Executive Functions in Motor Imagery: Support for the Motor-Cognitive Model over the Functional Equivalence Model

Abstract

The Motor-Cognitive Model holds that motor imagery relies on executive resources to a much greater extent than do overt actions. According to this view, engaging executive resources with an interference task during motor imagery or overt actions will lead to a greater lengthening of the time required to imagine a movement than to execute it physically. This model is in contrast to a currently popular view, the Functional Equivalence Model, which holds that motor imagery and overt action use identical mental processes, and thus should be equally affected by task manipulations. The two competing frameworks were tested in three experiments that varied the amount and type of executive resources needed to perform an interference task concurrent with either an overt or imagined version of a grasping and placing action. In Experiment 1, performing a concurrent calculation task led to a greater lengthening of the time required to execute motor imagery than overt action relative to a control condition involving no interference task. Further, an increase in the number of responses used to index performance affected the timing of motor imagery but not overt actions. In Experiment 2, a low load repetition task interfered with the timing of motor imagery, but less so than a high load calculation task; both tasks had much smaller effects on overt actions. In Experiment 3, a word generation task also interfered with motor imagery much more than with overt actions. The results of these experiments provide broad support for the Motor-Cognitive Model over the Functional Equivalence Model in showing that interfering with executive functions had a much greater impact on the timing of motor imagery than on overt actions. The possible roles of different executive processes in motor imagery are discussed.

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

“…It was constrained by funding, which meant that the reliability of obtained point estimates (means) would be relatively low. However, given the very large effect sizes previously reported for the action tasks used here [11,12] we could be reasonably certain that these effects would replicate within even a limited sample.…”

“…Relative to Experiment 1, we eliminated the variables Order, Session , and Precision , and added the variable Calc [for calculation]. The lattermost variable was introduced as it has previously been shown to have a large impact on the timing of motor imagery with only a minimal effect on overt action [11,12]. It also represents a classic example of a task that relies heavily on executive resources [74].…”

“…There, participants were required to perform interference tasks heavily dependent on executive resources, including a calculation task involving counting backwards by threes, or generating words from a single root letter, while simultaneously executing or imagining an action. In line with the Motor-Cognitive model, both calculation and word generation lengthened imagined movement times much more than overt movement times [11,12]. Glover and colleagues argued that the interference tasks used executive resources normally engaged by the motor imagery system, resulting in the latter being slower in generating imagined movements.…”

“…For example, if TMS had deleterious effects on motor imagery, but only some of the cognitive tasks, this would suggest that the processes involved in the affected task(s) contribute heavily to the role of the DLPFC in motor imagery. We also tested the effects of TMS on performance in the calculation task of Experiment 2, used previously by Glover and colleagues [11,12]. Notably, calculation relies heavily on executive functions, as it involves a series of conscious mental operations, and activates the right DLPFC [74].…”

Effect of task complexity on ipsilateral motor response programming to physically presented and imagined stimuli

“…Specifically, high fidelity representation (i.e., overpracticed skills) places low demands on the central resource pool, leading to similarities with overt action, whereas tasks that have a low fidelity representation (i.e., novel or complex tasks) require greater online control, leading to longer imagery times than are required to overtly perform the corresponding action. Greater executive control to generate novel or complex tasks contributes to more time needed to generate the image (Glover & Baran, 2017; Glover et al, 2020). Overt behaviour, alternatively, involves unconscious and automatic feedback processing in conjunction with forward models to monitor and fine-tune movement (Glover, 2004).…”