Impacts of spaceflight experience on human brain structure

Abstract

Spaceflight induces widespread changes in human brain morphology. It is unclear if these brain changes differ with varying mission duration or spaceflight experience history (i.e., novice or experienced, number of prior missions, time between missions). Here we addressed this issue by quantifying regional voxelwise changes in brain gray matter volume, white matter microstructure, extracellular free water (FW) distribution, and ventricular volume from pre- to post-flight in a sample of 30 astronauts. We found that longer missions were associated with greater expansion of the right lateral and third ventricles, with the majority of expansion occurring during the first 6 months in space then appearing to taper off for longer missions. Longer inter-mission intervals were associated with greater expansion of the ventricles following flight; crew with less than 3 years of time to recover between successive flights showed little to no enlargement of the lateral and third ventricles. These findings demonstrate that ventricle expansion continues with spaceflight with increasing mission duration, and inter-mission intervals less than 3 years may not allow sufficient time for the ventricles to fully recover their compensatory capacity. These findings illustrate some potential plateaus in and boundaries of human brain changes with spaceflight.

Figure 1

Pre- to post-flight ventricular volume changes associated with current mission duration. 2-week-long missions resulted in smaller increases (or in some instances decreases) in ventricle volume compared to missions lasting 6 months or longer. The duration of the current mission, in days, was used in the statistical models. To protect crewmember privacy, the plots show average values of subgroups of astronauts who completed 2-week-long (n-8), 6-month-long (n = 18) or 1-year-long missions (n = 4). Individuals gave permission for data presentation when group sizes are smaller than five. Subgroup sample sizes are indicated in gray on the leftmost plot. The lower right corner of each plot shows the ventricles overlaid on a rendered MNI standard space brain template, with the referent ventricle shown in green. Asterisks indicate results that survived the Benjamini–Hochberg FDR correction. Error bars represent standard error of the mean.

Figure 2

Pre- to post-flight FW volume changes associated with previous number of missions. Cluster located at the outer wall of the right lateral ventricle overlaid on a rendered MNI standard space template (left). The scatterplot shows that having completed a greater number of previous missions was associated with greater post-flight FW volume decreases within this cluster. The dotted line indicates the linear fit of the individual data points. Circular markers indicate the average of subgroups, the size of each subsample is indicated in gray. The 2 and 3 previous missions subgroups have been combined to protect crewmember privacy for data visualization; they were not combined for the statistical analyses. Error bars represent standard error. Results are FWE corrected at p < 0.05, two-tailed. R indicates the right hemisphere. FW, free water.

Figure 3

Pre- to post-flight ventricle volume changes associated with inter-mission intervals. Pre- to post-flight ventricle volume changes of the third ventricle were significantly associated with the number of years since the previous mission’s end. Actual inter-mission intervals were used in the statistical models. Crewmembers have been subgrouped for data visualization only to protect their privacy. The x axis shows exclusive ranges (i.e., encompassing inter-mission intervals up to, but excluding the upper bound). Markers indicate subgroup averages with subgroup sample sizes indicated in gray in the leftmost plot. Error bars represent standard error. Asterisks indicate results that survived the Benjamini–Hochberg FDR correction.