The association of motor imagery and kinesthetic illusion prolongs the effect of transcranial direct current stimulation on corticospinal tract excitability

Abstract

Background: A kinesthetic illusion induced by a visual stimulus (KI) can produce vivid kinesthetic perception. During KI, corticospinal tract excitability increases and results in the activation of cerebral networks. Transcranial direct current stimulation (tDCS) is emerging as an alternative potential therapeutic modality for a variety of neurological and psychiatric conditions, such that identifying factors that enhance the magnitude and duration of tDCS effects is currently a topic of great scientific interest. This study aimed to establish whether the combination of tDCS with KI and sensory-motor imagery (MI) induces larger and longer-lasting effects on the excitability of corticomotor pathways in healthy Japanese subjects.

Methods: A total of 21 healthy male volunteers participated in this study. Four interventions were investigated in the first experiment: (1) anodal tDCS alone (tDCSa), (2) anodal tDCS with visually evoked kinesthetic illusion (tDCSa + KI), (3) anodal tDCS with motor imagery (tDCSa + MI), and (4) anodal tDCS with kinesthetic illusion and motor imagery (tDCSa + KIMI). In the second experiment, we added a sham tDCS intervention with kinesthetic illusion and motor imagery (sham + KIMI) as a control for the tDCSa + KIMI condition. Direct currents were applied to the right primary motor cortex. Corticospinal excitability was examined using transcranial magnetic stimulation of the area associated with the left first dorsal interosseous.

Results: In the first experiment, corticomotor excitability was sustained for at least 30 min following tDCSa + KIMI (p < 0.01). The effect of tDCSa + KIMI on corticomotor excitability was greater and longer-lasting than that achieved in all other conditions. In the second experiment, significant effects were not achieved following sham + KIMI.

Conclusions: Our results suggest that tDCSa + KIMI has a greater therapeutic potential than tDCS alone for inducing higher excitability of the corticospinal tract. The observed effects may be related to sustained potentiation of resultant cerebral activity during combined KI, MI, and tDCSa.

Keywords: Corticospinal tract; Kinesthetic illusion; Motor imagery; Transcranial direct current stimulation; Transcranial magnetic stimulation; Visual stimulation.

Fig. 1

Schematic representation of multiple synchronized stimulations. Anodal tDCS was applied. An anode was placed above the motor hotspot of the left FDI and a cathode was placed above the contralateral orbit (a). A visual illusion was induced by having subjects view a movie of someone else’s index finger performing abduction/adduction (b). The subjects performed motor imagery of index finger abduction of their own index fingers (c)

Fig. 2

Experimental timeline. Interventions were conducted for 15 min each. Cortical excitability was examined using transcranial magnetic stimulation (TMS) prior to the intervention to establish control conditions (Baseline MEP = pre-intervention, PRE = 15 min after baseline MEP) and at timed intervals after the intervention (Post0 = 0 min post-intervention, Post30 = 30 min, and Post60 = 60 min). We calculated the ratio of MEP amplitudes at each post-intervention time-point relative to baseline MEPs [(MEP amplitude at each stage – baseline MEP amplitude)/baseline MEP amplitude × 100]

Fig. 3

The increase in MEP amplitudes under combination with tDCS. Superimposed raw EMGs of MEPs recorded from a single subject’s FDI in the tDCSa + KIMI and tDCSa conditions are shown (a). Results of the average MEP ratio from the FDI induced in each of the 4 conditions are shown for each time-point (b). Error bars indicate the standard deviation. *: p < 0.05 vs. PRE, **, ‡‡, §§: p < 0.01 vs. PRE (Dunnett’s post-hoc test). †: p < 0.05 vs. tDCSa (Tukey’s post-hoc test)

Fig. 4

The MEP ratio in the sham + KIMI Results of the average MEP ratio of FDI induced in the sham + KIMI conditions at each time-point. Error bars indicate the standard deviation