TMS Over Dorsolateral Prefrontal Cortex Affects the Timing of Motor Imagery but not Overt Action: Further Support for the Motor-Cognitive Model

Abstract

The Motor-Cognitive model suggests a functional dissociation between motor imagery and overt action, in contrast to the traditional notion of shared processes central to the Functional Equivalence view. According to the Motor-Cognitive model, motor imagery differs from overt action primarily through the use of executive resources to monitor and elaborate a motor image during execution. This can result in a lack of correspondence between motor imagery and its overt action counterpart. The present study examined the importance of executive resources in motor imagery using TMS to impair the function of the dorsolateral prefrontal cortex, while measuring the time to complete imagined versus overt actions. In two experiments, TMS over the dorsolateral prefrontal cortex slowed motor imagery but did not affect overt actions. TMS over the same region also interfered with performance of a mental calculation task, though it did not reliably affect less demanding cognitive tasks also thought to rely on executive functions. Taken together, these results were consistent with the Motor-Cognitive model but not with the idea of functional equivalence. The implications of these results for the theoretical understanding of motor imagery, and potential applications of the Motor-Cognitive model to the use of motor imagery in training and rehabilitation, are discussed.

Online versus Cognitive Control: A Dividing Line between Physical Action and Motor Imagery

“…The best-fitting model was a full model including the effects of Action, Vision and their interaction (Figure 2). A stastistical model including only an effect of Action fit the data much better than a null model (λ adj = 803), with longer movement times in the motor imagery group than in the physical action group, consistent with what is usually observed 30,31,34 . Adding Vision to the model further improved the fit (λ adj > 1000), with longer movement times in the Peripheral condition than in the Foveal condition, a finding in the opposite direction to what we predicted.…”

“…The general experimental setup is shown in Figure 4 and has been used previously 30,31,34 . Participants sat comfortably at a 50 × 90 cm table set inside a large room.…”

“…In opposition to the notion of functional equivalence, however, numerous studies have observed large discrepancies between the timing and affordances of real versus imagined movements [26][27][28][29][30][31][32][33][34][35][36][37][38][39] . Further, at the neural level, certain brain areas show greater activation during motor imagery than during physical action, most notably the dorsolateral prefrontal cortex 15,19,[40][41][42][43][44][45][46][47][48] , and connectivity patterns differ between physical and imagined action [49][50][51][52] , raising serious doubts as to the degree to which motor imagery and physical action can be described as functionally equivalent.…”

“…Given the vital importance of motor imagery to training and rehabilitation [53][54][55][56][57][58][59] , and the numerous divergences of findings from the Functional Equivalence view, it is important to develop more sophisticated and precise models of how motor imagery relates to physical action. In the present study, we test a new model of motor imagery, the Motor-Cognitive Model 30,31,34 . In this account, both physical and imagined movements consist of a planning and execution stage 60 .…”

“…Conversely, in motor imagery there is no feedback-based online control as the body remains stationary. Rather, monitoring and elaboration of the motor image is a conscious process that depends heavily on executive functions subserved by the dorsolateral prefrontal cortex 30,31,34 . Unlike the dedicated fast visuomotor modules that subserve online control 65,66,68 , executive resources are unequipped to simulate the fine-tuned adjustments that occur during physical movement 69 .…”