were observed as in Exp 1, for both back and palm views. LR task: the back view > palm view SD task: no difference Reaction time (ms)Error rate (%) SD Task Reaction time (ms) Error rate (%) Back ViewPalm ViewBack View Back vs Palm LR Task Comparison between LR & SD Degree of Rotation Task View Task Supported by research grants to HJS from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI) and the National Key Subject Projects of Ministry of Education of China – Basic Psychology in Southwest University (Grant Number: XGZ413062).