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The State Scientific Center of the Russian Federation –IBMP of the Russian Academy of Sciences performed the Mars 500 project at the IBMP in Moscow, which consisted of three isolation studies with six crewmembers each: a 14-day pilot study (completed in November 2007), a 105-day pilot study (completed in July 2009), and the main 520-day study simulating a mission to Mars (completed in November 2011), which is the focus of this manuscript. The high fidelity of the simulation to actual spaceflight was reflected in the following features of the experiment: (i) a multinational crew of N = 6 healthy adult male volunteers selected by the Russian Federation (N = 3), the European Space Agency (N = 2), and the China National Space Administration (N = 1), who were trained together and who were similar in age (average age at hatch closing 32 years, range 27–38), careers, and education (e.g., engineers, physicians, military backgrounds) to astronauts/cosmonauts living on the ISS; (ii) 520 consecutive days of confinement (3 June 2010 to 4 November 2011) in a 550 m3 pressurized facility with a volume and configuration comparable to a spacecraft with interconnected habitable modules; (iii) facility modules equipped with life support systems and an artificial atmospheric environment at normal barometric pressure; (iv) activities that simulated aspects of the International Space Station with daily maintenance work, scientific experiments, and exercise; (v) isolation from Earth's daily environmental light-dark cycles, temperatures and seasonal conditions; (vi) a realistic Mars flight simulation based in orbital mechanics and under the direction of mission controllers, with a 30-day Mars orbiting phase (between mission days 244 and 273) and 3 of the 6 crewmembers simulating egresses on the Martian surface (between mission days 257 and 265); (vii) work throughout the 520-day mission that included both routine and simulated emergency events; (viii) changes in communication modes and time delays between mission days 54 and 470 that would occur in transit to and from Mars; (ix) limited consumable resources (food and water); and (x) the crew awareness of frequent publicity of the mission by media and the public. Thus, Mars 520 had many essential features of an isolated and confined environment (ICE) that had the fidelity necessary to study behavioral and psychological reactions to prolonged space flight. The crew lived on a 5-day work cycle, with two days off, except for simulation of special situations (e.g., emergencies). For the whole mission operations were organized around 24-h clock time. A typical workday would start with personal hygiene and breakfast at 8:00 followed by operative work (including facility inspection), operative meetings, and the preparation of scientific experiments. After lunch (served between 13:30 and 14:30), the crews performed the scientific experiments and exercised until supper was served at ca. 19:30. The rest of the evening could be used for personal time. A total of 91 experiments in the areas of physiology (N = 20), psychology (N = 21), biochemistry, immunology, and biology (N = 34), microbiology (N = 8), and operations and technology (N = 8) were performed during the 520 days of the mission. Not all of the experiments required the crew's active participation. Sampling frequency differed between experiments and ranged from three times during the mission to continuously throughout the mission, with most of the studies sampling data on a regular but discontinuous basis (e.g., once every month). The study was approved by the Institutional Review Board of the University of Pennsylvania, Philadelphia, Pennsylvania, USA. Prior to the start of the study, all subjects signed written informed consent forms. They were compensated for their participation in the study, and they were free to discontinue the study at any time. The crewmembers revealed their identities before, during and after the simulation. To ensure confidentiality in this manuscript, results were de-identified (i.e., crewmembers were randomly assigned English alphabetic letters a–f) and no data were reported relative to crewmembers' nationalities, ages, professions, or roles in the mission.
2.2 Experimental procedures and measurements: All instructions and subjective rating scales were translated and displayed in Russian for the three Russian participants, and in English for the non-Russian participants. 2.2.1.1 Social Desirability Scale 17 (SDS-17): Once during the two weeks prior to hatch closing, each crewmember filled out the Social Desirability Scale 17 (SDS-17), which measured social desirability bias [8]. The latter is defined as “distorting one's self-presentation to make a favorable impression upon others.” Social desirability may represent, among others, internalization of cultural values, the expression of ongoing personality traits, or an overly favorable self-evaluation [9]. The SDS-17 is composed of 16 true–false items (one item was dropped from the final scale), six of them reverse keyed. A score of 16 represents maximal distortion of one's self representation to make a favorable impression. 2.2.1.2 Visual Analog Scales (VAS): Immediately prior to and/or following each PVT-B test bout, crewmembers filled out several computerized questionnaires and rating scales. Crewmembers indicated their current status on 100 mm visual analogue scales (VAS) with the following binary anchors: happy – unhappy; healthy – sick; energetic – physically exhausted; mentally sharp – mentally fatigued; not stressed – very stressed; fresh/ready to go – tired; good sleep quality – poor sleep quality (morning only); and high workload – low workload (evening only). 2.2.1.3 Profile of Mood States - Short Form (POMS-SF): Crewmembers completed the POMS-SF [10], [11] in the morning once each week. POMS-SF is a measure of psychological distress in a variety of healthy, physically ill, and psychiatric populations. It consists of a list of 37 adjectives. Crewmembers had to indicate the degree to which each adjective described themselves at the moment they took the test using a 5-point Likert format. Standard scoring of the POMS yields a global distress score referred to as Total Mood Disturbance as well as scores for six subscales: Fatigue-Inertia, Vigor-Activity, Tension-Anxiety, Depression-Dejection, Anger-Hostility, and Confusion-Bewilderment. 2.2.1.4 Beck Depression Inventory (BDI-II): Once a week in the evening, crewmembers completed the modified Beck Depression Inventory (BDI-II) [12], a 21-question multiple-choice self-report inventory for measuring the severity of depression. The BDI-II was scored by summing the highest ratings for each item. Each item is rated on a 4-point scale ranging from 0 to 3, and the total scores can range from 0 to 63. The BDI-II can be separated in an 8-item affective subscale (pessimism, past failures, guilt feelings, punishment feelings, self-dislike, self-criticalness, suicidal thoughts or wishes, and worthlessness) and a 13-item somatic subscale (sadness, loss of pleasure, crying, agitation, loss of interest, indecisiveness, loss of energy, change in sleep patterns, irritability, change in appetite, concentration difficulties, tiredness and/or fatigue, and loss of interest in sex). Crewmembers were asked to indicate how they felt during the past week. The question on suicidal thoughts was removed from the BDI-II for reasons of cultural sensitivity, reducing the number of items to 20 and the maximum score from 63 to 60. 2.2.1.5 Conflict Questionnaire (CQ): Weekly, in the evening, crewmembers filled out a brief conflict questionnaire developed for the study. They were asked to indicate, both currently and in the last 7 days, whether they had a conflict with either another crewmember or mission controllers. If they indicated a conflict had occurred, they were to indicate whether or not it was resolved. They did not have to indicate the identity of the person(s) with whom they indicated they had a conflict. Data acquisition for psychological measures resulted in 100% completed tests (i.e., N = 6 for SDS-17; N = 444 for BDI-II; POMS-SF;CQ; VAS for workload and sleep quality ratings; and N = 888 for VAS scales for unhappiness, physical exhaustion, mental fatigue, stress, tiredness). 2.2.1.6 Post-mission debrief interviews: During individual crewmember's debrief interviews the second day post-mission confinement, crewmembers were asked to name the two crewmembers with whom they communicated most frequently.
2.2.2.1 Actigraphy: Actigraphy is a reliable, non-invasive method to validly assess rest-activity cycles [13]. Throughout the 520-day simulated mission to Mars, each crewmember continuously wore a wristwatch size actigraph (Actiwatch Spectrum, Philips/Respironics) on the wrist of the non-dominant arm. The device measured both average white light intensity (illuminance in Lux) and a calibrated activity level from movement-induced accelerations of the wrist. It also displayed clock time. In the Mars 520-day study actigraphs recorded one activity and one illuminance value per minute. A validated algorithm [14] was used to automatically classify 1-min actigraphy epochs into active wake, sleep, or waking rest (Respironics Actiware, Version 5.59.0015, standard settings). In cases of obvious misclassification, the automatic scoring was corrected manually (less than 2.8% of the automatic scoring was corrected this way). Epochs with off-wrist or missing data (due to data downloads or equipment failure) were classified accordingly. Overall, 4,396,333 min (73,272 h or 98.0%) of valid actigraphy data were obtained from the 6 crewmembers while they lived in the facility throughout the Mars 520-day study. For statistical analyses off-wrist or missing actigraphy epochs were imputed with averages of non-missing epochs calculated for each crewmember, each mission quarter, and each of the 1440 min of the day. 2.2.2.2 Psychomotor Vigilance Test (PVT-B): Once per week, each crewmember performed a 3-minute version of the Psychomotor Vigilance Test on a calibrated laptop computer (Pulsar Informatics, Inc.) to assess the effects of potential changes in sleep-wake behavior. The PVT-B measures vigilant attention by recording response time (RT) to visual stimuli that occur at random inter-stimulus intervals (ISI), and it has negligible aptitude and learning effects [15], [16]. The brief PVT (i.e., PVT-B) was validated against the standard 10-minute PVT [17] and shown to predict performance on a simulated luggage screening task [18]. Each crewmember performed the PVT-B on a different day of the week, once in the morning after waking up and once in the evening. The test required visually monitoring a red rectangular box on the computer screen, and pressing a response button as soon as a yellow stimulus counter appeared, which stopped the counter and displayed the RT in milliseconds for a 1 s period. ISIs varied randomly from 2–5 s. Data acquisition for PVT-B resulted in N = 888 completed tests, which was 100% of the data sought.
To analyze time in mission effects, mixed model ANOVAs (Proc Mixed, SAS Version 9.3, SAS Institute Inc., Cary, NC) with a random intercept for crewmembers and unstructured covariance were performed with mission quarter (MQ) as the only explanatory variable (MQ1, days 1–130; MQ2, days 131–260; MQ3, days 261–390; MQ4, days 391–520) and with the scores from the mood scales (BDI-II and POMS-SF) and visual analog scales as outcome variables. Although we could have justified many different hypotheses relative to time in mission (e.g., steadily increasing or decreasing effects, third quarter effect), we chose to keep our hypothesis as generic as possible (null hypothesis: no difference between mission quarters). This was partially driven by findings on the activity data that showed a steep decline in activity initially, a slow but steady decline during the second and third mission quarters, and a sharp rise at the end of the mission, which conformed to neither of the two above-stated hypotheses [19]. Our mixed model analyses took the clustered nature of the data into account and used all available data points based on repeated measures within subjects (N = 444 for measures sampled only in the morning or in the evening and N = 888 for measures sampled both in the morning and the evening). The models for outcomes sampled both in the morning and the evening were also controlled for administration time (morning or evening). If a type 3 test indicated a significant MQ effect (P<0.05), post-hoc tests comparing each MQ with each other MQ were performed. Post-hoc tests were Bonferroni corrected for Type I error inflation (α = 0.05/6 = 0.0083). To investigate individual differences between crewmembers, ANOVAs (Proc Mixed in SAS) were performed with crewmember as the only explanatory variable and with the scores from the mood scales (BDI-II and POMS-SF) and visual analog scales as outcome variables. Again, models with visual analog scale variables sampled twice daily were also controlled for administration time (morning or evening). If a Type 3 test indicated a significant crewmember effect (P<0.05), post-hoc tests comparing data from each crewmember with data from each of the other crewmembers were performed. Post-hoc tests were Bonferroni corrected for Type I error inflation (α = 0.05/15 = 0.0033). For ease of interpretation, all scales were transformed to a 0 to 100 range in Tables 1 and 2.
Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0093298.t001 Effects of time-in-mission on psychological measures. Table 1: Superscript number 1–4 indicate a significant difference to the respective mission quarter at α = 0.05 after Bonferroni correction of post-hoc tests. All scales were transformed to a 0 to 100 range. A score of 100 represents the maximal expression (e.g., maximal depression, maximal tension-anxiety, maximal unhappiness, etc.). Standard errors are reported in parenthesis. P-value for the main effect of mission quarter is reported in the last column. POMS: Profile of Mood States Short Form.
Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0093298.t002 Inter-individual differences in psychological measures. Table 2: Superscript letters a–f indicate a significant difference to the respective crewmember at α = 0.05 after Bonferroni correction for post-hoc tests. All scales were transformed to a 0 to 100 range. A score of 100 represents the maximal expression (e.g., maximal depression, maximal tension-anxiety, maximal unhappiness). Standard errors are reported in parenthesis. P-value for the main effect of crewmember is reported in the last column. POMS: Profile of Mood States Short Form; ICC: Intra-class Correlation;* indicates that the respective crewmember answered 0 on all items throughout the whole mission.
To investigate changes of individual differences with time in mission, graphs plotting cumulative scores of mood and visual analog scale outcomes relative to time in mission were generated for those variables with a significant (P<0.05) main effect for mission quarter. To further investigate individual differences, we calculated intra-class correlations (ICC) for each outcome measure as the ratio of between-subjects variance to the sum of the between- and within-subjects variances. The ICC is based on variance components analysis, involving the explicit separation of within-subjects variance and between-subjects variance in data derived from repeated measurements in individuals. The ICC expresses the proportion of variance in these data that is explained by systematic inter-individual variability. Stability of ICC values was interpreted using the following ranges: “slight” (0.0–0.2); “fair” (0.2–0.4); “moderate” (0.4–0.6); “substantial” (0.6–0.8); and “almost perfect” (0.8–1.0) [20]. We compared actigraphy scorings across subjects on a minute per minute basis. One minute epochs that were classified as missing or off-wrist for at least one crewmember were excluded from the analysis (86,068 min or 11.5% of the 520-day period). For each crewmember, those minutes were counted where the crewmember was either the only crewmember sleeping or the only crewmember active awake. We then calculated the cumulative time for both categories corrected for the amount of missing data (i.e., relative to the full 520-day or 12,480-h mission).
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