# Investigating the Potential Use of EEG for the Objective Measurement of Auditory Presence
## 1 Introduction  
Presence is the participant's physiological sense of \"being there\" in the virtual environment while physically situated in another place [1, 2]. As one of the factors that contribute most to the overall experience of virtual reality (VR), presence has been investigated in the context of VR in many studies [1, 3, 4, 5, 6, 7, 8, 9, 10, 11]. In addition to VR, research on presence could also benefit other fields such as video games, audio production, and immersive audio technologies.  
Generally, presence is measured using questionnaires, of which the two most widely used are the presence questionnaire (PQ) by [PERSON] and [PERSON] [2, 6] and the SUS questionnaire by [PERSON], [PERSON] and [PERSON] [1, 3, 7]. Subjective measurement based on questionnaires is both valid and reliable since it reflects the subjective evaluation of the participants. As an alternative, researchers have been exploring and improving objective methods of measuring presence. Objective measurement uses physiological signals from the body directly, and can be straightforward and in real time if conducted under strictly controlled conditions. Furthermore, it can prevent the break in presence (BIP) [3, 12, 13] caused by the act of answering the questionnaires, since participants are no longer required to leave the virtual environment.  
Since presence is a sensation generated by the brain, objective measurement could possibly utilize electroencephalography (EEG) as a tool to reveal physiological changes. EEG records brain activities through multiple electrodes placed on the scalp. It has already been used in research on presence [8, 9, 10, 11, 14, 15] and in other fields [16, 17, 18, 19, 20].  
In our previous study [21], we explored the possibility of measuring presence objectively using EEG. Two experiments were carried out, in which a loudspeaker array of eight loudspeakers was used to reproduce urban soundscapes as the only stimuli for presence. Subjective measurements (by questionnaires) and objective measurements (by EEG) wereboth conducted during the process. It was found that the presence score (the result of the questionnaire) and the theta/beta ratio (an index derived from EEG) could both indicate a change in the perceived presence level, and that they were positively correlated with each other.  
In addition to the theta/beta ratio, several other indices were also investigated in the previous study, and we observed a dramatic change in the power of the alpha band as the presence level varied. In this study, we therefore re-analyzed the original data, focusing on the alpha band.  
Alpha is the dominant oscillation in the brain's EEG. Numerous studies have demonstrated that the alpha band plays an important role in cognitive processing [22; 23; 24; 25]. Peak alpha frequency (PAF) is the predominant frequency of alpha-band oscillations (i.e., the maximal power value in the alpha band), and it can be viewed as a neurophysiological trait of cognitive functions [26; 27; 28]. It shows inter-individual differences [29; 30; 31; 32; 33], and also varies intra-individually with age [34; 35; 36], fatigue [37], and cognitive load or task demands [38; 39]. For this reason, the present study used PAF specifically as an EEG index to investigate the correlation between EEG and subjective presence level.  
In summary, the present study investigated the objective measurement of auditory presence using EEG on the basis of the previous study. Specifically, PAF was examined to determine whether it could reflect changes in the perceived presence level.  
The rest of the paper is organized as follows. Section 2 explains the methodology of this study, and Section 3 presents the results of the experiments. Following that, Section 4 provides a discussion of the results. The conclusion and suggestions for future work are then presented in Section 5.