## 3 Discussion
### SVM-ML Configuration
#### 3.1.2 EEG Channel Selection  
It was observed that the overall accuracies of the model with the 21-channel EEG were better than those of the 62-channel EEG when not considering arm differences (Table 2). The 21-channel EEG covers the sensorimotor cortex, which is the main response area to sensory stimulations [70, 71]. The accuracies based on the 21-channel EEG suggested that direct cortical processing from the sensorimotor cortex was sufficient to capture the sensory differences generated by different fabric samples through the SVM-ML model. Previous studies have demonstrated that significant RSP variations for different EEG bands during sensory stimulations were mainly captured in the sensorimotor cortex for both unimpaired and stroke populations [28, 72, 73]. On the contrary, involuntary attention activities beyond the sensorimotor cortex were involved in passive fabric stimulation experiments [28]. This could be a hurdle to the recognition of cortical responses to fabric stimulations using the SVM-ML model. Meanwhile, voluntary cognitive activities were also found to disturb measurement of cortical responses to sensory stimulations [73, 74]. For example, in post-stroke sensory evaluation by a subjective questionnaire, stroke persons with sensory impairments could distinguish different fabric stimulations because of the compensation of cognitive processing, e.g., individual experiences, to the residual sensory pathways [28]. In this work, the voluntary cognitive activities were minimized by asking the participants to stay awake but mentally inactive during the fabric stimulations. Therefore, the EEG RSP features of the sensorimotor cortex detected by the 21-channel EEG were sufficient for representing the differences in direct cortical responses to fine tactile sensations. Table 2 also shows that when identifying different fabric stimulations without considering arm differences, the model achieved better overall accuracies in higher bands, i.e., the beta and gamma bands. This was consistent with the results of previous neurophysiologic research into how the human brain reacts to tactile sensations induced by fabrics [74, 75]. The cortical responses of the brain to tactile sensations are elicited by skin-fabric interactions, which are characterized by the EEG beta and gamma band activations [76]. Beta oscillations have been shown to be involved in the phasic locking process between the primary and secondary somatosensory cortex in response to tactile sensation [77]. Meanwhile, it was observed that the neuronal assemblies of the sensorimotor cortex were joined in large-scale networks oscillating in the beta band during maintenance of a sustained hand lever press activity [78]. This phenomenon suggested that the primary somatosensory and primary motor cortex were bound together in a beta-synchronized cortical network [78]. Furthermore, [PERSON] et al. found that beta oscillations in the sensorimotor cortex served as an informative feature characterizing affective tactile stimulation by interactions with different fabrics [79]. [PERSON] et al. demonstrated that pleasant and unpleasant tactile sensations present different beta-oscillation patterns [74]. The gamma oscillations of the sensorimotor cortex were also observed in response to tactile sensation. This may reflect the timing code and temporal organization for higher-order somatosensory processing, which is important  
for sensory binding [80; 81]. In addition, the study by [PERSON] et al. showed that gamma oscillations were simultaneously evoked in the primary and secondary somatosensory cortex during sensory stimulations, thus suggesting that it is critical for forming functional cortico-cortical connections and for conveying somatosensory information from the primary and secondary somatosensory cortex [82]. [PERSON] et al. found that tactile-stimuli-induced gamma oscillations in the somatosensory cortex were enhanced and prolonged by spatial tactile attention [83].