Stimulus properties matter more than perspective: an fMRI study of mental imagery and silent reading of action phrases.

UNLABELLED: The role of the primary motor cortex (M1) in tasks involving action words remains controversial. Therefore, we investigated whether the previously reported involvement of M1 in processing of action words results from the semantic representation of action words per se, or if M1 activation may actually depend on whether or not subjects (explicitly or automatically) adopt a strategy of simulating the movements. Subjects silently read short phrases describing a situation which either involved a motor scene or not (STIMULUS: motor, non-motor phrases) and performed a secondary task: either they were explicitly asked to imagine the situation or they performed letter detection preventing them from using a simulation strategy (TASK: imagery vs. letter detection). In addition, phrases were presented both in 1st and 3rd person singular ( PERSPECTIVE: 1st vs. 3rd person). This allowed us to investigate the influence of the secondary tasks (letter detection versus explicit motor imagery) on the neural activity in M1 during the processing of motor and non-motor phrases. We found differential left M1 activity in the task by stimulus interaction with enhanced M1 activation for imagery in the presence of motor phrases (vs. non-motor phrases) compared to letter detection of motor vs. non-motor phrases. This M1-activity was not differentially modulated by perspective. Therefore, M1 activation previously found in experiments of silent reading of action words may have resulted from the subjects' strategy to mentally simulate the movements during the processing of action words.