Functional Equivalence Revisited: Costs and Benefits of Priming Action With Motor Imagery and Motor Preparation

Public Significance Statement Motor imagery is a promising tool for neurorehabilitation, so a better understanding of how motor imagery works is becoming increasingly important as a result. This work shows that motor imagery can influence subsequent action to a greater extent than preparation for action alone. This evidence provides a new and meaningful development of the functional equivalence hypothesis. The functional equivalence (FE) hypothesis suggests motor imagery (MI) is comparable with the planning stages of action. A strong interpretation of this hypothesis suggests MI can prime subsequent actions in a way that should be indistinguishable from motor preparation (MP). Alternatively, MI could involve more richly informative motor plans than MP, producing different effects on the performance of subsequent actions. Although past research has demonstrated MI can prime action, little research has directly compared it with MP, and so the value of a strong FE interpretation for understanding MI remains unclear. In the present study, a precueing paradigm was used in 4 experiments, and congruency effects of MI and MP on subsequent action were compared. Precues instructed participants to prepare for (MP condition) or imagine (MI condition) a corresponding response prior to making a real response to the imperative stimulus, which was either congruent or incongruent with the precue information. Experiment 1 provided first evidence favoring our alternative hypothesis that imagery-primed responses should result in larger response priming effects than prepared-responses, suggesting that MI might involve more richly informative motor plans than preparation for action alone. In experiment 2, we manipulated interstimulus foreperiods and replicated the MI-priming effect, showing it to be independent of differences in temporal uncertainty between MI and MP. Experiment 3 showed the MI-priming effect is present in both foot and finger responses, and Experiment 4 suggested the larger congruency effects in the MI condition could not be explained by differences in cognitive load between MI and MP. These results suggest that a strong FE hypothesis does not hold. Findings are discussed in line with the predictive processing models of action and MI.