Why is telling right from left more difficult than telling above from below?

Adults take longer to judge the locations of horizontal stimuli than to judge the locations of vertical stimuli. In order to determine the source of this difficulty with the horizontal dimension, the congruity between the locations of stimuli and verbal descriptions was judged in a reaction time (RT) task. Because bilateral symmetry of the nervous system may be related to the difficulty with horizontal stimuli, this was varied by using right-handed, left-handed, and ambidextrous subjects. However, this variable produced no significant effects in the RT task. Horizontal stimuli took longer than vertical stimuli whether the verbal description was encoded before or during the RT periods, suggesting that label encoding is not the entire source of the effect. However, when the verbal labels were eliminated entirely by having subjects learn and use stimulus-letter pairs, horizontal stimuli did not take longer than vertical stimuli. This suggests that perception of the stimulus is not the cause of the difficulty. Together, the experiments indicated that comparing horizontal labels to stimuli is the largest source of the difficulty in telling right from left. Reasons why adults have such a problem were discussed.