Pixel asymmetry predicts between-object differences in the object-based compatibility effect

When participants make left/right responses to unimanually graspable objects, response times (RTs) are faster when the responding hand is aligned with the viewed object's handle. This object-based compatibility effect (CE) is often attributed to motor activation elicited by the object's afforded grasp. However, some evidence suggests that the object-based CE is an example of spatial CEs, or Simon effects, elicited by the protruding nature of objects' handles. Moreover, recent work shows that the way in which objects are centred on-screen might attenuate or reverse CEs, perhaps due to differences in pixel asymmetry (the proportion of pixels either side of fixation) between centralities. In this study, we tested whether pixel asymmetry also contributes to between-object variation in object-based CEs. In experiment 1 (N = 34), we found that between-object differences in asymmetry predicted object-based CEs, such that objects with a greater proportion of pixels to the handle-congruent side of fixation produced larger CEs. In experiment 2 (N = 35), we presented participants with mug (low asymmetry) and frying pan (high asymmetry) images and found that between-object and within-object (due to stimulus centrality) differences in pixel asymmetry interact to moderate CEs. Base-centred stimuli (centred according to the width of the object's base) produced conventional CEs, whereas object-centred (centred according to the object's total width) stimuli produced negative CEs (NCEs). Furthermore, the effect of centrality was smaller for mugs than pans, indicating an interaction between within-object and between-object differences in pixel asymmetry.