MENTAL-ROTATION EFFECT - A FUNCTION OF ELEMENTARY STIMULUS DISCRIMINABILITY

It is well known that when humans have to decide whether two different ly oriented shapes are identical or mirror images their performance de teriorates as a function of the orientation disparity (mental-rotation effect). Here it is shown that the effect can also be obtained reliab ly with non-mirror-image, arbitrarily different polygons provided they are previously selected to be hard to discriminate. The slope of the decision speed versus orientation disparity functions was found to be inversely related to the discriminability of shapes under conditions o f no, ie 0 degrees, orientation disparity. Easily discriminable polygo n pairs yielded essentially flat, no-effect functions. The arbitrary p olygons that were more difficult to discriminate produced a rotation e ffect that was similar to those of mirror-image polygon pairs. Mirror images in this context may only be a special case of hard-to-discrimin ate stimuli. We also show that the speed of judging whether simple lin es were of the same or different length was similarly subject to a rot ation effect provided that the length differences were sufficiently sm all, ie when their baseline dicriminability was poor enough. It is sug gested that the mental rotation of complex shapes (eg polygons) may bu ild on rotation effects pertaining to the simpler elements of which th ey are composed. Further, some special effects associated with the rot ation of such simpler components may explain certain peculiarities app arent in orientation invariance functions obtained with complex stimul i.