Visual discrimination of abstract mirror-reflected three-dimensional objects at many orientations.

Abstract

This study examined people’s discrimination of identical and mirror-reflected pairs of abstract three-dimensional objects like those used by Shepard and Metzler (1971). In contrast to previous research, the axis of the orientation difference (OD) between a pair of objects was quite unconstrained by experimental design. Each stimulus pair differed in orientation by an angle of rotation up to 180° about one of 13 disparate axes. The axis of OD was randomly ordered across trials. Subjects reported attempting to imagine one object at the orientation of the other and then comparing their shapes. Reaction time (RT) depended on the extent of OD, confirming earlier results. However, RT also depended on the axis or plane of OD between a stimulus pair. Slopes for RT-OD functions for trials with identical objects varied by a factor of 3 for different axes. The rank order of functions from steepest to slightest slope was: (1) axes not in any principal plane of the observer’s visual reference frame, (2) frontal diagonal axes, (3) horizontal diagonal axes, (4) the line-of-sight axis, (5) midsagittal diagonal axes, (6) the vertical axis, and (7) the horizontal axis perpendicular to the line of sight. There was no difference in performance between subjects achieving high scores on pretests of spatial ability and those achieving moderate scores. The variation in slopes is likely due to variation in rate, path length, and/or initiation time of imagined reorientations. Various geometrical procedures could underlie the imagined spatial transformations used in the task. RT patterns are much more consistent with the class of procedures producing paths of relatively efficient length than the class of inefficient procedures, such as that using rotations through Euler angles.