Effect of altered gravity on the neurobiology of fish

In vertebrates (including humans) altered gravitational environments such as weightlessness can induce malfunction of the inner ears due to a mismatch between canal and statolith afferents. This leads to an illusionary tilt because the inputs from the inner ear are not confirmed by the other sensory organs, which then results in intersensory conflict. Vertebrates in orbit therefore face severe orientation problems. In humans the intersensory conflict may additionally lead to a malaise commonly referred to as space motion sickness (SMS). After the initial days of weightlessness the orientation problems (and SMS) disappear as the brain develops a new interpretation of the available sensory data. The present contribution reviews the neurobiological responses, particularly those of fish, observed under altered gravitational states concerning behavior and neuroplastic reactivities. Investigations employing microgravity (spaceflight, parabolic aircraft flights, clinostat) and hypergravity (laboratory centrifuges as ground-based research tools) provide insights for understanding the basic phenomena, many of which remain only incompletely explained.