## 1 Introduction
### Effects on the Neurologic System  
During the initial days of spaceflight, space motion sickness (SMS) is the most commonly encountered neurologic condition. Microgravity exposure during spaceflight commonly leads to alterations in spatial orientation and gaze stabilization (e.g., shape recognition [186], depth perception and distance [187; 188]). Postflight, impairments in object localization during pitch and roll head movements [189; 190] and fine motor control (e.g., force modulation [191], keyed pegboard completion time [192], and bimanual coordination [193]) are common. Anecdotally, astronauts also reported alterations in smell and taste sensations during their missions [27; 194; 195]. The observed impairment in olfactory function is perhaps due to elevated intracranial pressure (ICP) with increased cerebrospinal fluid outflow along the cribriform plate pathways [196]. However, to date, there have been no studies directly measuring ICP during spaceflight.  
Upon returning from spaceflight, studies have observed that astronauts experience decrements in postural and locomotor control that can increase fall risk [197]. These decrements have been observed in both standard sensorimotor testing and functional tasks. While recovery of sensorimotor function occurs rapidly following short-duration spaceflight (within the first several days after return) [192; 198], recovery after long-duration spaceflight often takes several weeks. Similar to SMS, post-flight motion sickness (PFMS) is very common and occurs soon after g-transition [30]. Deficits in dexterity, dual-tasking, and vehicle operation [199] are also commonly observed immediately after spaceflight. Therefore, short-duration astronauts are recommended to not drive automobiles for several days, and only after a sensorimotor evaluation (similar to a field sobriety test).  
Similarly to the effects seen following short-duration spaceflight, those returning from long-duration spaceflight can also experience deficits in dexterity, dual-tasking, and vehicle operation. Long-duration astronauts are recommended to not drive automobiles for several weeks, and also require a sensorimotor evaluation. While central nervous system (CNS) changes [53] associated with long-duration spaceflight are commonly observed, the resulting effects of these changes both during and immediately after spaceflight remain unclear [199]. Observed CNS changes include structural and functional alterations (e.g., upward shift of the brain within the skull [54], disrupted white matter structural connectivity [55], increased fluid volumes [56], and increased cerebral vasoconstriction [200]), as well as modifications to adaptive plasticity [53]. Adaptive reorganization is primarily observed in the sensory systems. For example, changes in functional connectivity during plantar stimulation have been observed within sensorimotor, visual, somatosensory, and vestibular networks after spaceflight [201]. In addition, functional responses to vestibular stimulation were altered after spaceflight--reducing the typical deactivation of somatosensory and visual cortices [202]. These studies provide evidence for sensory reweighting among visual, vestibular, and somatosensory inputs.  
Further research is required to fully understand the observed CNS changes. In addition, integrated countermeasures are needed for the acute effects of g-transitions on sensorimotor and vestibular function.