Ocular Counter-Rolling During Centrifugation and Static Tilt

Activation of the gravity sensors in the inner ear—the otoliths—generates reflexes that act to maintain posture and gaze. Ocular counter-rolling (OCR) is an example of such a reflex. When the head is tilted to the side, the eyes rotate around the line of sight in the opposite direction (i.e., counter-rolling). While turning corners, undergoing centrifugation, or making side-to-side tilting head movements, the OCR reflex orients the eyes towards the sum of the accelerations from body movements and gravity. Deconditioning of otolith-mediated reflexes following adaptation to microgravity has been proposed as the basis of many of the postural, locomotor, and gaze control problems experienced by returning astronauts. Evidence suggests that OCR is reduced postflight in about 75% of astronauts tested; but the data are sparse, primarily due to difficulties in recording rotational eye movements. During the Neurolab mission, a short-arm human centrifuge was flown that generated sustained sideways accelerations of 0.5-G and one-G to the head and upper body. This produces OCR; and so for the first time, the responses to sustained centrifugation could be studied without the influence of Earth’s gravity on the results. This allowed us to determine the relative importance of sideways and vertical acceleration in the generation of OCR. This also provided the first test of the effects of exposure to artificial gravity in space on postflight otolith-ocular reflexes. There was little difference between the responses to centrifugation in microgravity and on Earth. In both conditions, the induced OCR was roughly proportional to the applied acceleration, with the OCR magnitude during 0.5-G centrifugation approximately 60% of that generated during one-G centrifugation. The overall mean OCR from the four payload crewmembers in response to one-G of sideways acceleration was 5.7±1.1 degree (mean and SD) on Earth. Inflight one-G centrifugation generated 5.1±0.9 degree of OCR, which was a small but significant decrease in OCR magnitude. The postflight OCR was 5.9±1.4 degree, which was not significantly different from preflight values. During both 0.5-G and one-G centrifugation in microgravity, where the head vertical gravitational component was absent, the OCR magnitude was not significantly different from that produced by an equivalent acceleration during static tilt on Earth. This suggests that the larger OCR magnitude observed during centrifugation on Earth was due to the larger body vertical linear acceleration component, which may have activated either the otoliths or the body tilt receptors. In contrast to previous studies, there was no decrease in OCR gain postflight. Our findings raise the possibility that inflight exposure to artificial gravity, in the form of intermittent one-G and 0.5-G centripetal acceleration, may have been a countermeasure to deconditioning of otolith-based orientation reflexes. 11 Ocular Counter-Rolling During Centrifugation and Static Tilt Authors Steven Moore, Bernard Cohen, Gilles Clément, Ian Curthoys, Mingjia Dai, Izumi Koizuka, Takeshi Kubo, Theodore Raphan Experiment Team Principal Investigator: Bernard Cohen1 Co-Investigators: Gilles Clément2 Steven Moore1 Ian Curthoys3 Mingjia Dai1 Izumi Koizuka4 Takeshi Kubo5 Theodore Raphan6 1Mount Sinai School of Medicine, New York City, USA 2Centre National de la Recherche Scientifique, Paris, France 3University of Sydney, Sydney, Australia 4St. Marianna University School of Medicine, Kawasaki, Japan 5Osaka University, Osaka, Japan 6Brooklyn College, New York City, USA