# Investigating the Potential Use of EEG for the Objective Measurement of Auditory Presence
## 2 Materials and Methods  
This study used the same experiment as that described in our previous paper [21]. The previous study focused on investigating the relationship between presence and the theta/beta ratio, whereas this study used the same raw data (i.e., the recorded raw EEG data and the results of questionnaires) to investigate the relationship between presence and another EEG index, the peak alpha frequency (PAF). Therefore, this section summarizes the experimental design while focusing on introducing the specific method for acquiring PAF. Details about the other aspects of the experimental design can be found in [21]. This study used ESMA-3D Immersive Soundscape Recordings by Dr. [PERSON]'s team [40] as programs to stimulate presence. Due to its nature, the 8-channel signal was reproduced by an 8-channel double-layer quad-speaker array (i.e., cube array) [41] in the room, as shown in Figure 1. No visual stimuli were used in this study. To shorten the duration of the experiment, 6 out of 13 scenes were selected from the original file. With each scene lasting 30 s, the overall duration of the program was three minutes. As well as the original program described above (denoted Program A), two processed versions of it were also used to stimulate different levels of presence. Program B was created by combining signals from all eight channels of Program A and copying the sum to each channel. Since each channel had the same content, we should expect Program B to have a similar effect as a mono sound file. Program C was derived from Program A by muting the upper four height channels. Thus, the ESMA-3D degenerated into ESMA without height channels. We assumed that these three programs would stimulate presence levels in the following order: A > C > B. These three programs were then used in the subsequent experiments. The study was composed of two experiments. Experiment 1 used Programs A and B for comparison, involving a total of 18 experienced listeners from the lab (16 males and 2 females, ages ranging from 23 to 27 years old), and Experiment 2 used Programs A and C for comparison, involving a total of 25 naive listeners (8 males and 17 females, ranging in age from 21 to 28 years old). During each experiment, two programs were presented in succession. In the meantime, EEG was recorded using a portable EEG device (Emotiv EPOC X [42]). After that, participants were required to fill in the questionnaire at the end of the experiment. The questionnaire used in the study contained four questions as follows:  
1. Please rate your sense of being in the virtual environment on a scale of 0 to 10, where 10 represents your normal experience of being in a place;
2. During your experience, did you often think to yourself that you were actually in the virtual environment? Please rate it on a scale of 0 to 10, where 10 represents you almost felt you were actually in the virtual environment;
3. How well could you identify sounds? Please rate it on a scale of 0 to 10, where 10 represents you could clearly identify different kinds of sounds;
4. How well could you localize sounds? Please rate it on a scale of 0 to 10, where 10 represents you could easily detect the location of each sound. The first two questions were taken from the SUS questionnaire. The first question focuses on assessing how closely the virtual environment resembles the real world, and the second question aims to evaluate the participants' subjective impressions of their experience within the virtual environment. The last two questions were chosen from the presence questionnaire to evaluate the realism of the sound field of the virtual environment. The final presence score was the sum of the answers to these four questions. As the questions suggest, the higher the presence score, the greater the level of presence. Thus, the subjective assessment of presence (represented by the presence score) and the objective measurement (represented by EEG) were both acquired.