Exploring the response code in a compatibility effect between physical size and left/right responses: The hand is more important than location.

The spatial-size association of response codes (SSARC) effect refers to the finding of better performance with the left hand to small stimuli and with the right hand to large stimuli, as compared to the reverse mapping. In the present study, we investigated which response coding is responsible for the emergence of the SSARC effect. We observed a SSARC effect only with response selection between hands but not between fingers of one hand, indicating that the responses are coded relative to the body midline. Furthermore, we observed a SSARC effect with parallel arms but not with crossed arms, suggesting that both the anatomical side of the effector and its external spatial position contribute to the response code. However, using a reaching task as compared to keypresses, the SSARC effect followed the arms, suggesting that the crucial spatial response code refers more strongly to the anatomical side of the effector rather than to the external spatial response position. These findings document a strong influence of anatomically- or body-based coding on the SSARC effect, are at odds with the proposition of a generalized magnitude system that utilizes a common, external spatial metric, and point toward a categorical nature of response codes underlying the SSARC effect. (PsycInfo Database Record (c) 2023 APA, all rights reserved).