## 4 Discussion  
### Conclusions  
The current study investigated what cortical mechanisms are involved when auditory selective attention is improved by perceptual training. For this purpose, we further analyzed our previous dataset [11] which demonstrated the efficacy of our neurofeedback training paradigm for improving auditory selective attention [4; 5; 6; 7; 8; 9; 10; 11]. By examining induced cortical oscillations, in the Experimental Group (who received neurofeedback), we found that alpha (~10 Hz) and beta (~25 Hz) oscillations in the right parietal cortex increase over the course of training. The Placebo Group (who did not receive neurofeedback) did not show any significant change in cortical oscillations. There was no significant change in alpha and beta oscillations in the Placebo Group. These findings provide novel perspectives of the brain plasticity related to enhanced auditory selective attention following neurofeedback training, which was not provided by our previous report [11]. We claim that the stronger beta oscillation in the Experimental Group indicates more strongly engaged top-down processing to predict forth-coming sounds to attend in advance [45; 46]. Beta oscillation is believed to be related to top-down prediction based on the context of  
Figure 3: Spectrograms and topographies of induced activities in the Placebo Group. (**A**) The t-score spectrograms of auditory modulation between “Up” and “Down” in week 1, week 4, and between week 4 and week 1 are obtained during the neurofeedback training from the Placebo Group. No significantly different clusters are found in the Placebo Group. To indicate where the regions of interest are, the dotted line boxes indicate significantly different clusters within the alpha and beta wave ranges (8–13 Hz and 24–29 Hz, respectively), obtained by the cluster-based permutation tests from the Experimental Group [38]. (**B**) The mean topographies of the beta wave cluster taken from the Placebo Group are obtained in the preparatory time range before target speech onset (from \(-\)0.50 to \(-\)0.45 s) at the sensor space and source space. (**C**) The topographies of the alpha wave cluster taken from the Placebo Group are obtained in the preparatory time range before target speech onset (from \(-\)0.48 to \(-\)0.43 s) at the sensor space and source space. stimuli [46; 47]. Especially, enhancement in beta is observed during the tasks that require top-down attention [48; 49]. [PERSON] et al. [50] showed that for the audiovisual onset synchrony task, the preparatory beta oscillations in the central, parietal, and temporal lobes increased after training. Our result indicates that this is also true in more auditory-centered tasks. Our result provides more support with contrasting outcomes in the Placebo Group where no neurofeedback was given. Likewise, the increase in alpha oscillation in the right parietal region can be interpreted as enhanced inhibitory processing to ignore competing sounds. According to [PERSON] et al. [51], single-trial speech intelligibility in speech-in-noise tasks considerably correlates with, and independently contributes to the overall magnitudes of alpha power in parieto-occipital EEG channels and beta power in frontal channels. Furthermore, [PERSON] et al. [52] demonstrated that accuracy in SiN is predicted by the alpha modulation between clean and noise-degraded speech. Similarly, in this study, SiN results were improved when alpha power modulation was shown across the training period in the Experimental Group, and SiN results were not improved when alpha power modulation was not shown across the training in the Placebo Group [11]. Moreover, as in [PERSON] and [PERSON] [53], the enhancement of alpha in the parietal region can be used as a predictor of the SiN results. The right parietal stimulation of alpha was observed, but rightward spatial attention was not enhanced, consistent with the results by [PERSON] et al. [54]. It is also consistent with the finding by [PERSON] et al.