# Effects of Viewing Displays from Different Distances on Human Visual System
## Materials and Methods
### Response Measures
#### 2.5.2 EEG Signal Response  
The electrical activity in the brain is generated by billions of brain cells called neurons ([PERSON] _et al._, 2016). [PERSON] and [PERSON] [PERSON] and [PERSON] (2010) suggested that the EEG data recorded from the occipital lobe, either left or right occipital lobes (either O1 or O2) are suitable to measure visual fatigue. Moreover, [PERSON] and [PERSON] [PERSON] and [PERSON] (2016) found that the inhibition function and the electrical activity of the cerebral cortex are relevant to visual fatigue. In this study, the EEG signals were recorded from the occipital lobes (O1 or O2). The positions of the occipital lobes (O1 and O2) were determined by calculating 10% for Oz from inion and at 5% from Oz on the left and right hemisphere of the head as O1 and O2, respectively, by adhering to the 10-20 international standard for EEG electrode placement ([PERSON] _et al._, 2016). Before placing the Ag/AgCl electrodes, the electrode sites (occipital lobes, forehead, and mastoid) were cleaned using 70% isopropyl alcohol swab, and subsequently, air dried for two minutes before performing the experiment. Thereafter, the Ag/AgCl disk electrodes were filled with super gel and placed on the prepared sites, which were held in place using the emotive headset. The reference and ground electrodes were placed over the mastoid region (behind right ear) and on the forehead of the participant, respectively ([PERSON] _et al._, 2016). Figure 2 shows the placement of the EEG electrodes. The ground, reference, O1, and O2 electrodes were connected to a four-channel EEG preamplifier for ME 6000; GND port, Ref port, ch-3, and ch-4, respectively. The four-channel EEG preamplifier was connected to ch1 and ch2 of ME 6000 device.  
The EEG data were amplified and recorded by Mega win 3.1b-13 system (Mega Electronics Ltd., Kuopio, Finland), with a high-quality four-channel digital EEG amplifier for ME 6000. The EEG data was recorded from occipital lobes (both O1 and O2) with a sampling rate of 1000 Hz. Before performing any analysis on the EEG waves, the noise was separated to eliminate undesired signals. The noise was filtered using a low-pass filter (four poles Elliptic filter was used) with a cut-off frequency of 32 Hz to  
Figure 2: Electroencephalogram (EEG) electrodes positions.  
remove the power-line noise and another high-frequency noise. Subsequently, the signal amplitude was carefully adjusted. The EEG waves were extracted by decomposition of the EEG data using multilevel discrete wavelet transform (DWT), where DWT contains sub-band of the signal. Four levels of DWT using Debauches 4 were implemented and the information about the frequencies of the EEG waves is divided into several wavelet levels. Moreover, a Fast Fourier transform (FFT) was employed to identify the frequency contained in each level. The relative powers of the \(\beta\) band and \(\alpha/\beta\) power ratio were the responses selected as the indicators of visual fatigue, which are response variables related to the EEG based on previous studies [28; 29]. The relative power of the \(\beta\) band was calculated using the integrated power of a particular band divided by the sum of the integrated powers of the \(\theta\), \(\alpha\), and \(\beta\) rhythms. For example, the basic index formula of \(\beta\) rhythm is expressed as a basic index \(\beta\) = 100 \(\times\) (power of \(\beta\))/(power of \(\alpha\) + power of \(\beta\) + power of \(\theta\)). The ratio indices are defined as the proportion of the basic indices, i.e., \(\beta/\alpha\), \((\alpha+\theta)/\beta\)[17; 29].