Input-output modality compatibility denotes the idea that some combinations of stimulus- and response-modality lead to superior performance (modality compatible mappings, e.g., visual-manual; auditory-vocal) compared to other combinations (modality incompatible mappings, e.g., visual-vocal; auditory-manual). However, the preconditions under which these effects arise remain largely elusive.
Therefore, we tested for modality compatibility effects in blocks in which either the stimulus-modality, or the response-modality, or both could vary. We found modality compatibility effects only when both varied, i.e., when several stimulus- and response-modalities were mixed within the same block. These results confirm a necessary and concrete precondition for modality compatibility effects that let us further specify the cognitive mechanisms underlying these effects.