In order to study if proprioceptive input manipulation influences spatial representation and, consequently, can contribute to develop a "human-centered design” for future Moon & Mars habitats, we simulated a microgravitational environment in which subjects performed a visual discrimination experiment while immersed in water.
To investigate this issue, a computer-based length-comparison task was proposed under partial water immersion conditions of participants. The task consisted in comparing the longest dimension of pairs of rectangles presented on a pc screen. The rectangles were located either to the left or to the right, or above or below with respect to a central fixation cross and could be oriented along the vertical or the horizontal meridian. All possible combinations composed of different pairs were created and presented in a pseudo-random order. The task was executed under two different conditions: water immersion (the head surfaced) and outside water. We observed that immersion in water modulated vertical-horizontal comparisons. In particular, the verticality bias, i.e. the normal tendency to overestimate the vertical relative to the horizontal dimension, significantly decreased when the vertical element was located to the right of the fixation point.
The results show that proprioceptive input modulation (i.e. a decreased body weight-related proprioceptive input) influences spatial representation, inducing an underestimation of the vertical relative to the horizontal dimension. We interpret this finding hypothesizing that spatial representation varies according to proprioceptive input intensity in order to optimize information processing for possible action execution. Along this line of reasoning, a reduced proprioceptive input would require by the sensory-motor system an adaptive response in terms of force and trajectory vectors applied to possible movements directed to potential targets. In order for the system to optimally adjust to this change, also spatial representation needs to be reconfigured accordingly. Specifically, a reduced body weight would need a reduction of movement-related force/trajectory vectors in order to precisely aim for targets; a correspondent reduction of the vertical dimension –a visual cue for depth perception- of the underlying spatial representation is compatible with this interpretation, subserving the motor system with a more adaptive spatial framework to execute more precise aiming movements.
Ongoing research in collaboration with ILEWG (International Lunar Exploration Working Group), DLR (German Space Agency) and KIT (Karlsruhe Institute of Technology) is investigating walking patterns and interactions of astronauts inside and outside the habitat using the swimming pool of the Neutral Buoyancy Facility at the European Astronaut Centre and at KIT. Moreover, similar data will be collected in simulated hypogravity with a modified treadmill where subjects will be held by a wide padded belt, with the upward forces applied to the belt simulating different levels of hypogravity.
In conclusion, we believe that the above mentioned studies can contribute to the safety and performance of future Moon and Mars human missions.