Spatial attention, the ability to allocate attentional resources to specific spatial locations, is crucial for effective interaction with our environment. It involves both automatic (stimulus-driven, bottom-up, exogenous) and voluntary (goal-directed, top-down, endogenous) components. Neuropsychological studies and vestibular manipulations in healthy individuals suggest that the vestibular system plays a role in spatial attention, but its specific contribution to automatic and voluntary components remains unclear. The vestibular system, located in the inner ear, encodes head position and contributes to postural balance by sensing gravity. In microgravity, the otolith organ's weight is significantly reduced or absent, providing a unique opportunity to investigate its role in spatial attention. Previous studies in microgravity, induced by parabolic or orbital flights, suggest that microgravity increases the reliance on visual information, potentially affecting attentional processes, but findings have been inconsistent. This study aimed to clarify the effects of microgravity on both automatic and voluntary components of visuospatial attention using parabolic flights.

Existing research highlights the interplay between automatic and voluntary components of spatial attention, often involving distinct neural circuits. Studies on stroke patients and individuals with vestibular disorders indicate a significant role for the vestibular system in spatial awareness and attention. However, the precise contribution of vestibular signals to the different components of visuospatial attention is not well understood. Previous research in microgravity environments has suggested altered visual processing, but the impact on attentional mechanisms remains inconclusive and controversial.

Seven healthy participants performed modified Posner cueing tasks during parabolic flights, measuring reaction times (RTs) and accuracy. The Posner task involved exogenous (peripheral, non-informative cues) and endogenous (central, informative cues) attention conditions. Participants performed the tasks in four conditions: 1 g before flight (PRE), 0 g (microgravity) during flight, 1 g during flight (1G), and 1 g after flight (POST). The study timeline is illustrated in Figure 1b. Scopolamine was administered to alleviate motion sickness. One participant experienced severe motion sickness and their data were excluded; hence data from 6 participants were analyzed. Reaction times (RTs) were measured to assess attentional engagement, disengagement, and shifting. Data analysis employed repeated-measures ANOVAs and post hoc analyses (Newman-Keuls).

Participants demonstrated a validity effect (faster RTs for valid trials) in both exogenous and endogenous tasks. However, the effect of gravity conditions differed significantly between the two tasks. In the exogenous task, microgravity (0G) significantly sped up RTs for valid trials compared to all 1g conditions (PRE, 1G, POST). Invalid trials in the 0G condition were slower than the 1g conditions (PRE, 1G, POST). The validity effect (difference between invalid and valid trial RTs) was larger in 0G than in 1G conditions. In the endogenous task, microgravity significantly sped up RTs for invalid trials compared to 1g conditions, indicating faster disengagement from invalid cues. The validity effect was smaller in 0G than in 1G conditions. Figure 2 shows the mean RTs for valid and invalid trials across all conditions, and Figure 3 illustrates the validity effect for both tasks across conditions.

The findings indicate that microgravity differentially affects automatic and voluntary attention. The enhanced validity effect in the exogenous task under microgravity suggests that reduced otolith input might enhance stimulus saliency and automatic attentional capture. Conversely, the reduced validity effect in the endogenous task suggests impaired voluntary attention maintenance. These results align with the known existence of separate neural circuits underlying automatic and voluntary attention. The authors propose that the reduced otolith input might alter the balance between egocentric and allocentric reference frames in spatial processing, potentially explaining the observed effects. The changes in attentional processing under microgravity might contribute to perceptual alterations and increased cognitive fatigue reported by astronauts.

This study demonstrates that microgravity significantly impacts the balance between automatic and voluntary visuospatial attention, enhancing reflexive attention to salient stimuli while impairing voluntary attention control. These findings are relevant for space exploration, suggesting the need for countermeasures to mitigate potential performance decrements in microgravity. Future research should investigate the interplay between egocentric and allocentric reference frames in microgravity and explore the implications for cognitive rehabilitation strategies for patients with vestibular disorders.

One limitation is the relatively small sample size (n=6). The use of parabolic flights introduces other variables, including the effects of hypergravity and the potential for motion sickness. While scopolamine was used, one participant still experienced severe motion sickness and was excluded. Future studies with larger samples and control for potential confounding factors are warranted.