A new study published in Frontiers in Neural Circuits is the first to analyze the structural connectivity changes that happen in the brain after long-duration spaceflight.
The results show significant microstructural changes in several white matter tracts such as the sensorimotor tracts. The study can form a basis for future research into the full scope of brain changes during human space exploration.
Our brain can change and adapt in structure and function throughout our lives. As human exploration of space reaches new horizons, understanding the effects of spaceflight on human brains is crucial. Previous research has shown that spaceflight has the potential to alter both the shape and function of an adult brain.
Through a collaborative project between the European Space Agency (ESA) and Roscosmos, a team of international researchers, led by Dr. Floris Wuyts of the University of Antwerp, have been studying the brains of humans traveling to space.
Wuyts and his colleagues have, for the first time, investigated structural changes in the brain after spaceflight at the level of deep-brain white matter tracts.
White matter refers to the parts of the brain that are responsible for communication between gray matter and the body and between various gray matter regions. In short, white matter is the channel of communication of the brain and gray matter is where information processing is done.
The learned brain
To study brain structure and function after spaceflight, the researchers used a brain imaging technique called fiber tractography.
“Fiber tractography gives a sort of wiring scheme of the brain. Our study is the first to use this specific method to detect changes in brain structure after spaceflight,” explained Wuyts.
Wuyts and his team acquired diffusion MRI (dMRI) scans of 12 male cosmonauts before and right after their spaceflights. They also collected eight follow-up scans, seven months after spaceflight. The cosmonauts all engaged in long-duration missions of an average length of 172 days.
The researchers found proof of the concept of ‘the learned brain’; in other words, the level of neuroplasticity the brain has to adapt to spaceflight. “We found changes in the neural connections between several motor areas of the brain,” said first author Dr. Andrei Doroshin, of Drexel University. “Motor areas are brain centers where commands for movements are initiated. In weightlessness, an astronaut needs to adapt his or her movement strategies drastically, compared to Earth. Our study shows that their brain is rewired, so to speak.”
The follow-up scans revealed that after seven months of returning to Earth, these changes were still visible.
“From previous studies, we know that these motor areas show signs of adaptation after spaceflight. Now, we have a first indication that it is also reflected at the level of connections between those regions,” continued Wuyts.
The authors also find an explanation for anatomical brain shifts observed after spaceflight.
“We initially thought to have detected changes in the corpus callosum, which is the central highway connecting both hemispheres of the brain,” explained Wuyts. The corpus callosum borders the brain ventricles, a communicating network of chambers filled with fluid, which expand because of spaceflight.
“The structural changes we initially found in the corpus callosum are actually caused by the dilation of the ventricles that induce anatomical shifts of the adjacent neural tissue,” said Wuyts. “Where initially it was thought that there are real structural changes in the brain, we only observe shape changes. This puts the findings in a different perspective.”
The future of spaceflight research
The study illustrates a need for understanding how spaceflight affects our body, specifically via long-term research on the effects on the human brain. Current countermeasures exist for muscle and bone loss, such as exercising for a minimum of two hours a day. Future research may provide evidence that countermeasures are necessary for the brain.
“These findings give us additional pieces of the entire puzzle. Since this research is so pioneering, we don’t know how the whole puzzle will look yet. These results contribute to our overall understanding of what’s going on in the brains of space travelers. It is crucial to maintain this line of research, looking for spaceflight induced brain changes from different perspectives and using different techniques,” concluded Wuyts.
There are well-documented changes in human sensorimotor performance following spaceflight, including post-flight declines in locomotion, balance, and fine motor control (Thornton and Rummel, 1977; Paloski et al., 1992, 1994; Reschke et al., 1994a,b, 1998; Black et al., 1995; McDonald et al., 1996; Bloomberg et al., 1997; Layne et al., 1997, 1998; Newman et al., 1997; Bock et al., 2003; Campbell et al., 2005; Rafiq et al., 2006).
However, the effects of spaceflight on human cognition and other motor behaviors have not been as thoroughly investigated (Strangman et al., 2014; Garrett-Bakelman et al., 2019). Performance of whole-body postural control typically returns to pre-flight levels within approximately 2 weeks of return to Earth (Wood et al., 2015; Ozdemir et al., 2018), however, it is not clear whether the same is true for other sensorimotor or cognitive behaviors.
Vestibular inputs are altered during spaceflight; in particular, otolith (small structures within the inner ear that senses linear accelerations and tilt) signaling of head tilt, which rely upon gravity, is absent and is likely down-weighted (Paloski et al., 1992, 1994; Reschke et al., 1994a,b, 1998; Black et al., 1995, 1999; Clément et al., 2020). The central nervous system adapts to altered vestibular inputs in-flight due to microgravity with as little as 2 weeks spent in spaceflight (Layne et al., 1998). Upon return to Earth, however, these adaptive changes may become maladaptive, resulting in difficulties with whole-body motor control.
Post-flight impairments have been reported during locomotion (McDonald et al., 1996; Bloomberg et al., 1997; Layne et al., 1998; Miller et al., 2018; Mulavara et al., 2018), balance (Paloski et al., 1992, 1994; Reschke et al., 1994a,b, 1998; Black et al., 1995, 1999), jumping (Newman et al., 1997), obstacle navigation (Mulavara et al., 2010; Bloomberg et al., 2015), and eye-head coordination (Reschke et al., 2017). Sensorimotor re-adaptation to the Earth’s gravity occurs in the weeks following return, with performance returning to pre-flight levels with about 6 days on a variety of functional tasks (Miller et al., 2018) to 15 days for the functional mobility test (FMT; Mulavara et al., 2010).
In-flight changes in performance of fine motor tasks have also been identified. For instance, astronauts maintained their manual dexterity while performing survival surgery on rats during a Neurolab shuttle mission. However, there was a significant increase in operative time, in some cases taking 1.5–2 times longer than on Earth (Campbell et al., 2005), which may be indicative of a speed-accuracy trade-off or slowing down to avoid compromising in accuracy.
Indices of movement variability, reaction time, and movement duration also increased on a hand pointing task executed without visual feedback during Neurolab shuttle missions (Bock et al., 2003), in addition to a significant increase in movement amplitude shortly following landing. During Skylab missions, impairments in reaching and grasping were also documented (Thornton and Rummel, 1977). Additionally, decreases in both force regulation and performance quality while tying surgical knots were identified in the low gravity phase of parabolic flight (Rafiq et al., 2006).
Recently, it has been shown that long duration spaceflight results in decreases in fine motor control, as seen by an increase in completion time on a grooved pegboard test (Mulavara et al., 2018). Here we evaluate bimanual motor coordination pre- and post-flight using the bimanual Purdue Pegboard Test, in which astronauts were asked to place small metal pegs into fitted holes as quickly as possible using both hands simultaneously (Tiffin and Asher, 1948).
Several spaceflight stressors have the potential to impact cognition in-flight, including the effects sleep loss, motion sickness, and social isolation. Astronauts anecdotally report so-called “space fog,” which includes attention lapses, short term memory problems, confusion, and psychomotor problems (Clément et al., 2020).
To date, empirical evidence for cognitive effects of spaceflight have been equivocal (c.f. Strangman et al., 2014). One study showed crewmembers were better able to mentally rotate the visual image of their environment as their exposure to microgravity increased, yet also a decreased ability in spatial orientation of written letters during the first 5 days in-flight (Clement et al., 1987). The authors posited that the absence of a gravitational reference field (e.g., the ground) may affect the central representation of movements.
Manzey et al. (1995) and Manzey and Lorenz (1998) have also reported declines in crewmembers ability to perform simultaneous cognitive and motor dual-tasking in-flight. The authors suggested that an increased demand for cognitive control of movement in microgravity may interfere with simultaneous cognitive task performance. Deficits in dual-tasking was further supported by Bock et al. (2010), who found higher tracking error inflight in both the single and dual-task conditions as well as higher dual-task cost in a rhythm production reaction-time task compared to a visuospatial reaction-time task and a choice reaction-time task.
The authors suggested that there may a scarcity of neural resources required for complex motor programming due to sensorimotor adaptation to microgravity. Dual-tasking deficits in astronauts post-flight were also identified when astronauts performed a tracking task whilst responding and entering numerical codes with their non-dominant hand (Moore et al., 2019).
In addition, NASA’s “Twins Study” also showed increased risk-taking on a cognitive task throughout spaceflight, as well as decreased accuracy in a visual object learning task, decreased abstract shape matching, and decreased cognitive speed for all measures on a subset of tasks from the Penn Computer Neurocognitive Battery, except for the digit symbol substitution task post-flight (Garrett-Bakelman et al., 2019).
However, the Twins Study only tested one astronaut in-flight and compared performance to that of their Earth-bound twin, and other previous investigations similarly were case studies (Manzey et al., 1995; Manzey and Lorenz, 1998) or had small sample sizes (n = 3; Bock et al., 2010).
Thus it remains unclear whether or how cognitive function is impacted by spaceflight. Spaceflight analog environments, such as extended isolation (Stahn et al., 2019) have been shown to reduce spatial cognition. Moreover, head-down tilt bed rest (HDBR) analogs has been shown to result in an overall cognitive slowing (Basner et al., 2021). Moreover, spatial orientation and distance estimation are impaired during both hypergravity and microgravity phases of parabolic flight (Clément et al., 2016). Here we also evaluated performance on a range of cognitive assessments pre- and post-flight.
As NASA’s goals shift from the International Space Station (ISS) to the Moon and Mars, mission duration will increase. It is imperative that we understand how other factors may interact with microgravity to affect sensorimotor and cognitive function, particularly flight duration, age and sex.
Exploration missions to Mars’ surface are estimated to take around 30 months in total (Clément et al., 2020), making it important to understand how mission duration interacts with changes in sensorimotor and cognitive function with spaceflight. Associations between mission duration and the magnitude of brain structural changes, free water shifts, and ventricular enlargement have been previously reported (Roberts et al., 2015; Alperin et al., 2017; Hupfeld et al., 2020a).
There is also evidence that longer flight duration results in prolonged brain and behavior recovery profiles (Bryanov et al., 1976; Hupfeld et al., 2020a). Flight duration may also be correlated with the magnitude of sensorimotor and cognitive changes that occur with spaceflight, or that effects of flight duration may be due to an interaction of microgravity with isolation and confinement hazards.
As age increases, sensorimotor adaptability declines (Seidler et al., 2010; Anguera et al., 2011). Astronaut training requires years to complete, and the average age for an astronaut at the onset of their first mission is 39.8 (±5.28) years (Smith et al., 2020).
It is important to consider the impact of age on behavioral changes with spaceflight; thus we include age as a covariate in all statistical models for exploratory purposes. Sex differences in the effects of microgravity have rarely been considered [as the Astronaut Corps has been historically male (Reschke et al., 2014)], but with the future Artemis program having equal representation of the sexes, it is important to identify any sex related differences. While our sample size of 15 astronauts is not large enough for a well-powered investigation of sex effects, we include sex as a model covariate for exploratory purposes.
Here we aimed to investigate how spaceflight impacts sensorimotor and cognitive performance. We included several assessments of whole-body sensorimotor behaviors including the Functional Mobility Test (FMT; Mulavara et al., 2010) in which astronauts completed a short obstacle course and the Sensory Organization Test-5 (SOT-5; Reschke et al., 2009; Wood et al., 2012), which was implemented using computerized dynamic posturography and required astronauts to maintain upright posture.
We also assessed fine motor control using the bimanual Purdue Pegboard Test (Tiffin and Asher, 1948). Finally, we assayed multiple aspects of cognitive function including processing speed, mental rotation, spatial working memory and cognitive-motor dual-tasking. Most tests were administered pre- and post-flight, with a subset of the test battery performed on three occasions on the ISS. Follow-up performance measurements were obtained over 6 months post-flight to characterize the trajectory of re-adaptation following return to Earth.
Based on prior investigations of behavioral changes with spaceflight (Mulavara et al., 2010; Wood et al., 2015), we hypothesized that performance on all sensorimotor tasks would decline from pre- to post-flight, and then recover to pre-flight levels within 1 month following return to Earth. We further hypothesized that performance on cognitive tasks would decrease from pre- to post-flight, with a similar recovery profile as the sensorimotor tasks.
Finally, we hypothesized that astronauts’ sensorimotor and cognitive (i.e., dual-tasking and spatial working memory) performance would be disrupted following their arrival to the ISS, and would then resolve throughout the flight as they adapted to the microgravity environment.
reference link : https://www.frontiersin.org/articles/10.3389/fncir.2021.723504/full
More information: Frontiers in Neural Circuits (2022). DOI: 10.3389/fncir.2022.815838 , www.frontiersin.org/articles/1 … cir.2022.815838/full