Vision for Perception and Vision for Action in Space Travelers
Microgravity challenges the human body and brain in many different ways. One of the most evident challenges is the altered functioning of sensory systems, including absent support afference, degraded proprioceptive feedback and unreliable vestibular input. The result is a conflict between the input from different channels, which is resolved through sensory realignment and reweighting (Horak et al., 1990; Block and Bastian, 2012). Vision remains informative and gains priority in the planning and feedback for locomotion and manual actions during spaceflight (Berger et al., 1997). The greater role of visual input in sensorimotor coordination is usually analyzed in the context of compensatory strategies or sensory reweighting and in comparison with other sensory modalities (Kornilova and Kozlovskaya, 2003; Clément, 2007). Alterations and adaptations within the visual system per se, including the transformed interplay of its subsystems, has not been the focus of previous analyses. As we demonstrate in the present paper, experimental studies of the changes in sensorimotor coordination during spaceflight or in ground-based microgravity models usually address performance in specific visually guided motor tasks rather than more basic neurocognitive mechanisms of the adaptive processes.
We suggest that the theory of two visual systems (Goodale and Milner, 1992; Rizzolatti and Matelli, 2003; Kravitz et al., 2011) is a promising framework for facilitating the understanding of sensorimotor coordination in space. Two streams, or pathways, in the neural processing of visual information, both originate in the primary visual cortex (Mishkin and Ungerleider, 1982). The ventral stream (the “what” pathway) ascends to the anterior part of the temporal lobe and provides information for visual awareness, or conscious perception (the “vision for perception” system). Meanwhile, the dorsal stream (the “where” or “how” pathway) ascends to the parietal lobe and propagates to the premotor cortex; it is considered to be the “vision for action” system since it is involved in the processing of spatial information critical for visually guided actions such as object and tool manipulation, and it is believed to be immune to visual illusions and to function independently of subjective states of consciousness (Milner and Goodale, 1995; Giese and Rizzolatti, 2015).
In this opinion paper, we briefly review the facts that are potentially relevant for predicting alterations in the ventral and dorsal pathways during the actual spaceflight and in ground-based microgravity analogs. We also discuss the idea that the dorsal and ventral streams may be affected differently during a microgravity-stimulated recalibration of sensorimotor coordination.