Preparation for horizontal or vertical dimensions affects the right-left prevalence effect

When stimulus and response simultaneously vary in both horizontal and vertical dimensions, the stimulusresponse compatibility effect is often larger for the horizontal dimension. We investigated the role of preparation for each dimension in this right-left prevalence. In Experiment 1, tasks based on horizontal and vertical dimensions were mixed in random order, and the relevant dimension in each trial was cued with a variable cue-target stimulus onset asynchrony (SOA). A right-left prevalence effect was observed only when participants prepared for the upcoming task. Experiment 2 replicated the absence of the prevalence effect for the simultaneous presentation of cue and target using a fixed SOA of 0 msec. In Experiment 3, the right-left prevalence emerged with a 0-msec: SOA when participants prepared for each dimension based on its frequency. These results suggest that participants' internal set can be greater for the horizontal dimension, leading to the right-left prevalence effect.