Data from two experiments investigating the impact of handedness and effector strength on a compatibility effect between physical stimulus size and left/right 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 (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 where we manipulated the mapping between physical stimulus size and response hand. In addition, we measured the strengths of participants’ left and right effectors (i.e., finger, hand, arm). In Experiment 2, we measured the SSARC effect in vocal responses in the same sample. There were four main results. First, participants’ dominant effectors were stronger than their non-dominant 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, 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 multiple size-space assocations may exist in parallel, or that space-size associations have generalized across motor systems, and formed a modality-independent association.