*3.2.2. Results*

The LF/HF ratio during MI under two MI conditions was significantly greater than that at rest (50% MI vs. Rest, 100% MI vs. Rest, \**p* < 0.05) (**Tables 12**, **13**). The LF/HF ratio immediately after MI under two MI conditions was reduced to rest level (**Tables 12**, **13**).

The relative values of the LF/HF ratio did not exhibit significant differences between two MI conditions (**Table 14**).

#### **3.3. Discussion**

Our previous works demonstrated significant increase of the LF/HF ratio during MI at various imagined muscle contraction strengths (i.e., 10% MVC, 50% MVC, and 100% MVC) [40, 44]. Thus, MI of isometric thenar muscle activity can increase the sympathetic nerve activity as with previous researches [42]. The central command is defined as a feed-forward mechanism by which activation of cardiovascular and respiratory centers is accomplished by descending signals from central nervous system [45]. TMS delivered over the primary motor cortex


**4. The application of MI to neurorehabilitation**

30–60% of patients have difficulty in using their affected upper limb after stroke [49]. Nakayama et al. [50] reported that recovery of upper limb function related activity of daily living mainly took place within the first 2 months after stroke. Further they reported that 79% of patients with mild upper limb paresis could achieve full upper limb function, whereas, in case with severe upper limb paresis, only 18% of patients who could achieve full upper limb function.

The Application of Motor Imagery to Neurorehabilitation

http://dx.doi.org/10.5772/intechopen.75411

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Depending on alteration of peripheral and central inputs, cortical connections and responses are continuously reorganized [51]. Motor cortex excitability will be decreased in post-stroke due to the damage of neural substrates, loss of sensory inputs, and disuse of the affected limb [52]. Described in introduction of this chapter, various brain areas including primary motor cortex corresponding to motor planning, preparation and execution were activated during MI [9–13]. Pascual-Leon et al. [53] employed TMS in the healthy subjects to map the primary motor cortex targeting the contralateral hand muscles pre- and post-MI training. Cortical representation of hand muscles in contralateral the primary motor cortex increased after MI training. Similarly, MI induced an enhancement of hand muscle cortical representation in post-stroke [54]. Thus, MI can induce the cortical plasticity after neural damage. Additionally, Wrigley et al. [55] reported that the corticospinal excitability was decreased following the significant decline of both size and number of the corticospinal neurons. Also, the spinal motor neuron excitability was significantly reduced in the post-stroke acute phase [8]. Ruffino et al. [56] indicated that neural adaptation following MI training, such as cortical reorganization, the reinforcement of synapse conductivity, and the decline of pre-synaptic inhibition, would be occurred at cortical and spinal level. Thus, in post-stroke patients, facilitating the corticospinal excitability, including the spinal motor neuron excitability should be important for improvement motor function. MI can increase the corticospinal excitability [14–16]. Further, Grosprêtre et al. [57] reported that during MI, the amplitude of cervico-medullar-evoked potentials (CMEPs) can measure directly pyramido-motoneuronal junction was significantly increased. The H-reflex amplitude, however, was unchanged. Conversely, the H-reflex amplitude was increased during MI [29]. Further, we showed significant increase the F-wave during MI [24–26, 28]. In regard to difference between two techniques, the H-reflex size can be influenced by pre-synaptic interneuron, whereas the F-wave is solely dependent on the spinal motor neuron excitability [58]. Although effect of MI on the spinal motor neuron excitability is still under debate, MI can be considered

to be an effective method for improvement upper limb function in post-stroke.

Brain-machine interface (BMI) is thought to be a potentially useful technology in neurorehabilitation. BMI can supplement for the lost motor function by bypassing disabled neuromuscular system, and improve brain plasticity and restoration of motor function by using external feedback [59, 60]. Various neurophysiological technologies, such as electroencephalography (EEG), magnetencephalography (MEG), and NIRS, have been used to measure and analyze brain activities. Among, the mu (μ) rhythm (ranged from 10-12 Hz) has been commonly used to monitor brain activities [61]. The event-related desynchronization (ERD) of the μ-rhythm was observed during MI. MI plays an important role in neurorehabilitation using EEG triggered-BMI. However, many people have difficulty in performing MI. Especially MI ability was significantly decreased in post-stroke patients [62]. They have no feedback about whether MI did perform correctly, because MI is a mental rehearsal of movement without any overt

**Table 11.** Comparison of ANS activity between 10% MI and 50% MI condition.


**Table 12.** Changes in LF/HF ratio under 50% MI condition.


**Table 13.** Changes in LF/HF ratio under 100% MI condition.


**Table 14.** Comparison of ANS activity between 50% MI and 100% MI condition.

increased the skin sympathetic nerve activity [46]. Furthermore, transcranial direct current stimulation (tDCS) delivered over the primary motor cortex increased the LF/HF ratio [47]. Thus, the corticospinal pathway including the primary motor cortex may affect the sympathetic nerve activity. The rostral ventromedial medulla is also part of the reticulospinal tract [48]. The activation of central nervous system during MI may increase the sympathetic nerve activity via the corticospinal and reticulospinal tracts.

The imagined muscle contraction did not affect the change of the sympathetic nerve activity. This is very similar with the result of the spinal motor neuron excitability during MI at various imagined muscle contraction strengths [24–26, 28]. If central command during MI affects the sympathetic nerve activity via the corticospinal pathway, the imagined muscle contraction strength may affect the sympathetic nerve activity. Park and Li [35] reported that the imagined muscle contraction strength did not affect the corticospinal excitability. Thus, it considered that the imagined muscle contraction strength might not be involved in change of the sympathetic nerve activity.

#### **3.4. Conclusion**

Our previous woks showed significant increase of the sympathetic nerve activity during MI of isometric thenar muscle activity. However, the imagined muscle contraction strength was not involved in change of the sympathetic nerve activity.
