## 1 Introduction  
Emotions as the reaction to the internal or external incident, is consisted with multiply feelings, thoughts and actions, which mixed up with psychological and physiological correspondence [1]. Therefore, the emotions which caused by the internal or external stimuli can then be observed through the biological signals such as electrocardiogram (ECG), electromyogram (EMG), galvanic skin response (GSR), electroencephalogram (EEG) and etc. [2; 3; 4; 5], which directly reflect on the emotional status of individuals. And different emotions might cause different changes in the biological signals. Homeostasis is been studied to prove that different emotions might be caused by internal or external stimuli, emotions such as love, anger and fear might be activated by the environment stimuli, while fatigue, pain and hunger are due to the body needs. Moreover, neurologist discovered that anger, fear, sadness and disgust have their own nervous system activities [6], moreover, emotions are associated with neurotransmitters, such as anger is caused by the combination of low serotonin, high dopamine and high norepinephrine, yet when experiencing the feeling of shame, those three neurotransmitters are in a low level, and all three in a high level when excitement is felt [7]. Moreover, evidence shows that the positive emotions are more active in the left prefrontal cortex [8], interestingly, anger which regarded as negative emotion, also activates the left prefrontal cortex [9]. Some neuropsychologists suggest that the both regions of the brain are responsible for the emotions, however, the left frontal brain is more associated with positive emotions and social functions, while the right frontal brain is dealing with the emotions that related to survival, which caused the asymmetric frontal brain activity [10]. Since evidences support that diverse emotions might be caused by different brain activities, from the characteristic of EEG, emotions can be recognized by the differentia of respondence. Nowadays, the wearable EEG devices contained with one or several electrodes that attached on the individual's skin can precisely record the electrical activity from the brain which can be convenient for further analysis. Generally, the analysis of EEG signal is based on different frequency bands as: delta, theta, alpha, beta and gamma. Each frequency band represents different function of body [11; 12] as delta is regarded associated with sleep and concentrated task, theta is associated with inhibition of elicited responses, alpha can represent the status of relaxation and also the inhibition control, beta ranges from calm to stressed and obsessive, including thinking, focus, alert and anxious, while gamma is associated with cognitive competence and revealed a denser network connection when involved with different emotional states than other frequency bands. Studies show that the beta and gamma have been observed with a higher performance on recognizing emotions compared to other three bands. Other than the different frequency bands, selection of electrodes also might bring about different results. Expect EEG signal, the diversity of emotion itself is also complex. [PERSON] [13] once proposed an emotional wheel which suggests that emotions are formable, with eight basic emotions placed on the emotional wheel, and each two as the polarity of one and another, the follow-up emotions can be then combined within those basic emotions in different degree. Therefore, emotions are regarded as relatable to each other, and psychologists attempt to quantify emotions by using methods such as factor analysis to map those emotions into a more limited dimension [14]. Generally, a two-dimensional coordinate map is applied with valence and arousal to quantify emotion, where valence is to describe the emotion degree of negative to positive, while arousal is to describe how energized or dispassionate the emotion is.