Gilles Clément

Translational otolith-ocular reflex during off-vertical axis rotation in humans.

Two characteristics of otolith-ocular responses - linear vestibulo-ocular reflex and vergence - were examined during constant velocity off-vertical axis rotation (OVAR) in the dark. Sixteen subjects were rotated about their longitudinal axis when tilted 30° relative to the direction of gravity. Rotational velocities were 36 and 288/s corresponding to frequencies of 0.1 and 0.8Hz, respectively. Subjects were asked to imagine stationary targets located at 0.5m, 1m, and 2m in the straight-ahead direction. Binocular eye movements were recorded in the dark using infrared videography. The modulation of horizontal slow phase velocity during OVAR was larger at 0.8Hz than at 0.1Hz, and the modulation at the high frequency was larger for the near target than for the mid and far targets. These characteristics confirm that the horizontal slow phase velocity during yaw OVAR represents a translational otolith-ocular reflex in response to acceleration along the inter-aural axis that is dependent on imagined fixation distance.

Electrical stimulation of the superior colliculus induces non-topographically organized perturbation of reaching movements in cats

Besides its well-known contribution to orienting behaviors, the superior colliculus (SC) might also play a role in controlling visually guided reaching movements. This view has been inferred from studies in monkeys showing that some tectal cells located in the deep layers are active prior to reaching movements; it was corroborated by functional imaging studies performed in humans. Likewise, our group has already demonstrated that, in cats, SC electrical stimulation can modify the trajectory of goal-directed forelimb movements without necessarily affecting the gaze position. However, as in monkeys, we could not establish any congruence between the usual retinotopic SC map and direction of evoked forelimb movements, albeit only a small portion of the collicular map was investigated. Therefore, the aim of the current study was to further ascertain the causal contribution of SC to reaching movement by exploring the whole collicular map. Our results confirmed that SC electrical stimulation deflected the trajectory of reaching movements, but this deviation was always directed downward and backward, irrespective of the location of the stimulation site. The lack of a complete map of reach directions in the SC and the absence of congruence between the direction of evoked forelimb movements and the collicular oculomotor map challenge the view that, in the cat, the SC causally contributes to coding forelimb movements. Interestingly, the very short latencies of the effect argue also against the interruption of reaching movements being driven by a disruption of the early visual processing. Our results rather suggest that the SC might contribute to the reach target selection process. Alternatively, SC stimulation might have triggered a postural adjustment anticipating an upcoming orienting reaction, leading to an interruption of the on-going reaching movement.

Neurovestibular Symptoms in Astronauts Immediately after Space Shuttle and International Space Station Missions

Objectives (1) To assess vestibular changes and related sensorimotor difficulties, especially instability of posture and gait, among astronauts immediately after they return from space and to compare the effects experienced after short- and long-duration space missions. (2) To determine whether any difficulties experienced were severe enough to impair the astronauts’ ability to leave the spacecraft in the event of an emergency. Study Design Prospective cohort study. Setting National Aeronautics and Space Administration’s Kennedy Space Center and Johnson Space Center. Subjects and Methods Fourteen crewmembers of 3 Space Shuttle missions that lasted about 1 week and 18 crewmembers of 8 International Space Station missions that lasted about 6 months were given brief vestibular examinations 1 to 5 hours after landing. These examinations focused on the presence of vestibular and motor coordination difficulties, as well as motion sickness and motion sensations. Standardized tests included the observation of abnormal eye movements, finger-to-nose pointing, standing up from a seated position, postural stability, and tandem gait. Results Unsteady walking and postural instabilities were observed after short- and long-duration missions. Motion sickness symptoms were observed after long-duration missions but not after short-duration missions. The symptom most frequently reported by the astronauts was an exaggerated perceived motion associated with sudden head movements during reentry and after landing. Conclusion The severity of the observed abnormalities would limit the ability of crewmembers during the first 5 hours after landing and increase the time required to leave the spacecraft during this period.

Chapter 59 – Space Physiology

Publisher Summary nThis chapter discusses space physiology, describing space motion sickness as a special form of motion sickness. It include such symptoms as depressed appetite, a nonspecific malaise, gastrointestinal discomfort, nausea, and vomiting. Many astronauts returning to Earth after long-duration stays on board the International Space Station now experience symptom recurrence at landing. The severity of the symptoms and the functional recovery after the flight seem to be directly proportional to the time on orbit. A sensory conflict theory of motion sickness postulates that motion sickness occurs when patterns of sensory inputs to the brain are markedly re-arranged, at variance with each other, or differ substantially from expectations of the stimulus relationships in a given environment. The extent of orthostatic intolerance postflight is variable and depends on the duration of the flight, individual differences in cardiovascular function among the astronauts, and the elapsed time after landing and method of postflight testing. Astronauts in orbit experience a headward shift of fluid at the onset of weightlessness resulting in facial puffiness and distended neck veins.