Hemispheric sensitivity to body stimuli in simple reaction time.

Previous research indicates that people respond fastest when the motor response is (spatially, functionally, anatomically, or otherwise) congruent to the visual stimulus. This effect, called ideomotor compatibility, is thought to be expressed in motor areas. Congruence occurs when the stimulus and response share some dimensions in their internal representations. If the ideomotor compatibility hypothesis were true, we would expect facilitation when right hand stimuli are presented to the left hemisphere, or left hand stimuli are presented to the right hemisphere. To address this issue, we conducted a simple reaction time experiment with lateralized targets. Participants were instructed to press a button as soon as a target was observed. The target stimulus was a left hand, a right hand, or a neutral control. Each hemisphere showed faster responses to contralateral hand stimuli as compared with ipsilateral hand stimuli, consistent with the ideomotor compatibility hypothesis. The results support an automatic and implicit processing of visual stimuli within motor representations even when no recognition of, or decision about, the stimulus is necessary.