## 2 Materials and Methods  
### Data Acquisition of Self-Acquired Dataset (Dataset 1)  
Subjects were 16 healthy, both undergraduate and graduate students (6 women, 10 men; age range 20-23 years, mean 21 \(\pm\) 0.89). Surveys were given to subjects and reported that no subjects had disorders relating to neurology. None of them had any prior experience with BCI. Two to three preparatory runs were provided to all subjects before data acquisition and were paid USD 2 per measurement for taking part in. Before and after each session, subjects were asked to fill out some information relating to their conditions to ensure the consistency of awareness during the experiment. Throughout the whole procedure, subjects had to comply with some regulations: (i) no caffeine or any stimulants consumption a few hours before experiments; (ii) restrict negative feelings; (iii) avoid sleep deprivation. During the experiment, participants were asked to sit on a chair in a Faraday-shielded room that blocks electromagnetic radiation and observe a computer screen. Minimizing eye movements, body adjustments, and avoiding any undesired movements during a cue onset. Subjects were allowed to give signs to pause or stop the experiment should any unexpected conditions show up (cough, scratch, sleepy, tired, etc.). Data were recorded by NicoletOne V32 (Natus Medical Inc., Pleasanton, CA, USA) with monopolar montage with electrodes C3, Cz, C4, P3, Pz, and P4 according to the International 10-20 electrode placement system [54] (Figure 1). A reference and a ground electrode were attached to the left and right earlobes accordingly. All electrodes' impedance was kept below 5 k\(\Omega\) and the sampling frequency was 500 Hz. Nuprep was used to clean the scalp to reduce skin impedance, and conductive gel Ten20 was used for electrode attachment (Figure 1). Six electrode positions were chosen based on the fact that the most noticeable mu and beta ERD/ERS can be recognized when performing imagery tasks [55; 56]. There were five measurements that occurred on five different days for each participant. There were five 8 min sessions in each measurement and a 10 min break was provided between each session. In each session, subjects were asked to pay attention to a MATLAB simulation program for 30 segments. Each segment lasted for 16 s (Figure 2): the first 6 s, the fixation cross appeared on the screen denoted relaxation state (no physical movements or mental activities); the next 6 s were for kinesthetic motor imagery tasks accompanied with cue denoted by a red arrow pointing left, right, and downward (for left hand, right hand and feet respectively); the last 4 s were for cue offset and subjects returned to relaxation. A cue onset and offset were indicated by a deep sound. In total, each subject has around 220-250 trials for each class. However, some were unable to complete the full experiment due to an inappropriate schedule. Subjects \"ID03\", \"ID06\", \"ID10\", \"ID12\" have exceptionally fewer trials, approximately 180, 195, 140, and 134 trials for each class, respectively. Most MI-BCI research conducted worldwide follows the same measuring procedure: when the subject is at resting state and observing the screen, the arrow appears in a random order after a cue, and the subject performs the imagining; lastly, the subject rests for random durations. The time required for arrows to appear, the duration required to complete the task, and the resting period are the variations between the measured procedures in these studies. The procedure for measuring in [57] began with the subject observing a cross in the center of the screen, and then the subject observed an arrow from the 2 nd to the 6 th second.