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\begin{document}

\title{A Longitudinal study on the impact of high-altitude hypoxia on
perceptual processes}



\author[1]{Fumei GUO}%
\author[2]{Changming Wang}%
\author[1]{Getong Tao}%
\author[3]{Hailin Ma}%
\author[4]{Jia Zhang}%
\author[1]{Yan Wang}%
\affil[1]{Institute of Psychology Chinese Academy of Sciences}%
\affil[2]{Xuanwu Hospital Capital Medical University}%
\affil[3]{Affiliation not available}%
\affil[4]{Xiamen University Faculty of Medicine and Life Sciences}%


\vspace{-1em}



  
  \date{June 04, 2024}


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\selectlanguage{english}
\begin{abstract}
This study aimed to explore the neural mechanism underlying
high-altitude (HA) adaptation and deadaptation on perceptual processes
in lowlanders. Eighteen healthy lowlanders were administered a facial
S1-S2 matching task that included incomplete face (S1) and complete face
(S2) photographs, combined with ERP technology. Participants were tested
shortly before they went to HA at sea level (Test 1), twenty-five days
after entering HA (Test 2), one week (Test 3) and one month (Test 4)
after returning to lowlands. Compared with sea level baseline, shorter
latencies of P1 and N170 and larger amplitudes of complete face N170
were found in HA. After returning to sea level, compared with HA, the
amplitude of the incomplete face P1 was smaller after one week and the
complete face was smaller after one month. The right hemisphere N170
amplitude was larger after entering HA and one week after returning to
sea level compared to baseline, but it returned to baseline after one
month. Taken together, the current findings suggest that HA adaptation
increases visual cortex excitation to accelerate perceptual processing.
More mental resources are recruited during the configural encoding stage
of complete faces after HA exposure. The perceptual processes affected
by HA exposure is reversible after returning to sea level, but the
low-level processing stage is different between incomplete and complete
faces due to neural compensation mechanisms. The configural encoding
stage in the right hemisphere is affected by HA exposure and requires
more than one week but less than one month to recover to baseline%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{introduction}}

The human brain, especially the cerebral cortex, is extremely sensitive
and vulnerable to hypoxia . Oxygen supply is essential to maintaining
the normal function of brain's cognitive processes, which can be
conceived as a hierarchy of several serial phases, from early
sensory-perceptual stages, to higher-order cognitive processes (Mayevsky
et al., 1986). Hypoxia, whether environmental or pathological, such as
exposure to a hypobaric chamber or living at high altitude (HA), chronic
obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA),
has been found to broadly impact cognitive functions . Among them, HA is
an important residential region for human beings. Despite thousands of
years of struggle, hypoxia remains a significant public health problem.
Extant studies have documented neurologic deficits, cognitive
dysfunctions, and brain abnormalities, such as hallucinations under HA
conditions , which directly impair people's living quality and work
efficiency and thus cannot be ignored. Exposure to HA provides a
natural, convenient to observe, low-cost and reproducible model for
studying hypoxia in healthy individuals, which not only has guiding
significance on plateau activity, but also might provide new insights
for understanding hypoxia in clinical settings.

A number of studies have shown that HA exposure exerts a range of
deleterious effects on cognitive processes, verbal fluency, cognitive
flexibility, executive function, metacognition, etc. (Taylor et al.,
2016; Yan, 2014), while others indicating the presence of compensation
mechanisms to help maintaining cognitive performances . The
inconsistence across studies may be attributed to the cognitive domain
being interrogated . Since the human brain is inherently plastic, its
functional structure can be reorganized in response to environmental and
physiological alterations , those cognitive processes, e.g. higher-order
cognitive functions which involve the prefrontal lobe, may employ an
adaptive process to compensate for the neurocognitive impairment and
thus be less affected by hypoxia . Nonprefrontal lobe functions, such as
visual perception processing, reaction time (RT) , perceptive
discrimination, and color perception, demonstrated to be more sensitive
to hypoxia and are critical for the brain in adaptation to HA (Chen et
al., 2016), making it particularly suitable for monitoring hypoxia
adaptation of cognitive processes, which may help us understand how the
brain acclimates to harsh environments.

Previous studies indicated that brain electrophysiological activity in
the visual cortex may explain the alterations in visual cognition .
Among all visual inputs, face perception is one of the most developed
visual perceptual skills of human beings, and it plays a major role in
social interaction, which constitute the perceptual basis for
interpersonal communication. Nevertheless, few studies have directly
examined the neural processing of face information under HA hypoxia
conditions. Perceptual organization refers to the process of integrating
fragments of stimuli into a coherent pattern . ERPs are widely used in
perceptual organization studies. P1 and N170 are universally recognized
as two main face-sensitive brain potentials. The P1 component reflects
the low-level processing of face perception . The N170 component is
considered to index the perceptual categorization of faces , which
reflects high-level face perception and the configural encoding of
facial features as well as their integration into a holistic perception
. As a crucial cognitive function, perceptual alteration under HA can
help advance our understanding of the impact of HA hypoxia on
nonprefrontal tasks.

After prolonged HA exposure, the individual experiences a series of
physiological and biochemical adaptive responses, e.g., increases in
oxygen-carrying hemoglobin levels and ventilation, vasodilatation, and
so on, to compensate for hypoxia . Generally, the human beings could
acclimatize to a new altitude in one to three months . This gradual
acclimatization may counteract impaired cognitive performance during
prolonged exposure to HA . However, the results have been remarkably
mixed, with other studies reporting that the cognitive deficit under
long-term hypoxia is more severe than the short-term hypoxia (Sharma et
al., 2014; G. Zhang et al., 2013). Therefore, more empirical research is
needed to provide convincing evidence on how prolonged exposure to high
altitude environment affects cognitive processes.

Instead of settling in the plateau, many people return to sea level
after short periods of hypoxic exposure at HA. HA deadaptation refers to
the process of readaptation to a low-altitude environment involving a
series of changes in neural function, metabolism or even structure in
acclimatized HA residents. Physiological studies have shown that
returning to sea level requires approximately one week for the arterial
partial pressure of carbon dioxide (PCO2) to normalize . Biochemical
parameters such as Hypersensitive C-reaction protein (hsCRP) and
homocysteine returned to baseline level following return to sea level
one month compared to baseline . Other evidences suggests deficits in
neuropsychological functions may also last when sojourners return to sea
level . A study investigated visual spatial and visual non-spatial
discrimination abilities, found that the behavioral performances and ERP
measurements returned to baseline three months after sojourners returned
to the lowlands . Whether cognitive processes will recover accompanied
by physiological alterations during HA adaptation and deadaptation has
rarely been discussed. Investigating how cognitive processes change in
the HA environment and the possibility of long-lasting deficits after
returning to sea level could promote the understanding of our brain's
protective mechanism for survival and neuroplasticity, and ultimately
pave the way for neuroprotection.

Summing up, high altitude hypoxia/reoxygenation changes the neuronal
activity and physiological activities. Nevertheless, there are several
limitations in existing literatures. Firstly, most studies adopt a
cross-sectional design to compare HA residents or sojourners with
matched sea-level residents, lacking the comprehensive depiction
regarding alterations of cognitive processing under both HA adaptation
and deadaptation conditions. Secondly, there are abundant studies
concerning higher-order cognitive processes, while perceptual process is
often ignored. Thirdly, there are limited studies using ERP methods,
with high temporal resolution to index cognitive processing and provide
insight into the dynamic neural mechanisms of cognitive processes, which
are particularly suitable to monitor hypoxia adaptation.

To fill this knowledge gap, the present study adopted a S1-S2 paradigm
to evaluate the underlying perceptual processes electrophysiological
mechanisms of lowlanders who sojourned at an altitude and then returned
to the lowlands. Based on previous research , we anticipated that after
approximately four weeks of acclimatization to HA, compared with
baseline in the lowlands, the heart rate (HR) and diastolic pressure
(DBP) would be higher, while the peripheral capillary oxygen saturation
(SpO2) would be lower; the N170 amplitude would be significantly
increased, and the P1 and N170 latency would be significantly shortened
in the HA environment. For the deadaptation process, we speculated that
physiological and perceptual organization patterns persist as in the HA
environment upon return to sea level after one week, but after one
month, we expected them to return to baseline.

\section*{Methods}

{\label{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 \selectlanguage{ngerman}± 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).

\textbf{Experimental Procedure}

The physiological parameters and EEG were recorded at four stages. The
first test (Test 1) was performed at Xiamen (63 m) approximately one
week before participants ascended to HA. After three days on train, all
participants arrived in Lhasa (Altitudes: 3658 m) and stayed for two
days. Subsequently, the participants stayed in Damxung County
(Altitudes: 4300 m) for three weeks, then returned to Lhasa for three
days and performed the second test (Test 2) in Lhasa. The third test
(Test 3) was conducted within one week after the subjects returned to
Xiamen. The last test (Test 4) was performed one month after
participants returned to Xiamen (Figure 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1. Schematic illustration of the time and altitude of
data collection.}

Before the EEG recording, HR and SpO2 were measured with a finger clamp
pulse oximeter (model DEDAKJ-CMS50D) at rest, vital capacity (VC) was
assessed by a digital vital capacity tester (WQS-8888), and an Omron
sphygmomanometer (model HEM -7124) was applied to detect resting blood
pressure (systolic pressure, SBP and diastolic pressure, DBP).

The EEG experiment lasted approximately fifteen minutes. Participants
were seated in a comfortable chair approximately 70 cm in front of the
screen in a dimly lit cabin, which subtended 4.8\selectlanguage{ngerman}°×6° of visual angle.
The facial S1-S2 paradigm was adopted in this study. During the test,
120 trials were randomly presented, and 18 practice trials were used to
ensure that all the participants understood the experiment. Two face
photographs (S1 is an incomplete face, S2 is an complete face) were
presented in order in each trial. All photographs were black and white
and keep the same luminance. The probability that S1 and S2 are the same
face is 50\%. If S1 and S2 are identical, it is the congruent condition;
otherwise, it is the incongruent condition (Figure 2). The stimuli were
presented by E-Prime 2.0 (Psychology Software Tools, Pittsburgh, PA).
Each stimulus was presented 500 ms on the screen and followed by a blank
screen with a ``+'' in the center. Participants need to figure out
whether S1 and S2 are the same face and respond by pressing ``F'' or
``J'' on the keyboard. The ISI (interstimulus interval) was 800 ms, and
the ITI (intertrial interval) was randomly varied from 800 to 1200 ms.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2} . \textbf{Materials and Procedures} (A: Congruent
Condition; B: Incongruent Condition)

EEG Recording

EEG data were recorded from a 64 Ag/AgCl electrode cap (Neuroscan Inc.)
according to the extended International 10--20 system. The physical
reference electrode was placed approximately 2 cm posterior to the CZ
electrode. Vertical electrooculographic (VEOG) recordings are electrodes
placed above and below the left eye. Two electrodes were used at the
outer canthi of both eyes to record the horizontal electrooculogram
(HEOG). EEG signals were amplified in DC mode at 100~Hz and digitized at
a rate of 500 Hz. The scalp interelectrode impedances of each subject
were guaranteed to drop below 5 k\selectlanguage{greek}Ω.

\selectlanguage{english}Data Analysis

One-way repeated-measures analysis of variance (ANOVA) was conducted on
physiological data (SpO2, SBP, DBP, HR and VC) separately using time as
a within-subject factor.

A 4 (Time: Test 1, Test 2, Test 3, Test 4) \selectlanguage{ngerman}× 2 (Face stimulus :
incomplete, complete) within-subject design was conducted. Factors are
time and condition. The amplitudes and latencies of P1 and N170 are
dependent variables. Offline data from the EEG recordings were analyzed
using EEGLAB (http://sccn.ucsd.edu/eeglab/), software version 13.5.4b,
an open-source signal processing toolbox implemented in MATLAB (The
MathWorks, Inc, Natick, USA, Version 2018a). The data were referenced
offline to the algebraic mean of all cortical electrodes and filtered
offline with a bandpass filter of 1--90\selectlanguage{english} Hz and a notch filter (50 Hz).
Trials with various artifacts (body movements, ocular artifacts, or
muscle activity) were rejected, with a criterion of +-75 \selectlanguage{greek}μ\selectlanguage{english}V from the EEG
signal before proceeding to grand averages. Waveform averages for each
individual were calculated. Average waveforms were divided into epochs
of 1200 ms in length, including a baseline from -200 to 0 ms before the
onset of the first face in each trial. Based on previous research, four
posterior electrode sites (O1, O2, PO3, PO4) were considered for the
analysis of P1. Four electrode codes (P7, P8, PO7, PO8) from the
parietal and parieto-occipital regions were used for the analysis of
N170. Peak amplitudes and peak latencies were adopted for statistical
analyses for both components.

Three-way, repeated measure analysis of variances was conducted on P1
and N170 amplitudes and latencies separately. Hemisphere (left, right),
face stimulus (incomplete, complete) and time (Test 1, Test 2, Test 3,
Test 4) are within-subjects factors.

Data were analyzed using SPSS version 22 (Somers, Inc., Chicago) for
Windows. All statistical significance levels were two-tailed, with an a
priori alpha level for the significance of \emph{p} \textless{} 0.05. To
explore interaction effects, simple effect analyses were conducted.
Effect sizes were calculated through partial eta-squared or
Cohen's\emph{d} (\emph{d} ). The Greenhouse-Geisser epsilon correction
was employed when the data violated the sphericity assumption. A
Bonferroni corrected post hoc test was performed to determine pairwise
differences.

\section*{Results}

{\label{results}}

Physiological Results

The one-way repeated ANOVA revealed a significant main effect of time on
SpO2 {[}\emph{F} (3, 51) =111.21, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =0.87{]}. Post hoc comparisons
demonstrated that SpO2 in Test 2 was significantly lower than that in
the other tests. The main effect of time was significant for HR
{[}\emph{F} (3, 51) =8.80, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =0.34{]}, with higher HR for Test
2 than the other tests. The main effect of time on SBP was significant
{[}\emph{F}(3, 51) =7.49, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =0.31{]}, and SBP was
significantly higher in Test 2 than in Test 3. A significant main effect
of time was found for DBP {[}\emph{F} (3, 51) =26.75,\emph{p}
\textless{} 0.001, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2}=0.61{]}, with
larger DBP in Test 2 than in the other tests. There was a main effect of
time in VC {[}\emph{F} (3, 51) =13.08, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =0.44{]}, with a smaller VC in
Test 2 than in the other tests (Table 1).

\textbf{Table 1} . Descriptive statistical results of physiological
parameters (\emph{M} \selectlanguage{ngerman}±\emph{SD} )\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& \textbf{Test 1} & \textbf{Test 2} & \textbf{Test 3} & \textbf{Test
4}\tabularnewline
\midrule
\endhead
\textbf{SpO2(\%)} & 98.78\selectlanguage{ngerman}±1.17 & 92.06\selectlanguage{ngerman}±2.01 & 98.56\selectlanguage{ngerman}±0.98 &
98.67\selectlanguage{ngerman}±1.24\tabularnewline
\textbf{HR(bpm)} & 75.83\selectlanguage{ngerman}±11.16 & 88.78\selectlanguage{ngerman}±10.30 & 71.78\selectlanguage{ngerman}±11.78 &
74.00\selectlanguage{ngerman}±9.74\tabularnewline
\textbf{SBP(mmHg)} & 108.00\selectlanguage{ngerman}±14.63 & 113.84\selectlanguage{ngerman}±13.63 & 101.06\selectlanguage{ngerman}±11.32 &
104.28\selectlanguage{ngerman}±12.74\tabularnewline
\textbf{DBP(mmHg)} & 65.44\selectlanguage{ngerman}±8.25 & 76.31\selectlanguage{ngerman}±8.25 & 60.72\selectlanguage{ngerman}±5.34 &
62.67\selectlanguage{ngerman}±7.13\tabularnewline
\textbf{VC(ml)} & 3340.98\selectlanguage{ngerman}±976.25 & 2730.35\selectlanguage{ngerman}±760.17 & 3627.56\selectlanguage{ngerman}±1217.78 &
3671.50\selectlanguage{ngerman}±1216.89\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

\textbf{ERP Results}

\textbf{P1.} For P1 latency, the main effect of time was significant
{[}\emph{F} (2.11,35.83) = 9.53,\emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.36{]}, with shorter latency
for Test 2 than Test 1 (114.04 \selectlanguage{ngerman}± 2.68 ms\emph{vs.} 127.86 ± 2.73
ms,\emph{p} = 0.003), Test 3 (128.85 ± 3.19 ms, \emph{p} \textless{}
0.001) and Test 4 (125.72 ± 2.22 ms, \emph{p} = 0.001) (Figure 3). The
main effect of face stimulus was significant {[}\emph{F} (3,17) =
27.89\emph{p} \textless{} 0.001, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =
0.62{]}, with longer latency for the complete condition than for the
incomplete condition (129.02 \selectlanguage{ngerman}± 1.82 ms \emph{vs.} 119.22 ± 2.43 ms)
(Figure 4). No other significant main effects or interactions were
found.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3. The grand average waveform of P1 from Test 1 to Test
4} (black, red, green, blue line, separately). The two face stimulus
(complete/incomplete) and the four electrodes (PO3/PO4/O1/O2) were all
averaged.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\par\null

\textbf{Figure 4. The grand average waveform of P1 elicited in the
incomplete face condition and complete face condition} . The four tests
and four electrode sites (PO3/PO4/O1/O2) were averaged.

For the amplitude of the P1 component, the main effect of time was
significant {[}\emph{F} (1.72, 29.23) = 3.59, \emph{p} = 0.02,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2}=0.17{]}, and the amplitude of P1
in Test 2 was larger than that in Test 3 (4.70 \selectlanguage{ngerman}± 0.49 \emph{vs.} 3.92 ±
0.46, \emph{p =} 0.013) and Test 4 (3.91± 0.48, \emph{p =} 0.010)
(Figure 3). The main effect of hemisphere was significant {[}\emph{F}
(1,17) = 19.64, \emph{p} \selectlanguage{english}<0.001, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2}
= 0.54{]}, and the P1 amplitude in the right hemisphere was larger than
that in the left hemisphere (4.51 \selectlanguage{ngerman}± 0.48 \emph{vs.} 3.81 ± 0.41) (Table
2 and Figure 5). The main effect of face stimulus was significant
{[}\emph{F} (1,17) = 21.13,\emph{p} \selectlanguage{english}<0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.55{]}, and the P1 amplitude in
the complete condition was larger than that in the incomplete condition
(4.72 \selectlanguage{ngerman}± 0.43 \emph{vs.} 3.60 ± 0.48) (Table 2 and Figure 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5. The grand average waveform of P1 elicited in the left
hemisphere and right hemisphere} . The four tests and four electrode
sites (PO3/PO4/O1/O2) were averaged.

\textbf{Table 2.} The peak amplitude and peak latency of the P1
component (\emph{M} \selectlanguage{ngerman}±\emph{SD} )\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
\begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Face stimulus}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Hemisphere}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 1}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 2}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 3}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 4}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitudes} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitudes} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitudes} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitudes} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Incomplete\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
3.44\selectlanguage{ngerman}±1.99\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
3.44\selectlanguage{ngerman}±2.18\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
2.81\selectlanguage{ngerman}±1.88\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
3.07\selectlanguage{ngerman}±1.98\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3.87\selectlanguage{ngerman}±2.33\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.39\selectlanguage{ngerman}±2.67\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3.70\selectlanguage{ngerman}±2.44\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.12\selectlanguage{ngerman}±2.61\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Complete\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.46\selectlanguage{ngerman}±1.93\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
5.06\selectlanguage{ngerman}±1.93\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.43\selectlanguage{ngerman}±2.11\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3.80\selectlanguage{ngerman}±1.99\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.68\selectlanguage{ngerman}±2.02\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
5.91\selectlanguage{ngerman}±2.32\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.74\selectlanguage{ngerman}±2.03\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4.67\selectlanguage{ngerman}±2.30\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latencies} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latencies} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latencies} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latencies} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Incomplete\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
125.94\selectlanguage{ngerman}±22.29\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
109.11\selectlanguage{ngerman}±18.68\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
124.89\selectlanguage{ngerman}±21.62\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
123.39\selectlanguage{ngerman}±17.62\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
123.50\selectlanguage{ngerman}±18.45\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
104.94\selectlanguage{ngerman}±18.69\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
123.06\selectlanguage{ngerman}±22.16\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
118.89\selectlanguage{ngerman}±21.25\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Complete\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
130.06\selectlanguage{ngerman}±11.81\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
120.50\selectlanguage{ngerman}±12.96\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
136.22\selectlanguage{ngerman}±15.84\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
130.00\selectlanguage{ngerman}±15.24\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
131.94\selectlanguage{ngerman}±7.81\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
121.61\selectlanguage{ngerman}±8.10\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
131.22\selectlanguage{ngerman}±12.27\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
130.61\selectlanguage{ngerman}±11.74\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

The interaction between time and face stimulus was significant
{[}\emph{F} (3,51) = 3.84, \emph{p} = 0.015,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.18{]}. The simple effect test
showed that in the incomplete condition, the P1 amplitude was much
smaller in Test 3 than in Test 2 (3.25 \selectlanguage{ngerman}± 0.49 \selectlanguage{greek}μ\selectlanguage{english}V \emph{vs.} 3.92 \selectlanguage{ngerman}± 0.55
\selectlanguage{greek}μ\selectlanguage{english}V, \emph{p} = 0.042), while under complete conditions, the amplitude in
Test 4 (4.24 \selectlanguage{ngerman}± 0.49 \selectlanguage{greek}μ\selectlanguage{english}V \emph{vs.} 5.49 \selectlanguage{ngerman}± 0.48 \selectlanguage{greek}μ\selectlanguage{english}V, \emph{p} = 0.003) was
smaller than that in Test 2 (Figure 6 and Table 2). No other significant
main effects or interactions were found.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6. The grand average waveform of P1 elicited in the
incomplete face condition and complete face condition from Test 1 to 4}
. Four electrode sites (PO3/PO4/O1/O2) were averaged.

\textbf{N170.} The main effect of time was significant for N170 latency
{[}\emph{F} (2.01,34.08) = 32.87, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =0.66{]}, with shorter latency for
Test 2 than Test 1 (173.68 \selectlanguage{ngerman}± 2.56 ms \emph{vs.} 188.08 ± 2.29
ms,\emph{p} \textless{} 0.001), Test 3 (192.64 ± 2.44 ms,
\emph{p}\textless{} 0.001) and Test 4 (191.40 ± 2.59 ms, \emph{p}
\textless{} 0.001) (Figure 7). The main effect of face stimulus was
significant for N170 latency {[}\emph{F} (1,17)= 57.05, \emph{p}
\textless{} 0.001, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2}=0.77{]}, with
longer latency for the incomplete condition (195.57 \selectlanguage{ngerman}± 2.90 ms \emph{vs.}
177.33 ± 1.81 ms) than for the complete condition (Figure 8). The
interaction between time and face stimulus was significant {[}\emph{F}
(3,51) = 6.63,\emph{p} = 0.001, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} =
0.28{]}. In the incomplete condition, N170 latency was shorter in Test 2
than in Test 1 (179.25 \selectlanguage{ngerman}± 3.44 ms \emph{vs.} 198.06 ± 3.41 ms, \emph{p} =
0.001), Test 3 (201.89 ± 3.42 ms, \emph{p} \textless{} 0.001), and Test
4 (203.08 ± 3.52 ms, \emph{p} \textless{} 0.001), respectively. In the
complete condition, N170 latency was shorter in Test 2 than in Test 1
(168.11 ± 2.13 ms \emph{vs.} 178.11 ± 2.12 ms, \emph{p} = 0.001), Test 3
(183.39 ± 2.36 ms, \emph{p} \textless{} 0.001), and Test 4 (179.72.08 ±
2.10 ms, \emph{p} \textless{} 0.001) (Table 3 and Figure 9). No other
significant main effects or interactions were found.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 7. The grand average waveform of N170 from Test 1 to Test
4} (black, red, green, blue, separately). The two face stimulus
conditions (complete/incomplete) and the four electrodes (P7/P8/PO7/PO8)
were averaged.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\par\null

\textbf{Figure 8. The grand average waveform of N170 elicited in the
incomplete face condition and complete face condition} . The four tests
and four electrode sites (PO7/PO8/P7/P8) were averaged.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 9. The grand average waveform of N170 elicited in the
incomplete face condition and complete face condition from Test 1 to
4.}Four electrode sites (PO7/PO8/P7/P8) were averaged.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure 10. The grand average waveform of N170 elicited in the
left and right hemisphere.} The two face stimulus (Complete/ Incomplete
) and four electrode sites (PO7/PO8/P7/P8) were averaged.

With regard to the amplitude of the N170 component, the main effect of
time was significant {[}\emph{F} (3,51) = 5.09, \emph{p} = 0.004,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.23{]}, and the amplitude of
N170 in Test 2 was larger than that in Test 1 (-5.48 \selectlanguage{ngerman}± 0.59 \selectlanguage{greek}μ\selectlanguage{english}V\emph{vs.}
-4.17 \selectlanguage{ngerman}± 0.45 \selectlanguage{greek}μ\selectlanguage{english}V) (Figure 7). The main effect of hemisphere was
significant {[}\emph{F} (1,17) = 10.92, \emph{p} = 0.004,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.39{]}, and the amplitude of
N170 in the right hemisphere was larger than that in the left hemisphere
(-5.48 \selectlanguage{ngerman}± 0.54 \selectlanguage{greek}μ\selectlanguage{english}V \emph{vs.} -4.17 \selectlanguage{ngerman}± 0.55 \selectlanguage{greek}μ\selectlanguage{english}V) (Figure 10). The main effect
of the face stimulus was significant {[}\emph{F} (1,17) = 14.27,
\emph{p} = 0.002, \selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.46{]}, and
the amplitude of N170 in the complete condition was larger than that in
the incomplete condition (-5.50 \selectlanguage{ngerman}± 0.50 \selectlanguage{greek}μ\selectlanguage{english}V \emph{vs.} -4.15 \selectlanguage{ngerman}± 0.57 \selectlanguage{greek}μ\selectlanguage{english}V)
(Figure 8).

The interaction between time and face stimulus was significant
{[}\emph{F} (3,51) = 7.71, \emph{p} \textless{} 0.001,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0. 31{]}. In the complete
condition, the N170 amplitude was larger in Test 2 than in Test 1 (-6.39
\selectlanguage{ngerman}± 0.60 \selectlanguage{greek}μ\selectlanguage{english}V \emph{vs.} -4.55 \selectlanguage{ngerman}± 0.47 \selectlanguage{greek}μ\selectlanguage{english}V, \emph{p} = 0.001) (Figure 9 and
Table 3). The interaction between hemisphere and time was significant
{[}\emph{F} (3,51) = 3.40, \emph{p} = 0.024,
\selectlanguage{greek}η\selectlanguage{english}\textsubscript{p}\textsuperscript{2} = 0.17{]}; in the right
hemisphere, the N170 amplitude was larger in Test 2 (-5.92 \selectlanguage{ngerman}± 0.60
\selectlanguage{greek}μ\selectlanguage{english}V,\emph{p} = 0.023) and Test 3 (-6.05 \selectlanguage{ngerman}± 0.74 \selectlanguage{greek}μ\selectlanguage{english}V, \emph{p} = 0.014) than
in Test 1 (-4.40 \selectlanguage{ngerman}± 0.45 \selectlanguage{greek}μ\selectlanguage{english}V) (Fig 11 and Table 3). No other significant
main effects or interactions were found.

\textbf{Table 3.} The peak amplitude and peak latency of N170 (\emph{M}
\selectlanguage{ngerman}±\emph{SD} )\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
\begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Face stimulus}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Hemisphere}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 1}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 2}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 3}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Test 4}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitude} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitude} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitude} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Amplitude} \selectlanguage{greek}\textbf{(μV)}\selectlanguage{english}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Incomplete\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-3.37\selectlanguage{ngerman}±2.55\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-3.66\selectlanguage{ngerman}±3.35\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-3.92\selectlanguage{ngerman}±3.01\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-4.03\selectlanguage{ngerman}±2.92\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.50\selectlanguage{ngerman}±2.08\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-4.56\selectlanguage{ngerman}±3.11\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.48\selectlanguage{ngerman}±3.42\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-4.72\selectlanguage{ngerman}±2.72\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Complete\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-3.81\selectlanguage{ngerman}±1.94\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.49\selectlanguage{ngerman}±2.66\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-4.43\selectlanguage{ngerman}±2.33\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-4.69\selectlanguage{ngerman}±2.39\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-5.29\selectlanguage{ngerman}±2.29\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-7.29\selectlanguage{ngerman}±2.84\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-6.61\selectlanguage{ngerman}±3.24\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
-6.39\selectlanguage{ngerman}±2.35\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latency} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latency} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latency} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\textbf{Latency} \textbf{(ms)}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Incomplete\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
199.83\selectlanguage{ngerman}±19.91\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
178.29\selectlanguage{ngerman}±19.21\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
203.94\selectlanguage{ngerman}±18.17\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
204.67\selectlanguage{ngerman}±16.43\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
196.28\selectlanguage{ngerman}±12.14\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
180.22\selectlanguage{ngerman}±15.18\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
199.83\selectlanguage{ngerman}±17.65\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
201.50\selectlanguage{ngerman}±19.37\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Complete\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Left\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
178.17\selectlanguage{ngerman}±11.30\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
167.83\selectlanguage{ngerman}±10.79\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
183.67\selectlanguage{ngerman}±12.87\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
180.11\selectlanguage{ngerman}±9.43\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Right\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
178.06\selectlanguage{ngerman}±8.05\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
168.39\selectlanguage{ngerman}±8.90\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
183.11\selectlanguage{ngerman}±8.30\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
179.33\selectlanguage{ngerman}±9.57\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 11. The grand average waveform of N170 elicited in the
left and right hemisphere from Tests 1 to 4} (\textbf{A} :
Left;\textbf{B} : Right)\textbf{.} The two face stimulus (Complete/
Incomplete ) were averaged.

\section*{Discussion}

{\label{discussion}}

The present study used a follow-up method to investigate the change in
perceptual organization during HA adaptation and deadaptation. For the
adaptation process, compared with the sea level baseline, increased HR
and DBP and decreased SpO2 and VC were found when entering HA. Shorter
latencies of P1 and N170 showed a faster process of perceptual
organization, as well as a larger amplitude of complete face N170. For
the deadaptation process, all physiological parameters recovered to
baseline levels one week after returning to sea level. Compared with HA,
the amplitude of the P1 component of the incomplete face decreased after
one week, while that of the complete face decreased after one month. The
right hemisphere N170 amplitude increased after entering HA and one week
after returning to sea level but returned to baseline one month later.
The changes in physiological data and ERP components were out of sync;
the perceptual organization function recovery takes a relatively long
time. These findings offer novel insights into the neural mechanism of
HA adaptation and deadaptation on cognitive processes, especially in the
nonprefrontal domain.

After twenty-five days of HA exposure, shorter P1 and N170 latencies
were observed, consistent with previous chronic (two-year) HA exposure
study . The results suggested that higher levels of cortical excitation
within the visual cortex leads to a faster perceptual process when
entering HA. After staying at HA for approximately four weeks, the
organism is in a state of hypoxia stress. The reduced SpO2 and VC and
increased HR and DBP revealed a decrease in the partial pressure of
oxygen under a hypoxic environment at HA, causing sympathetic nerve
excitement, which may lead to faster perceptual processing. This
acceleration is compatible with previous studies that reported that the
resting EEG of people who first arrived at HA had enhanced \selectlanguage{greek}\emph{β} \selectlanguage{english}wave
activity, which is a manifestation of increased excitability and
excessive alertness in the cerebral cortex . According to a previous
study, hypoxia results in the facilitation of perception tasks . The
perception task involves activating sensory cortices, which are greatly
influenced by neurotransmitter activity, including dopamine (DA),
5-hydroxytryptamine (5-HT), and peptide corticotropin releasing factor
(CRF). HA exposure induces high concentrations of neurotransmitters,
leading to massive anoxic depolarization of neurons in cortical tissues
and causing brain hyperexcitability , which could positively affect
perceptual process speed. Existing studies have also suggested that
cognitive performance decreased obviously when SpO2 was 80\%, but
activation and increased performance were observed when SpO2 was 90\%,
and this activation could have a general stimulating effect on central
nervous system (CNS) functional adaptation to the impacts of hypoxia ,
which may reflect a mechanism attempting to restore/protect neuronal
homeostasis . Therefore, it might be interpreted as an adaptive trait
rather than a deficit, perhaps attenuating hypoxemia effects under mild
hypoxia. Further studies are needed to provide definitive conclusions on
the mechanisms relating to the acceleration of perceptual organization
processing under HA adaptation.

The N170 amplitude was larger than that at sea level after entering HA
for twenty-five days, which was also similar to the result of two years
of exposure . The increased N170 amplitude under HA adaptation may be
related to maintaining the facial configural encoding process efficiency
during hypoxic stress, as certain cortical areas were activated to a
greater extent to compensate for decreased processing efficiency due to
HA exposure . The increased N170 amplitude may indicate a higher level
of neural activity at HA . Interestingly, similar observations in terms
of the N170 enhancement phenomenon were also found among elderly adults,
which may be due to an increase in the sensitivity to visual stimuli or
a decline in brain adaptability with aging . A recent study revealed
that cognitive resources were insufficient under HA exposure during a
visual attention paradigm, which is similar to normal aging . Moreover,
hypoxia was found to accompany inflammatory brain states, such as
neurodegeneration . Evidence underscores that hypoxia triggers both the
accumulation of cerebral amyloidogenesis and tau phosphorylation, the
pathological markers of Alzheimer's disease, which are featured
cognitive aging . Chronic exposure to an HA environment may have a
similar neural mechanism to cognitive aging, or chronic exposure to HA
may accelerate brain aging, but this inference needs more evidence to be
confirmed.

After returning to sea level, the body makes adjustments to adapt to the
sea level, and all the physiological parameters (SpO2, VC, HR, SBP and
DBP) recover to baseline. Compared with HA, SBP decreased one week after
returning to sea level, even though it had not been significantly
altered by HA adaptation. These findings imply that the autonomic
nervous system outcomes induced by HA exposure are reversible within one
week after returning to low altitudes. P1 and N170 latency also returned
to baseline less than one week after descending to lowlands, suggesting
that the change in the perceptual process was also reversible after
returning to sea level. However, hypoxia was found to have a longer
lasting effect on brain activity than on blood, which might be because
the brain consumes the most oxygen and is more sensitive to hypoxia .
Cortical activation recovery under the HA deadaptation process always
took more than one week in previous studies . A neuroimaging study
showed that entering the HA or returning to sea level in one week were
both associated with a sustained greater level of excitement in the
posterior parietal cortex and occipital cortex, which are the foundation
of the perceptual processes . A longitudinal EEG study also conforms to
this view; increased beta power was found after one month of HA exposure
(3,800 m), and reoxygenation occurred seven days after returning to sea
level, which revealed a sustained higher level of cortical excitation
within the visual cortex during HA deadaptation, which may be explained
by the rise in capillaries induced by chronic hypoxia . The latency
recovered to baseline sooner in our study, which indicates that the
change in cortical activation may not be the direct cause of the change
in latency. The change in latency may be modulated by a more basic
physiological process, which may also regulate the activation of the
cortex. The subtle changes in perceptual organization observed here may
be a result of synergistic interactions between physiological
deadaptation and the aftereffect of hypoxia on neurons . The
physiological mechanism of latency changes and its relationship with
cortical activation need to be further studied.

During the deadaptation process, compared with HA, the incomplete P1
amplitude decreased one week after returning to sea level, while the
complete P1 amplitude decreased after one month. As the P1 amplitude was
associated with attention arousal , the decrease in the P1 amplitude may
reflect lower arousal to the low-level visual features during the HA
deadaptation process. An MRI study was conducted on participants who
carried out the same task following the same schedule and found that
after one week of reoxygenation exposure, cell activity was inhibited in
the visual cortex . This implied that the low-level face perception
process was inhibited for sojourners who went through the deadaptation
process because of the shortage of sufficient arousal. Although this
initial visual processing of faces under both the incomplete condition
and complete condition in the deadaptation process showed no difference
compared to the baseline at sea level, the low-level face perception
process of incomplete faces and complete faces was different after HA
deadaptation. This may be because compared with the complete face, the
processing of incomplete faces involves not only parietal lobe activity
but also frontal lobe activity to compensate for incomplete visual
information . After returning to sea level, this compensation mechanism
of incomplete faces soon disappeared, so the P1 amplitude decreased in
one week. For complete faces, attention arousal decreased gradually,
leading to progressively smaller P1 amplitudes.

Seven days after descending to the lowlands, the N170 amplitude remained
larger in the right hemisphere, but it was no longer detectable after
one month, indicating that it takes more than one week but less than one
month for configural face-encoding processing to recover to baseline.
This face processing configuration stage is also reversible, although
slower than autonomic nervous system recovery. Previous studies found
physiological retention (arterial blood gases and hemoglobin) of
adaptation after returning to sea level after seven days. During hypoxia
in rats for three weeks, microvessels in the brain significantly
increased, and remained elevated after reoxygenation . Therefore, the
found N170 retention might be relevant to physiological responses that
we had not measure or molecular and cellular responses in the brain .
Only a tentative interpretation is possibly based on the present data,
and further studies should be conducted to elaborate the neural
mechanisms.

Both P1 and N170 tend to exhibit a larger amplitude in the right
hemisphere than in the left hemisphere , and this dominance of face
perception in the right hemisphere did not change under HA exposure. Our
results also revealed that the complete face still evoked larger P1 and
N170 amplitudes than the incomplete face in the HA exposure condition.
Moreover, P1 latency for incomplete faces is shorter than that for
complete faces, while N170 latency is longer in incomplete faces than in
complete faces . This pattern represents the basic law of face
perception organization processing, which was not altered by HA
adaptation or deadaptation.

Notably, there are several limitations regarding the current study.
First, it lacks a sea level control cohort. Moreover, we did not collect
ERP data when participants arrived at HA immediately because hair
washing is not recommended within three days of arrival at HA to prevent
cold. Further study could assess the acute effects of HA exposure on
perceptual process and compare it with chronic HA exposure condition.
One more limitation is that we examined healthy, young, and prepared
university students during an incremental ascent. Before entering HA,
they had regular exercise training and took nutrition products in
advance to prevent any possible sickness. As such, some generalization
to a broader population may be limited.

Following the limitations outlined above, it would be advisable to
replicate our findings in different altitude exposure circumstances
(e.g., different HA durations, different altitudes, and different rates
of ascent). Another useful addition to the design would be to elucidate
the impact of HA adaptation and HA deadaptation on other cognitive
function types. Finally, future studies could broaden existing research
by investigating the specific relationships between physiological
parameters (such as cerebrovascular and cardiovascular parameters) and
cognitive alterations, especially how physiological and biochemical
activities regulate neuronal activities.

\section*{Conclusion}

{\label{conclusion}}

In summary, using ERP, we demonstrated that the perceptual process
changed under both HA adaptation and HA deadaptation. The present study
suggests that HA adaptation accompanies increased occipital lobe
excitation and more neural resources are recruited during the configural
encoding stage. Moreover, this change in perceptual organization is
reversible after returning to sea level, although low-level feature
processing for complete and incomplete faces is different after
deadaptation due to the compensation mechanism of incomplete faces,
which leads to quicker recovery. Simultaneously, the recovery of the
configural encoding stage requires more than one week but less than one
month. The present longitudinal study provides a persuading picture
clarifying how HA hypoxia/reoxygenation affect perceptual processes, and
enables us to shed light on how the human brain acclimates to hypoxia
environments.

\section*{Acknowledgments}

{\label{acknowledgments}}

This work was supported by the National Natural Science Foundation of
China {[}grant numbers 31771247, and 3110081{]}.

\section*{Reference}

{\label{reference}}

\selectlanguage{english}
\FloatBarrier
\end{document}

