Handedness and Effector Strength Modulate a Compatibility Effect Between Stimulus Size and Response Position With Manual and Vocal Responses

Humans respond more quickly with the left hand to a small stimulus, and with the right hand to a large stimulus, as compared to the reverse mapping (spatial-size association of response codes [SSARC] effect). We investigated the hypothesis that strength differences between the hands contribute to the origin of this effect. Therefore, 80 left-handers and 80 right-handers participated in two experiments. In Experiment 1, participants performed a manual choice-response task in which we manipulated the mapping between physical stimulus size and responding hand. In addition, we measured the strengths of participants' left and right effectors (i.e., finger, hand, and arm). In Experiment 2, we measured the SSARC effect in vocal responses of the same sample. There were four main results. First, participants' dominant effectors were stronger than their nondominant effectors. Second, the SSARC effect occurred in manual and vocal responses with similar size. Third, in both modalities, the SSARC effect was larger in right-handers than in left-handers. Finally, strength differences between effectors (fingers and hands) correlated with the size of the SSARC effect. In sum, results support the hypothesis that functional differences between the hands contribute to the origin of the SSARC effect. In addition, the results suggest that size-space associations have generalized across motor systems, and formed a modality-independent association.