Effects of stimulus-response probability on choice reaction time: Evidence from the lateralized readiness potential

The lateralized readiness potential (LRP) was measured in 2-choice reaction time (RT) tasks with unequally probable stimuli and responses. Responses were faster and more accurate in high- than in low-probability trials. LRPs indicated that participants prepared the high-probability response and suggested that this preparation speeded high-probability responses relative to low-probability responses. When stimulus onset was temporally predictable, the high-probability response was clearly prepared prior to stimulus onset. When stimulus onset was unpredictable, preparation of the high-probability response occurred just after stimulus onset, even on low-probability trials where this preparation was inappropriate. With unpredictable stimulus onsets, RT distributions were adequately represented by a mixture model with prepared versus unprepared states; with predictable onsets, they were not.