Interactions between imagined movement and the initiation of voluntary movement: a TMS study

Abstract

Objective: The purpose was to examine motor imagery-induced enhancement in corticospinal excitability during a reaction time (RT) task.

Methods: Nine young and healthy subjects performed an isometric finger flexion tasks in response to a visual imperative cue. In the pre-cue period, they were instructed to: (1) rest; (2) imagine flexing their fingers isometrically (ImFlex); or (3) imagine extending their fingers isometrically (ImExt). Surface EMGs from the finger flexors and extensors were monitored to ensure EMG silence before movement onset. Transcranial magnetic stimulation (TMS) was used to evaluate changes in motor-evoked potentials (MEP) in the finger flexor and extensor muscles during the response phase. TMS was delivered either with the imperative cue, or 120 ms before and after the imperative cue.

Results: RT was slower when they were imagining finger extension prior to the visual imperative cue. MEPs were significantly increased for the finger flexors during imagined finger flexion and for the finger extensors during imagined finger extension at both TMS delivery time points, reflecting movement specific enhancement in corticospinal excitability during motor imagery. When TMS was delivered 120 ms after the cue, finger flexor MEPs were further facilitated under the Rest and ImFlex conditions, but not under the ImExt condition, suggesting additive interactions between imagery-induced enhancement and early rise in corticospinal excitability during the initiation of a reaction time response.

Conclusions: Our results provide neurophysiological evidence mediating dynamic interactions between imagined movement and the initiation of voluntary movement.

Significance: Motor imagery can be integrated into a rehabilitation protocol to facilitate motor recovery.

Figure 1

The averaged reaction time (RT) with standard error bars. TMS_-120, TMS₀, TMS₁₂₀ indicates TMS delivery at 120 ms prior to, on, and 120 ms after the visual go signal. ImFlex: imagined finger flexion; ImExt: imagined finger extension. Asterisk indicates statistical significance.

Figure 2

The effect of TMS delivery on reaction time at rest. The rectified, filtered FDS EMG signals from typical trials of the same subjects are plotted. Arrows denotes the delivery of TMS, vertical line: the go-signal; Numbers: timing of TMS with respect to the visual go signal.

Figure 3

The motor-evoked potentials (MEP) (A: FDS, B: EDC). MEPs were normalized to individual maximal values. TMS_-120, TMS₀, TMS₁₂₀ indicates TMS delivery at 120 ms prior to, on, and 120 ms after the visual go signal. ImFlex: imagined finger flexion; ImExt: imagined finger extension. Standard error bars are shown. Asterisk indicates statistical significance. Dotted line indicates the baseline FDS and EDC MEP, respectively, at rest at TMS₀.

Figure 4

Conceptual illustration of motor imagery-reaction time dynamic interactions. During squeezing reaction time tasks, there is a natural facilitation course of finger flexor corticospinal excitability (solid curve line). Motor imagery of agonist (ImFlex) brings resting potentials of finger flexors closer to motor threshold (i.e., subthreshold enhancement), thus showing additive effects (thick dotted line); while motor imagery of antagonist (ImExt) leads to inhibitory effects on resting potentials of finger flexors (think dotted line).