Several studies have indicated that children have difficulty differentiating mirror-image stimuli. In the present study adults were required to classify pairs of horseshoe stimuli as same or different. Response times were compared for stimulus pairs that varied in orientation (left-right vs up-down) and spatial plane of the pair (horizontal vs. vertical). Stimulus pairs in which the orientation matched the spatial plane of the pair (i.e., horizontal and left-right or vertical and up-down) took longer to classify than stimulus pairs in which these two variables were crossed. These results are interpreted as reflecting the necessity of synthesizing two sources of information in order to compare the former pair types--temporally encoded visual information and directional information from the motor scanning process. Implication for the source of children's difficulty with mirror-image stimuli of this type are discussed. (Author)