## 4 Discussion  
In Experiment 1, we found that the brain activity in the observer varied depending on the type of UI (Figure 5). Figure 5: Mu rhythm suppression (LC and RC) in different conditions in Experiment 2. Asterisks (**_p_ \(<\) 0.01, * \(p\) \(<\) 0.05) mean the significant mu rhythm suppression from baseline to observation of each actions. Sharps (# \(p\) \(<\) 0.05) mean the significant difference between conditions. In the TUI/CAN condition, watching the UI operation resulted in an elevated activity in RC in the observer in comparison with the resting state. This is consistent with the report that watching the action of others results in mu rhythm suppression in the somatosensory cortex, which roughly corresponds to RC [23; 31]. Therefore, it is suggested that the brain activity was induced in this MNS-related brain area. Among brain areas, only RC showed the activity in this experiment. We suppose this was because the user action took place in the left visual hemifield of the observer [29; 32] rather than the observer imagined imitating the right hand action of the operator as a result of watching it. This activity tended to be higher in the TUI/CAN condition in comparison with the TUI/OBJECT condition and the GUI condition, indicating that it was characteristic of the TUI/CAN condition. The MNS activity reflects the immediate goal of an action [22]. Thus we had expected mu rhythm to be suppressed in the TUI/OBJECT condition, but found the suppression only in the TUI/CAN condition. It has been reported that the observation of a highly unfamiliar action results in an elevated activity in the motor cortex [33; 34]. In the TUI/CAN condition in the present study, the result of each action is unpredictable, which is not seen in ordinary UIs. We suppose this caused the elevated activity in the sensorimotor area. On the other hand, the response of the observer did not seem to differ between the TUI/OBJECT condition and the GUI condition, in spite of the presence/absence of the grasping. In this experiment, any of the two conditions did not even show activation in the same brain area, indicating that they did not activate the MNS. It has been reported that the observation of a familiar task results in a lower activity in the inferior frontal gyrus and the premotor cortex in comparison with an unfamiliar task [35], and that repeated grasping of the same object leads to weaker mu rhythm suppression [36]. We suppose that the repeated stimulus presentation in the present study hindered the effect of the grasping action from being detected. In Experiment 2, we found that the brain activity in the operator also varied depending on the type of UI (Figure 5). Areas around the somatosensory cortex were active in the TUI/OBJECT condition and the TUI/CAN condition, suggesting the possible induction of a brain activity related to the brain activity seen in Experiment 1 [23; 31]. The brain areas that were active in this experiment included not only LC, an area activated by performing a right hand action, but also RC. The visual information of each user action was presented primarily in the left visual hemifield, whereas the visual information of the output as the result of the action was presented primarily in the right visual hemifield. Processing of such visual information possibly had some effect [29; 32]. These activities tended to be higher in the TUI/OBJECT condition in comparison with the GUI condition and the ACTION condition. The TUI/OBJECT condition and the GUI condition differed in that only the former involved the grasping action. In an fMRI study, the action of reaching for an object and grasping it and the action of only reaching for that object differed in that the former resulted in an elevated activity in the anterior intraparietal sulcus (alPS; [37]). In our experiment, the aIPS activity due to grasping probably had an effect on the sensorimotor area.