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Acta Astronautica

Acta Astronautica 69 (2011) 969–974

0094-57

doi:10.1

n Corr

Psychol

Tel.: þ8

E-m

journal homepage: www.elsevier.com/locate/actaastro

The influence on individual working memory during15 days �61 head-down bed rest

Xin Zhao a,b, YiXue Wang a, RenLai Zhou a,b,n, LinJie Wang c, Cheng Tan c

a Beijing Key Lab of Applied Experimental Psychology, Beijing Normal University, Beijing 100875, Chinab State Key Laboratory of Cognitive Neuroscience and Learning, Beijing Normal University, Beijing 100875, Chinac China Astronaut Research and Training Center, Beijing 100094, China

a r t i c l e i n f o

Article history:

Received 13 April 2011

Received in revised form

6 July 2011

Accepted 6 July 2011Available online 3 August 2011

Keywords:

Head-down bed rest

Weightlessness

Working memory

Emotion

65/$ - see front matter & 2011 Elsevier Ltd. A

016/j.actaastro.2011.07.003

esponding author at: Beijing Key Lab of Ap

ogy, Beijing Normal University, Beijing 10087

6 10 58802021.

ail address: rlzhou@bnu.edu.cn (R. Zhou).

a b s t r a c t

The research evaluated the changes of verbal and spatial working memory with females

during 15 days �61 head-down bed rest. We used 2-back task to evaluate the working

memory ability on four time points: the fifth day before the rest, the fifth day and the

tenth day in the rest and the fifth day after the rest, as well as record the participants’

depression and anxiety feelings using Beck Anxiety Inventory (BAI) and Beck Depres-

sion Inventory (BDI) simultaneously. The results demonstrated that the trends of verbal

and spatial working memory performance were consistent with that of the control

group during the rest. Moreover, in the �61 head-down bed rest conditions, the

participants have performed no damage on the working memory ability, and any

clinically salient anxiety and depression. The research considered that, compared to the

real space environment, individuals’ undamaged cognitive functions probably have

something to do with the failure of evoking clinical anxiety and depression in the

stimulated weightless environment.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In August 2005, National Aeronautics and SpaceAdministration (NASA) launched the Human ResearchProgram (HRP) project, investigating the long-termhuman activities and operations in space and integratingresults from various spacecrafts about human character-istics. Subsequently, the European Space Agency, RussianSpace Agency and Japanese relative agency have proposedsimilar research programs attempting to resolve the keyissue ‘‘Human Factor’’ in the long-duration space flights.China has also established a ‘‘three-step’’ developmentalstrategy when formulating our own developmental stra-tegies of the manned space flight. With the success of

ll rights reserved.

plied Experimental

5, China.

Shenzhou V and Shenzhou VII, which symbolized break-throughs of key techniques such as reusable launch andspace extravehicular activity, China is currently workingsteadily towards the third step goal involving the con-struction of the space station and solution of large-scaleand long-term manned space applications. Furthermore,the implementation of the third step puts higher demandson astronauts, which required them to stay in space forweeks, months or even years. Only a profound under-standing of regular pattern and trends of astronauts’operation abilities in the long-duration on-orbit flight,can conquer the cutting-edge high grounds in mannedspace field and posses core competence in long-durationflight and deep space exploration.

Currently, researches on astronauts’ operation abilitiesin long-duration on-orbit flight mainly focused onphysiologically functional changes in the weightless orsimulated weightless environments. A large number ofresearches have revealed that the weightless or simulated

X. Zhao et al. / Acta Astronautica 69 (2011) 969–974970

weightless environment would widely affect, even threatdifferent physiological systems including sensory system,motor system, vestibular function, blood system, fluidsystem, muscular system, skeletal system and theimmune system. Based on these foundations, researchersfurther investigated the changes of physiological func-tions and mechanisms and proposed corresponding pro-tective measures [1–6].

On-orbit flight requires astronauts to accurately andefficiently complete large amount of operation tasksinvolving spacecraft systems operation management, loadtest, rendezvous and docking, and extravehicular activ-ities. In order to complete these successfully, besides thephysiological health of astronauts, it also makes highdemands on astronauts’ cognitive abilities. Faced withtransient changing space world, astronauts have to moni-tor the outside world and inside brain changes, imple-ment planned behaviors, form reasoning, solve problemsat the same time. However, so far, compared to thephysiological functions, researches on the changes andmechanisms of individuals’ cognitive functions are rare.

Results in those rare researches on individuals’ cogni-tive functions in weightless or simulate weightless con-ditions are inconsistent: some argued that weightlessnessor simulated weightlessness would affect an individuals’cognitive functions. For instance, Russian astronautsreported that in the first half month, their memory abilityhad impaired, they forgot things that happened 5 min agoand knowledge acquired on the ground [7]; Two out offour ‘‘STS78’’ astronauts in a short-duration flight hadrevealed ‘‘orbital effect’’ in a six cognitive ability test, onedecreased saliently in two tasks, the other dropped in onetask [8]; Manzey et al. found that in an 8d flight, anastronaut had impaired on Dual-task paradigm. However,others contented that the environment of weightlessnessand simulated weightlessness would not influence theindividuals’ cognitive functions [9]. For example, Shehabet al. have tested 8 male participants on attention, spatialmemory and tracking ability and other three cognitiveabilities in a 17d head-down bed rest conditions. Andresults showed that in the rest states, those 8 participantshave showed no damage on cognitive abilities [10];another research also found out that there was nocognitive impairment among 6 male participants in a28d simulated weightless conditions; Seaton et al.reported recently that long-duration head-down bed restconditions would not damage an individuals’ cognitiveabilities [11].

The reason, leading to the inconsistency of researches,that whether the cognitive functions are damaged or notin weightless and simulated weightless environment mayhave something to do with a third variance, which lacksdeep detection. After further analysis of these results, thecognitive functions will be damaged to some extent in thereal space flights [7–9], while in the head-down bed restconditions, individuals’ cognitive abilities will not beimpaired apparently [10–12]. The head-down bed restexperiment, which simulates real space flights theoreti-cally, has always been a key approach in aerospacemedical research, because cardiovascular disorders, mus-cle atrophy, osteoporosis, endocrine disorders, water and

salt metabolism, immune function decline and otherchanges caused by head-down bed rest conditions aresimilar to those caused by weightlessness in real space [6].However, besides weightlessness, real space flights have itsown special environment, such as faster day and nightchanges, social isolation, limited narrow space, reduced orexcessive sensory input, cumulative radiation, toxic sub-stance pollution, and potentially threatens and otherstressful factors, which is hard to simulate in head-downbed rest experiments. Yet, it is those very special factorsin real space flights that always lead to some sort ofnegative emotions of astronauts, which easily evokeanxiety and depression. Lots of psychological researcheshave revealed that emotions can influence individuals’cognition [13–17]. Thus, compared with the real spaceflights conditions, researchers unable to find the impair-ment of cognition in head-down bed rest simulatedconditions probably has something to do with the failureto evoke individuals’ anxiety and depression.

Emotion affecting cognition probably mediated byworking memory has been proved in a growing numberof researches recently [18]. Working memory is the abilityof maintaining and operating information temporarilywhen individual performs cognitive tasks; it is the coreof human high-level cognitive activities and an essentialcomponent in the processing of learning, reasoning,problem solving and intelligence [19–24]. Therefore, the2-back task, Beck Anxiety Inventory (BAI) and BeckDepression Inventory (BDI) have been used to measureparticipants’ working memory abilities and anxiety anddepression feelings at the same time in the �61 head-down bed rest conditions.

The research aimed at further profoundly and system-atically revealing changes and trends of astronauts’ cog-nitive functions in long-duration on-orbit flight andclarifying partly the controversy among individuals’ cog-nitive abilities in the real space conditions and thesimulating experiments.

2. Methods

2.1. Subjects

22 female participants who are 1 or 2 year collegestudents were recruited for the bed rest group, agedbetween 19 and 24 years (M¼20.91, SD¼1.54). All ofthem have no special medical history with vision orcorrected vision normal, and they were right-handedand non-athletes; the control group also recruited 22females who are 1 or 2 year college students, agedbetween 18 and 23 years (M¼20.68, SD¼1.29), they haveno special medical history, and their vision or correctedvision were normal with right-handed and they werenon-athletes. All participants have completed consentbefore the experiments.

2.2. Procedure

The bed rest group was �61 head-down bed rest, theduration of which is 15 days, and the measurement timepoints were set at the fifth day before the rest, the fifth

X. Zhao et al. / Acta Astronautica 69 (2011) 969–974 971

day and the tenth day in the rest and the fifth day afterthe rest. The test time was between 8 and 9 pm everytime. Participants were arranged to complete the test in aposition in the bed before and after rest. In the bed restperiod, participants did everything such as having meals,washing and bathing and urinating lying in bed exceptexcrement and weighing. And participants were allowedto change body position along the longitudinal axis. Theywere also free to surf online, watch movies, listen tomusic, play games or read books and other leisureactivities, and also they were free to use phones to contactthe outside world. Two or three bed rest group partici-pants shared one room. Every bed had been separatedwith movable curtain, and the room temperature rangedfrom 25 to 28 1C. The diets of participants followed therecuperative standards of crews by dietitian and there isno limitation for drinking water. In the bed rest period,participants were arranged to measure body temperature,pulse and brachial artery blood pressure before breakfast.The control group participants did not attend the bed restperiod, all tests were finished in behavioral lab, and theroom temperature also maintained around 25–28 1C. Theintervals between four time points were exactly the sameas the bed rest group and the test time was also at 8–9 pmevery time.

2.3. Materials

2.3.1. Working memory task

In the research, 2-back task has been used to measureworking memory ability, which has a sound validationand reliability [25–27], and the program was compiled byE-prime2.0. The task was divided into spatial workingmemory 2-back task and verbal working memory 2-backtask. When performing the task, there was a fixation ‘‘þ ’’in the center of the black screen, and a series of Arabicnumbers would present up/down/left/right to the ‘‘þ ’’one by one, the numbers ranges from 1 to 9. And thenumber’s font was ‘‘Times New Roman’’, color white, size 60,and the numbers and the positions they appeared at wererandom. In the spatial working memory task, participantswere required to ignore verbal information and merelyjudge whether the present number’s position was con-sistent with the second one before the present one;whereas in the verbal working memory task, participantswere required to ignore the spatial information and judgewhether the present number was consistent with thesecond one before the present one. The intervals betweenstimuli were 4500 ms, first presented the prompt ‘‘X’’,

Table 1Scores of verbal working memory on four time points of the two groups (M7

Before the rest Fifth day in the rest

The rest group

ACC 95.8072.43 96.4872.77

RT 940.847215.90 804.307231.02

The control group

ACC 94.1974.36 95.1675.04

RT 959.447225.08 816.597172.32

followed by 1300 ms delay, and present the stimuli for200 ms, in which participants were allowed to make areaction within 2500 ms.

Participants used response box and press keys to react,half of whom press ‘‘1’’ when the judgment was consis-tent whereas press ‘‘3’’ when the judgment was incon-sistent, and the other half was vice versa. Moreover, thesequence of performing the spatial and verbal workingmemory tasks was balanced within participants. Twosorts of tasks consisted of 80 trials, respectively, dividedinto 2 blocks, and each block had 40 trials, and every firsttwo trial in each block needed no responses. In every task,the ratio of consistent to inconsistent stimuli is 1:1. Thestimuli displayed on a 14 in. screen, the screen resolutionis 1024�768, and refreshing rate was 85 Hz. The distancebetween the participants and the screen was about 60 cm.

2.3.2. Beck Anxiety Inventory (BAI)

Beck Anxiety Inventory has been used to measureparticipants’ anxiety feelings in the research, the Chineseversion of which has well-established validation andreliability [28]. The inventory includes 21 different anxi-ety syndromes, which set the degree of anxiety syndromeof participants as judgment index. The Inventory adopts 4level scoring methods: 1 means no, 2 means mild, of littleannoyance, 3 means moderate, but still capable to endurediscomfort, 4 means severe and not able to barely stand.In analysis, we used standardized scores, namely calcu-lated the total raw scores of the 21 items and then use theequation Y¼ int(1.19X) to transform into standardizedscore. In general, a total score of BAI Z45 was diagnosedas clinical anxiety.

2.3.3. Beck Depression Inventory (BDI)

The research used Beck Depression Inventory to mea-sure participants’ depression feelings, the Chinese versionof which has well-established validation and reliability[28]. The inventory includes 21 items, each of whichrepresent a category and the description of each categorycan be divided into four levels, and participants wererequired to give the value ranging from 0 to 3 according tothe seriousness. In analysis, we added 21 items’ scores,and got the total raw scores ranging from 0 to 63. Higherscores means higher degree of depression, in detail, thetotal scores falling in 10–19 were judged as mild depres-sion, 20–29 for moderate depression and 30–51 for heavydepression.

SD).

Tenth day in the rest After the rest

97.4273.27 97.4371.67

701.217154.49 674.587146.94

96.1775.71 96.1574.02

722.487139.70 716.967138.34

X. Zhao et al. / Acta Astronautica 69 (2011) 969–974972

2.4. Statistical methods

Data has been analyzed by SPSS16.0, and in dealingwith the reaction time, the wrong response trials havebeen erased, and also outliers outside M73SD have beenexcluded.

3. Results

The mean reaction time (RT) and accuracy (ACC) rateof verbal working memory of the participants of the bedrest group and the control group are presented in Table 1.

We used repeated measure ANOVA analysis, theresults showed that the means of the accuracy of four

92

93

94

95

96

97

98

1 2 3 4

The

mea

AC

C r

ate

of v

erba

lw

orki

ng m

emor

y

Time

bed rest groupcontrol group

Fig. 1. Changes for the ACC of verbal working memory of the bed rest

group and the control group. Time 1: before the rest; Time 2: 5th day in

the rest; Time 3: 10th day in the rest; Time 4 : after the rest.

0

200

400

600

800

1000

1200

1 2 3 4

The

mea

RT

rat

e of

ver

bal

wor

king

mem

ory

Time

bed rest group control group

Fig. 2. Changes for the RT of verbal working memory of the bed rest

group and the control group. Time 1: before the rest. Time 2: 5th day in

the rest. Time 3: 10th day in the rest. Time 4: after the rest.

Table 2Scores of spatial working memory on four time points of the two groups (M7

Before the rest Fifth day in the rest

The rest group

ACC 90.26710.61 95.4173.92

RT 978.847224.89 800.517172.89

The control group

ACC 88.24711.90 95.2675.00

RT 1038.147233.54 825.027218.37

tests is significantly different F(3,126)¼3.184, Po0.05,Z2¼0.070, further analysis revealed that the first time

test’s accuracy is significantly lower than the third andfourth tests; there is no salient difference between thesetwo groups F(1,42)¼2.852, P40.05, Z2

¼0.064, the inter-action between groups and different time points is insig-nificant F(3126)¼0.028, P40.05, Z2

¼0.001.We used repeated measure ANOVA analysis, the

results showed that the means of the reaction time offour tests is significantly different F(3,126)¼59.831,Po0.001, Z2

¼0.588, further analysis revealed that thefirst time test’s reaction time is significantly shorter thanthe second, third and fourth tests, and the second testreaction time is significantly shorter than the third andfourth tests, and the third test reaction time is shorterthan the fourth one; there is no salient difference betweenthese two groups F(1,42)¼0.241, P40.05, Z2

¼0.006, theinteraction between groups and different time points isinsignificant F(3,126)¼0.186, P40.05, Z2

¼0.004.The changes for the mean reaction time (RT) and

accuracy (ACC) rate of verbal working memory of theparticipants of the bed rest group and the control groupare shown graphically in Figs. 1 and 2.

The mean reaction time (RT) and accuracy (ACC) rateof spatial working memory of the participants of the bedrest group and the control group are presented in Table 2.

We used repeated measure ANOVA analysis, theresults showed that the means of the accuracy of fourtests is significantly different F(3126)¼13.685, Po0.001,Z2¼0.246, further analysis revealed that the first time

test’s accuracy is significantly lower than the third andfourth tests, the third test’s accuracy is lower than thefourth one; there is no salient difference between thesetwo groups F(1,42)¼0.901, P40.05, Z2

¼0.021, the inter-action between groups and different time points is insig-nificant F(3,126)¼0.196, P40.05, Z2

¼0.005.We used repeated measure ANOVA analysis, the

results showed that the means of the four reaction timesof tests are significantly different F(3,126)¼58.206,Po0.001, Z2

¼0.581, further analysis revealed that thefirst time test’s reaction time is significantly shorter thanthe second, third and fourth tests, and the second testreaction time is significantly shorter than the third andfourth tests; there is no salient difference between thesetwo groups F(1,42)¼0.313, P40.05, Z2

¼0.007, the inter-action between groups and different time points is insig-nificant F(3,126)¼0.938, P40.05, Z2

¼0.022.The changes for the mean reaction time (RT) and

accuracy (ACC) rate of spatial working memory of the

SD).

Tenth day in the rest After the rest

97.3973.29 96.5174.85

749.637188.90 698.517186.59

96.1673.32 94.8776.12

731.797141.10 742.277128.99

86

88

90

92

94

96

98

1 2 3 4

The

mea

AC

C r

ate

of s

patia

lw

orki

ng m

emor

y

Time

bed rest group control group

Fig. 3. Changes for the ACC of spatial working memory of the bed rest

group and the control group. Time 1: before the rest. Time 2: 5th day in

the rest. Time 3: 10th day in the rest. Time 4: after the rest.

0

200

400

600

800

1000

1200

1 2 3 4

The

mea

RT

rat

e of

spa

tial

wor

king

mem

ory

Time

bed rest group control group

Fig. 4. Changes for the RT of spatial working memory of the bed rest

group and the control group. Time 1: before the rest. Time 2: 5th day in

the rest. Time 3: 10th day in the rest. Time 4: after the rest.

X. Zhao et al. / Acta Astronautica 69 (2011) 969–974 973

participants of the bed rest group and the control groupare shown graphically in Figs. 3 and 4.

We found that the anxiety scores on the fifth day in therest ranged from 24 to 40 and 24 to 35 on the tenth day inthe rest, and in the rest, all participants’ scores were under45 (in general BAI total scores Z45 is considered clinicalanxiety); meanwhile, the depression scores on the fifth dayin the rest ranged from 0 to 7 and 0 to 6 on the tenth day inthe rest, all participants scores were under 10 (in general BDItotal scores fall in 10–19 were judged as mild depression,20–29 moderate depression and 30–51 heavy depression).

4. Discussion

Our research firstly used large-scale female participants,exploring the working memory abilities’ change trendsbefore, during and after the �61 bed rest simulated weight-less conditions. The results demonstrated that verbal andspatial working memory scores have improved with the testsrepetition of both the bed rest group and the control group,which may have something to do with practice effectbecause the intervals between these four test times wererelatively short. Moreover, early research revealed that work-ing memory is plastic to some extent [29–30]. Thus, differenttest time scores’ improvement may have relations withcognitive functions. This research found that there was nosignificant difference among these four test time points, andthe change trends of the bed rest group were consistent withthe control group, which demonstrated that individuals’

working memory abilities have not been impaired in the�61 head-down simulated weightless conditions, which wasconsistent with earlier research results [10–12].

However, Eddy et al. and Manzey et al. found that in thereal space flight, individual’s cognitive abilities have beendamaged [8,9]. According to our research and analysis ofthe relevant earlier research, we concluded that in the realspace flight, individual’s cognitive functions would bedamaged [8,9], while be well in the simulated weightless-ness [10–12]. As discussed above, the inconsistency resultsmay be affected by a third variance. We considered thatemotion may play an essential role in the real space flights’cognitive performance. Hence, our research measured par-ticipants’ anxiety and depression levels using Beck AnxietyInventory and Beck Depression Inventory simultaneously.Our results revealed that �61 head-down bed rest condi-tions did not evoke clinically significant anxiety anddepression. Thus, the fact that individuals’ cognitive func-tions have not been impaired in the �61 head-downsimulated weightless conditions probably has somethingto do with the failure of evoking clinical anxiety anddepression. In the real space flight environment, the cogni-tive function impairment has been detected [8,9]. One ofthe causes of the impairment is probably the specialenvironment of real space flight, such as faster day andnight changes, social isolation, limited narrow space,reduced or excessive sensory input, cumulative radiation,toxic substances pollution and potentially threatening andother stressful factors, which can evoke highly an indivi-duals’ depression and anxiety. Those negative emotions candistract and take up more attention sources, which in turnsconsume limited working memory resources [31–33]. Yet,this kind of special space environment is hard to simulatein the bed rest experiment on the ground. Further researchshould detect the working memory and emotion trendssimultaneously in the real space flight, which can combinethe present research and explore the influence betweenemotion and fundamental cognition. Moreover, we willconsider ERPs, fMRI and so forth technologies to detectthe weightlessness itself within redistribution of bloodvolume, which might affect cognitive function.

5. Conclusion

In 15 days �61 head-down bed rest simulated weightlessconditions, the trends of working memory have no significantdifference with the control group before and after the restand during the rest on the fifth and tenth days. The impair-ment of working memory has not been detected in the bedrest conditions. Participants have shown no clinical anxietyand depression feelings during the bed rest. Compared to thereal space environment, individuals’ cognitive functions havenot been damaged under the simulating weightlessness,which probably has something to do with the failure ofevoking highly anxiety and depression.

Acknowledgment

This work was supported by the National BasicResearch Program of China (2011CB711000) and Program

X. Zhao et al. / Acta Astronautica 69 (2011) 969–974974

for Changjiang Scholars and Innovative Research Team inUniversity (IRT0710).

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