## 1 Introduction  
Stroke is a major cause of adult disability, and the incidence of stroke is increasing significantly due to the extension of our lifespan [1]. Stroke survivors undergo a variety of rehabilitation treatments to minimize impairment of motor or cognitive function [2]. Among them, non-invasive brain stimulation (NBS), which repeatedly stimulates the brain, directly regulates neuroexcitability, and induces neuroplasticity, is widely implemented in clinical practice as it improves motor and cognitive function depending on the stimulation site [3; 4]. This directly helps neurorehabilitation and improves performance in the brain-computer interface or robot-assisted rehabilitation on motor recovery [5]. Representative NBS includes repetitive transcranial magnetic stimulation (rTMS) using a magnetic field and transcranial direct current stimulation (tDCS) using electrical current. The effect of NBS may vary depending on the stimulation site and intensity, but, in general, when lesions occur in the motor cortex, high-frequency rTMS stimulation is given to the ipsilesional primary motor cortex (M1) and cathodal tDCS stimulation to contralesional M1 [6]. Various neuroimaging techniques have been used to determine the effectiveness of these NBSs. These include functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). Although fMRI has a high spatial resolution, it has the disadvantage of low time resolution and expensive equipment. On the other hand, EEG has a low spatial resolution but a high temporal resolution. In addition, it is easy to measure due to its low cost [7]. Many studies focused on changes in neuromodulation according to NBS, so most of them used fMRI. However, EEG signals are often used to diagnose brain diseases because they can immediately reflect changes in neuronal activity in the cerebral cortex [8]. In this regard, EEG may be more suitable to observe changes in stimulation applied to neuronal cells. Many studies mainly explore changes in the neuromodulation effect in terms of brain connectivity because an abnormality in interhemispheric interactions characterizes it due to dysfunction of the lesion region in patients with stroke [9; 10]. High-frequency rTMS of the ipsilesional M1 can facilitate corticomotor excitability and motor skill acquisition [11]. In addition, the average strength of intrahemispheric connectivity over the contralesional M1 was slightly decreased after high-frequency rTMS using an fMRI study [12]. When anodal tDCS was applied to the ipsilesional M1 in patients with chronic stroke, functional connectivity increased enormously in the alpha band in the ipsilesional M1 with an increase in corticospinal excitability [8; 13]. On the other hand, delta and theta interhemispheric functional connectivity in rest-state EEG increased significantly after intermittent theta burst stimulation (iTBS), which is a special form of rTMS [14], but there was no significant difference in current TMS-EEG based on iTBS [4]. However, concurrent TMS-EEG recordings, which indicate both intracortical inhibitory and excitatory circuits of M1 immediately, show no instant effects of iTBS in patients with stroke [4]. Recently, dual-mode stimulation, which applies tDCS and rTMS together, has been found to be of great clinical use in neurorehabilitation. At this time, since the effect may vary depending on the order or location of the stimulus, many studies have been conducted to find a way to maximize the effect of NBS. In healthy subjects, the preconditioning mode with tDCS over the contralateral M1 followed by a high frequency of rTMS over the ipsilateral M1 increased the cortical excitability and motor function [15]. However, in patients with stroke, it was found that the simultaneous mode was more effective in increasing neural activity compared to the preconditioning mode.