Abstract
Motor imagery refers to the phenomenon of imagining performing an action without action execution. Motor imagery and motor execution are assumed to share a similar underlying neural system that involves primary motor cortex (M1). Previous studies have focused on motor imagery of manual actions, but articulatory motor imagery has not been investigated. In this study, transcranial magnetic stimulation (TMS) was used to elicit motor-evoked potentials (MEPs) from the articulatory muscles [orbicularis oris (OO)] as well as from hand muscles [first dorsal interosseous (FDI)]. Twenty participants were asked to execute or imagine performing a simple squeezing task involving a pair of tweezers, which was comparable across both effectors. MEPs were elicited at six time points (50, 150, 250, 350, 450, 550 ms post-stimulus) to track the time course of M1 involvement in both lip and hand tasks. The results showed increased MEP amplitudes for action execution compared to rest for both effectors at time points 350, 450 and 550 ms, but we found no evidence of increased cortical activation for motor imagery. The results indicate that motor imagery does not involve M1 for simple tasks for manual or articulatory muscles. The results have implications for models of mental imagery of simple articulatory gestures, in that no evidence is found for somatotopic activation of lip muscles in sub-phonemic contexts during motor imagery of such tasks, suggesting that motor simulation of relatively simple actions does not involve M1.
Highlights
Motor imagery has been defined as ‘‘the mental rehearsal of simple or complex motor acts that is not accompanied by overt movement’’ (Porro et al, 1996, p.7688)
Post hoc comparisons show that motor execution motor-evoked potentials (MEPs) (Mean = 0.297, Standard Error = 0.097) were significantly larger than motor imagery (M = −0.177, SE = 0.122, p < 0.001) and baseline MEPs (M = −0.188, SE = 0.116, p < 0.001), but that there was no difference between motor imagery and baseline MEPs (p = 1; see Figure 3A)
This study aimed to examine whether primary motor cortex was facilitated for speech motor imagery by examining a simple motor imagery task in lip and hand muscles
Summary
Motor imagery has been defined as ‘‘the mental rehearsal of simple or complex motor acts that is not accompanied by overt movement’’ (Porro et al, 1996, p.7688). Functional imaging had by this time shown some overlapping activation for both types of processing (Porro et al, 1996; Roth et al, 1996) Since such measures suggest a significant level of shared processing between the two behaviors, the extent to which motor plans are used in motor imagery compared to motor execution has been the subject of debate. Jeannerod (2001) proposed that covert and overt action stages represent a continuum where execution suggests the existence of a covert stage, but covert actions do not evolve into the overt stage In this sense, it can be said that a continuum of action processing, and so the use of motor plans for potential simulation of action, exists from action observation through covert simulation of motor plans to overt execution of motor plans. Such models often implicate primary motor cortex (M1) as an important element in simulation processing (Tian et al, 2016), but the central question remains at what level, and to what degree, motor imagery makes use of motor plans in the absence of overt action execution
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