Astronauts return to Earth from spaceflight missions with impaired mobility and balance; recovery can last weeks postflight. This is due in large part to the altered vestibular signaling and sensory reweighting that occurs in microgravity. The neural mechanisms of spaceflight-induced vestibular changes are not well understood. Head-down-tilt bed rest (HDBR) is a common spaceflight analog environment that allows for study of body unloading, fluid shifts, and other consequences of spaceflight. Subjects in this context still show vestibular changes despite being in Earth’s gravitational environment, potentially due to sensory reweighting. Previously, we found evidence of sensory reweighting and reduced neural efficiency for vestibular processing in subjects who underwent a 70-day HDBR intervention. Here we extend this work by evaluating the impact of HDBR paired with elevated carbon dioxide (CO2) to mimic International Space Station conditions on vestibular neural processing. Eleven participants (6 males, 34 ± 8 years) completed 30 days of HDBR combined with 0.5% atmospheric CO2 (HDBR + CO2). Participants underwent six functional magnetic resonance imaging (fMRI) sessions pre-, during, and post- HDBR + CO2 while we measured brain activity in response to pneumatic skull taps (a validated method of vestibular stimulation). We also measured mobility and balance performance several times before and after the intervention. We found support for adaptive neural changes within the vestibular system during bed rest that subsequently recovered in several cortical and cerebellar regions. Further, there were multiple brain regions where greater pre- to post- deactivation was associated with reduced pre- to post- balance declines. That is, increased deactivation of certain brain regions associated with better balance post-HDBR + CO2. We also found that, compared to HDBR alone (n = 13 males; 29 ± 3 years) HDBR + CO2 is associated with greater increases in activation of multiple frontal, parietal, and temporal regions during vestibular stimulation. This suggests interactive or additive effects of bed rest and elevated CO2. Finally, we found stronger correlations between pre- to post- HDBR + CO2 brain changes and dependence on the visual system during balance for subjects who developed signs of Spaceflight-Associated Neuro-ocular Syndrome (SANS). Together, these findings have clear implications for understanding the neural mechanisms of bed rest and spaceflight-related changes in vestibular processing, as well as adaptation to altered sensory inputs.

中文翻译：

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在二氧化碳 (CO2) 浓度升高的航天模拟中前庭处理的神经相关性：一项试点研究

宇航员从太空飞行任务返回地球时，机动性和平衡性受损；飞行后恢复可以持续数周。这在很大程度上是由于在微重力环境下发生的前庭信号和感觉重新加权的改变。航天引起的前庭变化的神经机制尚不清楚。头朝下倾斜的卧床休息 (HDBR) 是一种常见的航天模拟环境，可用于研究身体卸载、流体转移和航天飞行的其他后果。尽管处于地球的重力环境中，但在这种情况下的对象仍然表现出前庭变化，这可能是由于感觉重新加权。以前，我们发现了在接受 70 天 HDBR 干预的受试者中感觉重新加权和前庭处理神经效率降低的证据。在这里，我们通过评估 HDBR 与升高的二氧化碳 (CO2) 配对以模拟国际空间站条件对前庭神经处理的影响来扩展这项工作。11 名参与者（6 名男性，34 ± 8 岁）完成了 30 天的 HDBR 与 0.5% 大气 CO2 (HDBR + CO2) 的结合。参与者在 HDBR + CO2 之前、期间和之后接受了六次功能性磁共振成像 (fMRI) 会话，同时我们测量了响应气动颅骨敲击（一种经过验证的前庭刺激方法）的大脑活动。我们还在干预前后多次测量了机动性和平衡能力。我们发现在卧床休息期间前庭系统内的适应性神经变化的支持，随后在几个皮质和小脑区域恢复。更多，存在多个大脑区域，在这些区域中，失活前后较大的区域与前后平衡下降的减少有关。也就是说，与 HDBR + CO2 后更好的平衡相关的某些大脑区域的失活增加。我们还发现，与单独的 HDBR（n = 13 名男性；29 ± 3 岁）相比，HDBR + CO2 与前庭刺激期间多个额叶、顶叶和颞叶区域的激活增加有关。这表明卧床休息和二氧化碳升高的相互作用或累加效应。最后，我们发现 HDBR + CO2 前后大脑变化与平衡期间对视觉系统的依赖之间存在更强的相关性，这些受试者出现了航天相关神经眼综合征 (SANS) 的迹象。一起，