Why are left-right spatial codes easier to form than above-below ones?

Previous studies have shown that spatial compatibility is stronger in the left-right than in the above-below dimension. This left-right dominance cannot be attributed to a better representation of the effectors in left-right than in above-below locational codes or to the fact that incompatible left-right stimulus-response pairings cross the body midline, whereas incompatible above-below ones do not. Nicoletti and Umiltà (1985) proposed that the left-right dominance should be attributed to the allocation of attention to the more difficult discrimination, which, in vision, is that concerning the left-right dimension. This attentional hypothesis was tested in the present study, in which we used the auditory modality. We reasoned that because in the auditory modality the above-below discriminations are more difficult than left-right ones, attention should be preferentially allocated to the former. Therefore, in audition an above-below dominance should replace the left-right dominance observed in vision. Experiments 1 and 2 showed a clear-cut compatibility effect in the auditory modality for both the left-right and above-below dimensions. Experiment 3 showed that spatial compatibility was still stronger for the left-right than for the above-below dimension. Since the left-right one proved to be the more discriminable dimension, this finding rules out the attentional hypothesis, at least in the version originally proposed.