With the National Aeronautics and Space Administration’s (NASA) plans for long-term spaceflights in the near future, cognitive performance in spaceflight must be analyzed to ensure an optimum level of safety and performance. This paper reviews the current literature in this field. Although few deficits in basic cognition (logical reasoning, memory, sustained attention, psychomotor speed) have been found, there are significant deficits in perceptual-motor performance and divided attention. Perceptual-motor deficits appear to be related to microgravity effects on motor controls while divided attention deficits appear to be the result of stress effects. Spatial processing declines in space but compensatory actions mask deficits. More long-term studies and better pre- and post-flight controls must be used in order to draw accurate conclusions.
A Review of Human Cognitive Performance During Long-Term Spaceflight
The National Aeronautics and Space Administration (NASA) has major plans for the future of space exploration. Within the next 30 years, these include extended periods of human inhabitation on the International Space Station (ISS), the surface of the Moon, and eventually the surface of Mars (Tomko, 2005). These environments undoubtedly differ from ours here on Earth. They include a lack of gravity, danger, social isolation, confinement, and disturbances of sleep (Kansas, 1987; Monk, Buysse, Billy, Kennedy, & Willrich, 1998; Zulley, 2000). It is important to understand these environmental conditions and how they affect cognitive functioning as any deficits can compromise the performance of the astronauts and their safety while performing vital tasks.
Although a great amount of research has been conducted on the medical aspects of spaceflight, cognition has received less consideration (Benke, Koserenko, Watson, & Gerstenbrand, 1993; Fowler, Comfort, & Bock, 2000). This paper will summarize some of the findings that have been made in the areas of basic cognitive functioning (logical reasoning, memory, sustained attention, psychomotor speed), spatial processing, perceptual-motor functioning, and divided attention. Also, limitations of these studies will be presented, along with some ideas for future researchers to consider in helping humans prepare for the ISS and beyond.
Most of the studies that will be cited herein were conducted using actual astronauts as participants. Typically, the participants complete a series of cognitive tasks at several different sessions before the flight in order to set a baseline level of performance. These same tasks are then administered several times during the course of the flight in order to assess changes in the space environment. Finally, several follow-up sessions ensue post-flight to look for possible long-lasting effects on functioning.
Basic Cognitive Functioning
Basic cognitive functioning refers to some of the more traditional concepts looked at by cognitive psychologists (memory, reasoning, basic attention processes). Researchers in these areas have generally found stable performance across flight conditions (Manzey & Lorenz, 1998). In an 8-day space flight, Manzey, Lorenz, Schiewe, Finell, and Thiele (1995) used a battery of tests created by the Advisory Group for Aerospace Research and Development (AGARD) called the Standardized Tests for Research with Environmental Stressors (STRES). They found no significant effects on memory search capabilities or grammatical reasoning. A 438-day trip to space showed initial declines in the same memory search and grammatical reasoning tasks before takeoff but returned to normal upon entering space. These results were attributed to the great amount of stress and anticipation preceding the extraordinarily long mission (Manzey, Lorenz, & Poljakov, 1998). Benke et al. (1993) studied sustained attention and psychomotor speed using reaction time tasks and working memory on a six-day flight and found no significant effects.
A study conducted by Ceausu, Miasnikov, and Kozerenko (1982) did reveal some in-flight deficits. A test of basic arithmetic calculations showed a tradeoff of speed for accuracy in the first 3 days of an eight-day trip, and then an increase in both speed and accuracy beyond baseline levels by the fifth day. The initial decline appeared to be fatigue-related.
There are inherent changes in spatial processing in a weightless environment because the main reference cue used to determine position sense on Earth is gravity (Friederici & Levelt, 1990; Mitani, Horii, & Kubo, 2004). Despite this, spatial processing systems have been found to be flexible enough to use other sensory cues to compensate for the lack of gravity in order to maintain performance (Friederici & Levelt, 1990; Mitani et al., 2004). Friederici and Levelt (1990) found that participants could quickly compensate for the lack of gravity in space by using head-retinal cues in determining orientation. Other cues that can be utilized are touch, pressure, and the surrounding architectural lines (Lackner & DiZio, 1993; Benke et al., 1993).
There have been significant spatial processing deficits documented in the form of “inversion illusions” (Lackner & DiZio, 1993). These include sensations of feeling upside-down in relation to the aircraft or sensing that the entire aircraft is upside-down in relation to what is perceived as the upright position. Experience tends to decrease these perceptions under normal vision. This shows the flexibility of spatial processing and the ability of visual cues to compensate for the lack of gravitational cues.
Perceptual-motor processes include perception of a moving stimulus and integration of this information with motor control systems to produce a motor response that corresponds to the movement of the stimulus. This has largely been studied using manual tracking tasks, which generally include visually following a cursor on a display monitor and then making the corresponding arm motor movements to follow or control the cursor. Evidence suggests there are significant deficits in spaceflight. Manzey et al. (1995) used a tracking task from the AGARD STRES battery of tests that involved a cursor moving horizontally across a screen. The participant controlled the cursor with a joystick and had to maneuver it into a target area in the center of the screen. Performance levels significantly decreased shortly after launch but returned to the pre-flight baseline after a few days. The levels then dropped significantly again towards the end of the flight. Manzey et al. (1998) made an interesting discovery on the 438-day spaceflight. Using the same tracking task as Manzey et al. (1995), the authors discovered a significant decrease in tracking abilities during the first three weeks in-flight and the first two weeks post-flight. The authors mention that these may be significant adaptation periods when the astronaut must make physiological and psychological changes to accommodate the vastly different environment. This idea will be addressed in subsequent paragraphs.
Not all research supports a decline in perceptual-motor abilities. Fowler, Bock, and Comfort (2000) used a tracking task that differs from the AGARD STRES protocol. Their task involved a cursor that moved on a screen in one of three different sized circular-paths and at one of three different speeds. Without vision of the hand, each participant had to follow the cursor with his or her finger using a glove with a light emitting diode (LED) that displayed a point on the screen that represented where the participant was pointing. Some significant effects were found but were influenced by outliers and discounted by the authors. This finding should be questioned, however, because the apparatus and procedure used may not be sensitive enough to detect deficits. The level of difficulty of the paradigm used by Manzey et al. (1995) and Manzey et al. (1998) would seemingly be much higher because maneuvering a joystick is less natural than using a finger.
Ceausu et al. (1982) utilized a task called “Sta-Ball.” The participant viewed a ball on an oscilloscope that swayed around in a circle. He controlled the ball with a joystick, keeping it in a target zone in the center of the circle for as long as possible. The movements of the joystick were delayed, including a condition in which the ball shifted opposite of the joystick movement. Results showed that the participant did not decline in the ability to stabilize the ball. The findings from this task are difficult to consider, however, because the paradigm and apparatus were conceived by the authors themselves and have not been investigated to determine their validity in assessing perceptual-motor functioning.
Divided attention (sometimes referred to as “higher attentional functioning”) alludes to the ability to allocate attention resources between multiple simultaneous tasks (Manzey & Lorenz, 1998). Manzey et al. (1995) introduced a dual-task situation using the tracking task described earlier with a simultaneous memory search. Two different memory loads (two and four item recalls) were used to create two different dual-task conditions. The results indicated that there were significant declines in dual-task performance shortly after takeoff and before the return of the spacecraft across both memory loads. The authors explained this phenomenon as the effect of a “heightened attentional selectivity,” described as a reduction in the amount of sensory information that can be attended to at a given time. It has its greatest effects when an individual encounters more stress or fatigue than usual. One can easily imagine the moments after takeoff and before landing as being much more stressful and tiring, considering the preparations that crew members must make.
Manzey et al. (1998) used the same procedure in the 438-day spaceflight and found similar deficits during the first month of the spaceflight. Also included in this study were measures to assess the subjective emotional balance and fatigue of the participant. An analysis showed that these measures were correlated with dual-task performance, suggesting a heightened attentional selectivity explanation for the deficits.
Much like the tracking studies, Fowler, Bock et al. (2000) used a different design to study dual-task performance. They used the aforementioned LED glove-tracking task with an embedded reaction time task. The participants each held a handle with a button in the opposite hand and responded to a change in the cursor by pressing the button as quickly as possible. The results did not indicate any changes before, during, or after flight. As with the tracking task, this dual-task assessment may not be sensitive enough to changes in performance abilities. As mentioned before, the tracking task itself is seemingly less difficult than the AGARD STRES version used by the Manzey studies. Also, the reaction time task was built into the tracking task of the Fowler, Bock et al. study. Because attention is already focused on the cursor, this may not sufficiently tax the attention processes enough to elicit an effect. The independence of the memory search tasks from the tracking tasks used by the Manzey studies appears to be more challenging on the attention processes and more sensitive to declines.
Explanation of Perceptual-Motor Functioning Effects
Explaining the deficits found in perceptual-motor functioning is a very complex task. Although deficits in dual-task performance appear to be related to fatigue and isolation-related stressors, the causes of perceptual-motor deficits are less clear. They could be the result of fatigue and isolation-related stressors, microgravity effects on perceptual functioning, microgravity effects on motor functioning, or any combination of these factors. The previously-mentioned 438-day spaceflight study by Manzey et al. (1998) suggested two interesting phenomena. First, deficits in tracking were correlated with deficits in subjective mood ratings. This suggests that fatigue and isolation-related stressors may be the cause of the tracking deficits. Second, deficits in tracking occurred during the first three weeks in space and the first two weeks after landing. The authors suggest possible phases of physiological and psychological adaptation as the cause of deficits. The idea of psychophysiological adaptation periods receives support from literature reviews of neurological changes in space (Correia, 1998; Newberg, 1994). Animal studies have shown neurological changes in the vestibular organs during spaceflight, which persisted for about 9 to 11 days after landing. This may correspond to the two-week re-adaptation phase discussed by Manzey et al. (1998), however more research is needed to draw this conclusion.
Microgravity effects on motor abilities may provide the most salient explanations for the deficits in tracking. Heuer, Manzey, Lorenz, and Sangals (2003) discovered that declines in tracking performance were correlated with declines in muscle stiffness. Also, there have been some reports of astronauts having to tense their muscles upon awakening, which shows declines in proprioception (Lackner & DiZio, 1993). Bock, Howard, Money, and Arnold (1992) found that arm movements caused participants to point higher than a presented target. They attributed this to an attempt to use the same motor control system used on Earth, while decreases in the actual weight of the arm in space cause the movements to be inaccurate. Bock (1994) found that elbow movements consistently overshot a target position while only the arm was exposed to a weightless environment. This is interesting in that it shows a decline in motor abilities independent of any microgravity effects on the central nervous system.
There are some methodological issues that limit confidence in the research findings. The main concerns with these studies include limited generalizability and an inability to accurately explain the causes of observations (Manzey and Lorenz, 1998).
Generalizability is hindered by a number of issues. One consequence of studying performance in space is the inherently small sample sizes that can be studied at one time. Small n is almost impossible to alleviate because of the limitations in the number of individuals who are able to travel to space with the current transportation technology. Almost all studies have consisted of less than 8 participants; most have considerably fewer (Casler & Cook, 1999). This is a concern due to the difficulties in making comparisons between previous studies. Differences in terminology and methodology make meta-analyses complicated. For example, Casler and Cook (1999) performed a literature review of 29 studies and found 35 different cognitive elements that were discussed. Also, the number of studies conducted in spaceflight, especially long-term flights, is very limited. The lack of long-term studies is significant because any effects on cognitive abilities should be more intense during longer stays (Casler & Cook, 1999; Manzey & Lorenz, 1998).
Another limitation to human spaceflight research has been the internal validity of the studies. A legitimate question is whether observed deficits are the result of microgravity effects on physiological functioning or the result of fatigue and isolation-related stressors (Fowler, Comfort, et al., 2000; Manzey & Lorenz, 1998). These stressors are confounded with microgravity because both are absent in baseline assessments. Some of these stressors include heightened danger, confinement, isolation from loved ones, and tensions between fellow colleagues (Kansas, 1987). Space motion sickness (SMS) is another stressor that may also hinder performance. SMS affects about 67 percent of astronauts during the first few days in-flight and the first few days upon returning to Earth. Symptoms include nausea, headache, dizziness, and sensitivity to movements (Fuji & Patten, 1992; Newberg, 1994). Sleep patterns are also disturbed in spaceflight. The average amount of hours and the amount of delta sleep was found to decline in astronauts during a 17-day mission (Monk, Buysse, Billy, Kennedy, & Willrich, 1998). This has been well replicated in the literature and is a legitimate issue for astronauts. Sleep problems can lead to decreases in vigilance and mood (Zulley, 2000).
Although there appears to be few declines in basic cognitive functions in space, significant deficits in perceptual-motor functioning and divided attention have been found. Divided attention deficits seem to be caused by the increase in fatigue and isolation-related stressors in the flight environment, which decreases the amount of cues that can be attended to at once. Perceptual-motor deficits appear to be related to the effects of microgravity on motor controls, although possible microgravity effects on the vestibular system or effects from an increase in work and isolation-related stressors cannot be overlooked. Spatial processing systems appear to change drastically, although flexible, compensatory actions help alleviate most effects on performance. All of the performance findings are relatively inconclusive because they lack replication, especially in long-term studies. There is also a problem with internal validity because the space environment contains stressors that confound results.
With plans for human beings to inhabit the moon and beyond for long periods of time, more research is necessary to ensure their safety and performance. Above all, research needs to be cross-validated so that a significant body of knowledge can be built. Because of the small sample sizes, there is an increased risk of having Type II errors and of missing potentially important findings. Standardization of measures and procedures is necessary so that researchers may be able to use meta-analysis with confidence to pool data and increase statistical power. An example of a standardized battery of tests is the AGARD STRES battery used by Manzey et al. (1995) and Manzey et al. (1998); however, there have been some questions related to the validity of these tests (Draycott & Kline, 1996). The concern is that this battery of tests reflects a limited number of factors and probably does not assess a thorough domain of cognitive processes. Furthermore, the ecological validity of these assessments is not entirely clear. Future research may be necessary to develop a standardized battery of assessment that taps a wide range of human abilities necessary to conduct spaceflight operations and also has adequate ecological validity to the real-life tasks faced by astronauts.
Longer studies are necessary to fully understand the effects of spaceflight on cognition. These studies should seek to examine the adaptation periods noted by Manzey et al. (1998). In an effort to aid this examination, Manzey and Lorenz (1998) suggested collecting psychophysiological data along with task data to determine any relationships that might exist. This could potentially indicate the presence of neurological changes previously found in animal studies (Correia, 1998; Newberg, 1994).
Control environments should be improved to create better comparisons with in-flight findings. The use of isolated and confined environments (ICE) may be one way to achieve this (Casler & Cook, 1999). These are environments that are similar to the isolated and stressful space environment, yet lack the weightlessness that is associated with it. Examples include polar camps, submarines, and hyperbaric chambers. Use of these environments could help differentiate occurrences resulting from fatigue and isolation-related stressors from occurrences resulting from microgravity and its effects on the body; however, other artifacts unique to these environments must be considered (lighting and differences in air pressure, for example). Additionally, in accordance with the Manzey et al. (1998) design, in-flight assessments should include measures to determine how much each participant is influenced by fatigue or isolation-related stressors. Relationships with performance can be identified and further analyses can then occur.
Interestingly, alertness and vigilance have been virtually ignored by researchers thus far (Casler & Cook, 1999). This especially becomes an issue because of the sleep disturbances that have been found to occur in space (Monk et al., 1998; Zulley, 2000). Studies should be conducted to determine the possible effects these sleep disturbances may have on alertness and vigilance and how their potential declines may hinder performance.
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