Methods

Participants
A total of 23 students who came from an aid education team of Xiamen University (Fujian province, China P.R.) were recruited as paid volunteers. Five participants were removed due to noisy EEG data and drug use, which affected the central nervous system. Thus, the final sample consisted of 18 individuals (5 males, mean ± SD age: 19.15 ± 0.99 years). According to the self-report, all participants had normal or corrected-to-normal vision and were right-handed. None of them had any history of alcohol consumption, neurological or psychiatric diseases, hypertension, heart disease, lung disease or chronic altitude sickness. None of the participants had been to HA areas before, except for two team leaders who went to HA for a short stay one year ago. Oxygen supply equipment was not used throughout the study period. Informed consent was obtained from all the participants prior to the experiment.
Stimulus
Sixty natural neutral facial pictures were quoted from the Chinese Standard Emotional Face Picture System . Half of them were male, and half were female. Adobe Photoshop was used to randomly occlude pictures, with an occlusion ratio of 50% for each picture, which makes the perceptual process more difficult. The picture pixel resolution is 260×300 (width× height). Each stimulus appeared in the center of the 17-inch LCD computer screen (resolution 1024×768 pixels, refresh rate 75 Hz).