## 4 Discussion
### Limitations of the Study  
There are several limitations of this study. First, the current experimental paradigm divided the Experimental Group and the Placebo Group depending on whether visual neurofeedback was given or not. A future study will also be conducted on how attentional modulation would change when neurofeedback is intentionally provided to participants as incorrect results by randomizing visual feedback.  
The concurrent speech streams were disparate in location, speaker identity, and tempo in order to maximize stream separation. Both parietal alpha oscillation and temporal beta oscillation were observed during the preparatory period when the subjects were focused, and valid feedback was provided. This indicates both spatial and pitch-related mechanisms contributed. In order to specify key auditory cues that contribute to the training effect, future research should test the effectiveness of various auditory cues for selective attention training and determine whether these cues are also present in a more general SiN task.  
Pearson correlation coefficients between the grand-average cortical evoked responses and a single-trial EEG waveform were used for decoding selective attention in this study. The intraclass correlation coefficients might be used for the decoder. For the follow-up study, it would be considered if the decoding algorithm is updated to improve the mean decoding accuracy by using machine learning algorithms. Also, the whole 64-channel EEG data from a smaller number of subjects in the Experimental Group were used for the induced activity analysis, and the small sample size in one group was a limitation of this study. A future study will recruit a greater number of subjects.  
This study did not reveal whether the training impact endures after the training time or disappears. A follow-up study is anticipated to investigate this question. Lastly, because selective listening in background noise is hampered by peripheral hearing loss [56], additional research is required to examine the training effect in clinical populations (e.g., hearing aid or cochlear implant users [11]).  
**Author Contributions:** Conceptualization, I.C.; methodology, I.C. and H.S.; software, I.C. and H.S.; validation, H.S., J.H., L.G., K.R. and S.K. ([PERSON]); formal analysis, I.C. and H.S.; investigation, S.K. ([PERSON]); resources, I.C.; data curation, L.G. and K.R.; writing--original draft preparation, H.S.; writing--review and editing, H.S., L.G., J.H. and I.C.; visualization, H.S.; supervision, I.C.; project administration, I.C.; funding acquisition, I.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the American Otolological Society Research Grant and the Department of Defense Hearing Restoration Research Program Grant (W81 XWH-19-1-0637) awarded to I.C., as well as NIDCD P50 (DC000242 31).  
**Institutional Review Board Statement:** The work was finished in compliance with the International Medical Association's Code of Ethics, and we acquired written informed consent (Declaration of Helsinki). The Institutional Review Board at the University of Iowa examined and approved all study methods.  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.