## 2 Materials and Methods  
### Data Acquisition  
#### 2.1.1 Experimental Procedure  
The EEG data used in this study were acquired between August and October 2022. Ethical approval for the study was given by the ethical commission of human sciences of Vrije Universiteit Brussel (ECHW_364.02). The subject group included 15 participants (14 male, 1 female) aged between 18 and 50 years (mean \(28\pm 7\)). Participants had no prior experience with MI. After giving their informed consent, each participant took part in 5 sessions, which took place on separate days. The first 2 sessions of the experiment were familiarization sessions, where the participant was alternately asked to perform and imagine movements without feedback. Before starting the first session, the concept of MI was introduced and the participant's MI ability was assessed through the MIQ-3 questionnaire [33]. After filling in the questionnaire, participants were asked to choose whether they preferred kinesthetic MI (feeling the movement) or visual MI (visualizing the movement). MIQ-3 outcomes were not used in the current study as this falls outside of the scope of the article. Finally, at the end of the first session, the participants were once again asked if they would use kinesthetic or visual MI. For the second session, following a brief summary of MI concepts and the participant's previous experience, a final executed movement run was performed followed by an MI run. At the end of the second familiarization session, an MI decoding model was trained on data from the imagined movement run to perform a first feedback run. Feedback runs were introduced to train users to get better at generating motor imagery [34] and to assist them in staying focused, as suggested by general human-computer interaction research [35]. The last 3 sessions each started with an imagined movement calibration run and three or more feedback runs. The design of this experiment expands the procedure used to acquire MI data for the BCI competition IV dataset 2a [36] by incorporating feedback. Each session consisted of multiple runs where movements were requested while the participant was seated in front of a screen. The sessions always began with a baseline run where participants had to do nothing while observing the visual cues for the movements or performing certain actions such as blinking and moving their head. Following the baseline, there were two types of runs, depending on the session. During calibration runs, participants had to either perform or imagine the requested movement. The requested movement could either be closing the right hand, closing the left hand, curling the toes, or pushing the tongue against the top front teeth. The same procedure was employed for both executed and imagined movements. Figure 1 shows the experimental design for a single trial during such a calibration run. Each run consisted of 15 repetitions of each movement, resulting in 60 trials per run. Whether to perform or imagine the movement was orally communicated to the participant before the start of the run. At the start of each trial, a fixation cross was shown for 3 s, giving the participant time to get ready. Subsequently, a textual cue was displayed indicating the movement to perform or imagine. Depending on the movement, the cue appeared above, below, to the left, or to the right of the fixation cross. The order in which movements were presented was randomized for each run. After 1.5 s, the cue disappeared and the fixation cross turned green, giving the GO signal for the participant to start performing or imagining the movement. After 2.5 s, the cross turned red to indicate that the participant could stop. Finally, there was a break with a randomized duration between 3 and 4 s where white noise was shown to the participant. After the break, the next trial of the run began or the run ended if all trials were completed. For feedback runs, a BCI decoding model was trained on previously acquired MI calibration data to give feedback after each movement imagination.