Effect of jump exercise training on long-term head-down bed rest-induced cerebral blood flow responses in arteries and veins.

NEW FINDINGS: What is the central question of this study? What is the effect of an exercise countermeasure on microgravity-induced change in cerebral blood flow? What is the main finding and its importance? Jump exercise training as a countermeasure did not modify the heterogeneous cerebral blood flow response to head-down bed rest, suggesting that this method is effective in preventing cardiovascular system deconditioning but is not good for cerebral haemodynamics. ABSTRACT: This study aimed to examine the effect of an exercise countermeasure on cerebral blood flow (CBF) response to long-term -6° head-down bed rest (HDBR) in all cerebral arteries and veins. Twenty male volunteers were exposed to HDBR for 60 days with (training group, n = 10) or without (control group, n = 10) jump exercise training as a countermeasure to spaceflight. The blood flow in the neck conduit arteries (internal carotid and vertebral artery; ICA and VA) and veins (internal jugular and vertebral veins; IJV and VV) was measured, using ultrasonography before (baseline) HDBR, on the 30th and 57th day of HDBR. Long-term HDBR causes a heterogeneous CBF response between the anterior and the posterior brain or between arteries and veins. Long-term HDBR decreased anterior cerebral arterial and venous blood flow, while posterior cerebral arterial and venous blood flows were well maintained. However, exercise jump training did not change each arterial and venous CBF responses to HDBR (control vs. training; ICA, P = 0.643; VA, P = 0.542; external carotid artery, P = 0.644; IJV, P = 0.980; VV, P = 0.999). These findings suggest that jump exercise training did not modify the heterogeneous CBF response to long-term HDBR.

Arterial and venous cerebral blood flow responses to long-term head-down bed rest in male volunteers.

NEW FINDINGS: What is the central question of this study? A heterogeneous cerebral blood flow (CBF) response in the cerebral arteries has been demonstrated in several physiological conditions, and it might be attributable to different physiological properties. However, the whole cerebral haemodynamic response to weightlessness remains unknown. What is the main finding and its importance? Long-term head-down bed rest caused a heterogeneous CBF response between the anterior and posterior cerebral arteries and between the cerebral arteries and veins. Especially, in contrast to the anterior cerebral circulation, the posterior arterial and venous CBFs were well maintained throughout weightlessness. ABSTRACT: In this study, we investigated the whole cerebral haemodynamic response to long-term head-down bed rest (HDBR). We hypothesized that long-term exposure to weightlessness influences cerebral blood flow (CBF) or CBF distribution among cerebral arteries and veins because of the different physiological roles of each cerebral vessel. To test this hypothesis, 10 male volunteers were exposed to -6 deg HDBR for 60 days. Blood flows in the internal carotid artery, external carotid artery and vertebral artery or internal jugular vein and vertebral vein were measured using ultrasonography before and on days 30 and 57 of the HDBR. The internal carotid artery blood flow was reduced on day 30 (P = 0.019) and had returned to the baseline level by day 57. In contrast, the vertebral artery blood flow remained unaltered throughout the HDBR (P = 0.626). The internal jugular vein blood flow was reduced on day 30 (P = 0.009), whereas the vertebral vein blood flow remained unaltered (P = 0.397). These findings suggest that long-term HDBR causes a heterogeneous CBF response between the anterior and posterior cerebral circulation in the both arteries and veins. The posterior arterial and venous CBFs were well maintained throughout HDBR, and these CBF responses to HDBR were different from the anterior cerebral circulation.

Internal carotid, external carotid and vertebral artery blood flow responses to 3 days of head-out dry immersion.

NEW FINDINGS: What is the central question of this study? The extent to which weightlessness associated with a fluid shift from the peripheral to the central circulation influences the blood flow in each cerebral artery remains unknown. The present study was designed to explore the effect of short-term weightlessness conditions on both anterior and posterior cerebral blood flow. What is the main finding and its importance? Short-term weightlessness affects both anterior and posterior cerebral vasculature. However, a heterogeneous cerebral blood flow response in each cerebral artery did not occur during 3 days of dry immersion. We have recently demonstrated that a heterogeneous cerebral blood flow (CBF) response in each cerebral artery might contribute to the maintenance of circulatory homeostasis in the brain. However, the extent to which weightlessness associated with a fluid shift from the peripheral to the central circulation influences the distribution of CBF in each cerebral artery remains unknown. We hypothesized that a dry immersion-induced fluid shift (weightlessness conditions) would cause a heterogeneous CBF response in each cerebral artery. During and after 3 days of dry immersion, the blood flows in the internal carotid (ICA), external carotid (ECA) and vertebral arteries (VA) were measured by Doppler ultrasonography using an 8 MHz linear transducer. Although the 3 days of dry immersion and the 2 days recovery period did not change the blood flow in each cerebral artery, the conductance in both ICA and VA decreased during dry immersion on days 2 and 3 (ICA, 2.95 and 3.23 ml min-1  mmHg-1 ; VA, 1.10 and 1.05 ml min-1  mmHg-1 , respectively) from the baseline (ICA, 3.47 ml min-1  mmHg-1 , P = 0.027; VA, 1.23 ml min-1  mmHg-1 , P = 0.004). In addition, Pearson correlation analysis demonstrated that the 3 days of dry immersion induced a decrease in cardiac output (P = 0.004) that was associated with changes in ICA (P = 0.046) and VA blood flow (P = 0.021), but not ECA blood flow (P = 0.466). These findings suggest that short exposures to weightlessness, acting via a cephalad redistribution of fluid volume and blood flow in the human body, influenced the cerebral vasculature in each cerebral artery but did not cause a heterogeneous CBF response in each cerebral artery.

Hippocampal and striatal M1 -muscarinic acetylcholine receptors are down-regulated following bilateral vestibular loss in rats.

Permanent vestibular loss has detrimental effects on the hippocampus, resulting in a disruption to spatial learning and memory, hippocampal theta rhythm and place cell field spatial coherence. Little is known about the vestibular system-related hippocampal cholinergic transmission. Since the pharmacological blockade of muscarinic acetylcholine (ACh) receptors within the hippocampus produces deficits in learning and memory, we hypothesized that ACh receptors may at least partly support the integration of vestibular input. Consequently, we examined the expression of M1 muscarinic ACh receptors in the hippocampus at 7 and 30 days following bilateral vestibular lesions (BVL) in rats using autoradiography. Animals were divided into sham (n = 12) and BVL (n = 11) groups. BVL animals received intratympanic injections of sodium arsanilate (30 mg/0.1 ml) under isoflurane anesthesia and sham animals received the same volume of saline. Analysis of the brain tissue revealed a significant reduction in the number of M1 receptors throughout the hippocampus and striatum at 30 days (P ≤ 0.0001), but not at 7 days following BVL. This suggests that the changes in learning and memory seen following vestibular damage may be in part due to the loss of M1 muscarinic receptors in the hippocampus and striatum. © 2016 Wiley Periodicals, Inc.

Effect of an acute increase in central blood volume on cerebral hemodynamics.

Systemic blood distribution is an important factor involved in regulating cerebral blood flow (CBF). However, the effect of an acute change in central blood volume (CBV) on CBF regulation remains unclear. To address our question, we sought to examine the CBF and systemic hemodynamic responses to microgravity during parabolic flight. Twelve healthy subjects were seated upright and exposed to microgravity during parabolic flight. During the brief periods of microgravity, mean arterial pressure was decreased (-26 ± 1%, P < 0.001), despite an increase in cardiac output (+21 ± 6%, P < 0.001). During microgravity, central arterial pulse pressure and estimated carotid sinus pressure increased rapidly. In addition, this increase in central arterial pulse pressure was associated with an arterial baroreflex-mediated decrease in heart rate (r = -0.888, P < 0.0001) and an increase in total vascular conductance (r = 0.711, P < 0.001). The middle cerebral artery mean blood velocity (MCA Vmean) remained unchanged throughout parabolic flight (P = 0.30). During microgravity the contribution of cardiac output to MCA Vmean was gradually reduced (P < 0.05), and its contribution was negatively correlated with an increase in total vascular conductance (r = -0.683, P < 0.0001). These findings suggest that the acute loading of the arterial and cardiopulmonary baroreceptors by increases in CBV during microgravity results in acute and marked systemic vasodilation. Furthermore, we conclude that this marked systemic vasodilation decreases the contribution of cardiac output to CBF. These findings suggest that the arterial and cardiopulmonary baroreflex-mediated peripheral vasodilation along with dynamic cerebral autoregulation counteracts a cerebral overperfusion, which otherwise would occur during acute increases in CBV.

Vestibular loss promotes procedural response during a spatial task in rats.

Declarative memory refers to a spatial strategy using numerous sources of sensory input information in which visual and vestibular inputs are assimilated in the hippocampus. In contrast, procedural memory refers to a response strategy based on motor skills and familiar gestures and involves the striatum. Even if vestibular loss impairs hippocampal activity and spatial memory, vestibular-lesioned rats remain able to find food rewards during complex spatial memory task. Since hippocampal lesions induce a switch from declarative memory to procedural memory, we hypothesize that vestibular-lesioned rats use a strategy other than that of hippocampal spatial response to complete the task and to counterbalance the loss of vestibular information. We test, in a reverse T-maze paradigm, the types of strategy vestibular-lesioned rats preferentially uses in a spatial task. We clearly demonstrate that all vestibular-lesioned rats shift to a response strategy to solve the spatial task, while control rats use spatial and response strategies equally. We conclude that the loss of vestibular informations leading to spatial learning impairments is not offset at the hippocampus level by integration process of other sense mainly visual informations; but favors a response strategy through procedural memory most likely involving the striatum, cerebellum, and motor learning.

Sensorial countermeasures for vestibular spatial disorientation.

Spatial disorientation is defined as an erroneous body orientation perceived by pilots during flights. Limits of the vestibular system provoke frequent spatial disorientation mishaps. Although vestibular spatial disorientation is experienced frequently in aviation, there is no intuitive countermeasure against spatial disorientation mishaps to date. The aim of this review is to describe the current sensorial countermeasures and to examine future leads in sensorial ergonomics for vestibular spatial disorientation. This work reviews: 1) the visual ergonomics, 2) the vestibular countermeasures, 3) the auditory displays, 4) the somatosensory countermeasures, and, finally, 5) the multisensory displays. This review emphasizes the positive aspects of auditory and somatosensory countermeasures as well as multisensory devices. Even if some aspects such as sensory conflict and motion sickness need to be assessed, these countermeasures should be taken into consideration for ergonomics work in the future. However, a recent development in aviation might offer new and better perspectives: unmanned aerial vehicles. Unmanned aerial vehicles aim to go beyond the physiological boundaries of human sensorial systems and would allow for coping with spatial disorientation and motion sickness. Even if research is necessary to improve the interaction between machines and humans, this recent development might be incredibly useful for decreasing or even stopping vestibular spatial disorientation.

Changes in cerebral oxygenation during parabolic flight.

Assessing changes in brain activity under extreme conditions like weightlessness is a desirable, but difficult undertaking. Results from previous studies report specific changes in brain activity connected to an increase or decrease in gravity forces. Nevertheless, so far it remains unclear (1) whether this is connected to a redistribution of blood volume during micro- or hypergravity and (2) whether this redistribution might account for neurocognitive alterations. This study aimed to display changes in brain oxygenation caused by altered gravity conditions during parabolic flight. It was hypothesized that an increase in gravity would be accompanied by a decrease in brain oxygenation, whereas microgravity would lead to an increase in brain oxygenation. Oxygenized and deoxygenized haemoglobin were measured using two near infrared spectroscopy (NIRS) probes on the left and right prefrontal cortex throughout ten parabolas in nine subjects. Results show a decrease of 1.44 µmol/l in oxygenized haemoglobin with the onset of hypergravity, followed by a considerable increase during microgravity (up to 5.34 µmol/l). In contrast, deoxygenized haemoglobin was not altered during the first but only during the second hypergravity phase and showed only minor changes during microgravity. Changes in oxygenized and deoxygenized haemoglobin indicate an increase in arterial flow to the brain and a decrease in venous outflow during microgravity.

Time perception in astronauts on board the International Space Station.

We perceive the environment through an elaborate mental representation based on a constant integration of sensory inputs, knowledge, and expectations. Previous studies of astronauts on board the International Space Station have shown that the mental representation of space, such as the perception of object size, distance, and depth, is altered in orbit. Because the mental representations of space and time have some overlap in neural networks, we hypothesized that perception of time would also be affected by spaceflight. Ten astronauts were tested before, during, and after a 6-8-month spaceflight. Temporal tasks included judging when one minute had passed and how long it had been since the start of the workday, lunch, docking of a vehicle, and a spacewalk. Compared to pre-flight estimates, there is a relative overestimation for the 1-min interval during the flight and a relative underestimation of intervals of hours in duration. However, the astronauts quite accurately estimated the number of days since vehicle dockings and spacewalks. Prolonged isolation in confined areas, stress related to workload, and high-performance expectations are potential factors contributing to altered time perception of daily events. However, reduced vestibular stimulations and slower motions in weightlessness, as well as constant references to their timeline and work schedule could also account for the change in the estimation of time by the astronauts in space.

Tackling Insomnia Symptoms through Vestibular Stimulation in Patients with Breast Cancer: A Perspective Paper.

Insomnia symptoms are common among patients with breast cancer (BC; 20-70%) and are predictors of cancer progression and quality of life. Studies have highlighted sleep structure modifications, including increased awakenings and reduced sleep efficiency and total sleep time. Such modifications may result from circadian rhythm alterations consistently reported in this pathology and known as carcinogenic factors, including lower melatonin levels, a flattened diurnal cortisol pattern, and lower rest-activity rhythm amplitude and robustness. Cognitive behavioral therapy and physical activity are the most commonly used non-pharmacological interventions to counter insomnia difficulties in patients with BC. However, their effects on sleep structure remain unclear. Moreover, such approaches may be difficult to implement shortly after chemotherapy. Innovatively, vestibular stimulation would be particularly suited to tackling insomnia symptoms. Indeed, recent reports have shown that vestibular stimulation could resynchronize circadian rhythms and improve deep sleep in healthy volunteers. Moreover, vestibular dysfunction has been reported following chemotherapy. This perspective paper aims to support the evidence of using galvanic vestibular stimulation to resynchronize circadian rhythms and reduce insomnia symptoms in patients with BC, with beneficial effects on quality of life and, potentially, survival.

Spaceflight alters reaction time and duration judgment of astronauts.

We report a study on astronauts aimed at characterizing duration judgment before, during, and after long-duration stays on board the International Space Station. Ten astronauts and a control group of 15 healthy (non-astronaut) participants performed a duration reproduction task and a duration production task using a visual target duration ranging from 2 to 38 s. Participants also performed a reaction time test for assessing attention. Compared to control participants and preflight responses, the astronauts' reaction time increased during spaceflight. Also, during spaceflight, time intervals were under-produced while counting aloud and under-reproduced when there was a concurrent reading task. We hypothesize that time perception during spaceflight is altered by two mechanisms: (a) an acceleration of the internal clock through the changes in vestibular inputs in microgravity, and (b) difficulties in attention and working memory when a concurrent reading task is present. Prolonged isolation in confined areas, weightlessness, stress related to workload, and high-performance expectations could account for these cognitive impairments.

Cognitive and balance functions of astronauts after spaceflight are comparable to those of individuals with bilateral vestibulopathy.

Introduction: This study compares the balance control and cognitive responses of subjects with bilateral vestibulopathy (BVP) to those of astronauts immediately after they return from long-duration spaceflight on board the International Space Station. Methods: Twenty-eight astronauts and thirty subjects with BVP performed five tests using the same procedures: sit-to-stand, walk-and-turn, tandem walk, duration judgment, and reaction time. Results: Compared to the astronauts' preflight responses, the BVP subjects' responses were impaired in all five tests. However, the BVP subjects' performance during the walk-and-turn and the tandem walk tests were comparable to the astronauts' performance on the day they returned from space. Moreover, the BVP subjects' time perception and reaction time were comparable to those of the astronauts during spaceflight. The BVP subjects performed the sit-to-stand test at a level that fell between the astronauts' performance on the day of landing and 1 day later. Discussion: These results indicate that the alterations in dynamic balance control, time perception, and reaction time that astronauts experience after spaceflight are likely driven by central vestibular adaptations. Vestibular and somatosensory training in orbit and vestibular rehabilitation after spaceflight could be effective countermeasures for mitigating these post-flight performance decrements.

Comprehensive assessment of physiological responses in women during the ESA dry immersion VIVALDI microgravity simulation.

Astronauts in microgravity experience multi-system deconditioning, impacting their inflight efficiency and inducing dysfunctions upon return to Earth gravity. To fill the sex gap of knowledge in the health impact of spaceflights, we simulate microgravity with a 5-day dry immersion in 18 healthy women (ClinicalTrials.gov Identifier: NCT05043974). Here we show that dry immersion rapidly induces a sedentarily-like metabolism shift mimicking the beginning of a metabolic syndrome with a drop in glucose tolerance, an increase in the atherogenic index of plasma, and an impaired lipid profile. Bone remodeling markers suggest a decreased bone formation coupled with an increased bone resorption. Fluid shifts and muscular unloading participate to a marked cardiovascular and sensorimotor deconditioning with decreased orthostatic tolerance, aerobic capacity, and postural balance. Collected datasets provide a comprehensive multi-systemic assessment of dry immersion effects in women and pave the way for future sex-based evaluations of countermeasures.

Differential respiratory control of the upper airway and diaphragm muscles induced by 5-HT1A receptor ligands.

BACKGROUND: Serotonin (5-HT) has a role in respiratory function and dysfunction. Although 5-HT affects respiratory drive to both phrenic and cranial motoneurons, relatively little is known about the role of 5-HT receptor subtypes in the control of upper airway muscle (UAM) respiratory activity. MATERIALS AND METHODS: Here, we performed central injections of 5-HT1A agonist (8-OHDPAT) or antagonist (WAY100635) in anesthetized rats and analyzed changes in the electromyographic activity of several UAM and other cardiorespiratory parameters. We also compared the pattern of Fos expression induced after central injection of a control solution or 8-OHDPAT. RESULTS: Results showed that 8-OHDPAT induced a robust increase in UAM activity, associated with either tachypnea under volatile anesthesia or bradypnea under liquid anesthesia. Injection of WAY100635 switched off UAM respiratory activity and led to bradypnea, suggesting a tonic excitatory role of endogenous 5-HT1A receptor activation. Co-injection of the agonist and the antagonist blocked the effects produced by each drug alone. Besides drug-induced changes in respiratory frequency, only slight increases in surface of diaphragm bursts were observed. Significant increases in Fos expression after 5-HT1A receptor activation were seen in the nucleus tractus solitarius, nucleus raphe pallidus, parapyramidal region, retrotrapezoid nucleus, lateral parabrachial, and Kölliker-Fuse nuclei. This restricted pattern of Fos expression likely identified the neural substrate responsible for the enhancement of UAM respiratory activity observed after 8-OHDPAT injection. CONCLUSIONS: These findings suggest an important role for the 5-HT1A receptors in the neural control of upper airway patency and may be relevant to counteract pharyngeal atonia during obstructive sleep apneas.

Twenty-four-hour variation of vestibular function in young and elderly adults.

Animal and human studies demonstrate anatomical and functional links between the vestibular nuclei and the circadian timing system. This promotes the hypothesis of a circadian rhythm of vestibular function. The objective of this study was to evaluate the vestibular function through the vestibulo-ocular reflex using a rotatory chair at different times of the day to assess circadian rhythmicity of vestibular function. Two identical studies evaluating temporal variation of the vestibulo-ocular reflex (VOR) were performed, the first in young adults (age: 22.4 ± 1.5 y), and the second in older adults (70.7 ± 4.7 y). The slow phase velocity and time constant of the VOR were evaluated in six separate test sessions, i.e., 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00 h. In both studies, markers of circadian rhythmicity (temperature, fatigue, and sleepiness) displayed expected usual temporal variation. In young adults, the time constant of the VOR showed variation throughout the day (p < .005), being maximum 12:25 h (06:00 h test session) before the acrophase of temperature circadian rhythm. In older adults, the slow phase velocity and time constant also displayed temporal variation (p < .05). Maximum values were recorded at 10:35 h (06:00 h test session) before the acrophase of temperature circadian rhythm. The present study demonstrates that vestibular function is not constant throughout the day. The implication of the temporal variation in vestibular system in equilibrium potentially exposes the elderly, in particular, to differential risk during the 24 h of losing balance and falling.

Vestibulo-Ocular Responses, Visual Field Dependence, and Motion Sickness in Aerobatic Pilots.

BACKGROUND: Aerobatic flight is a challenge for the vestibular system, which is likely to lead to adaptive changes in the vestibular responses of pilots. We investigated whether aerobatic pilots, as individuals who experience intense vestibular stimulation, present modifications of the vestibular-ocular reflex, motion sickness susceptibility and intensity, visual vertical estimation, and visual dependence as compared to normal volunteers.METHODS: To evaluate vestibulo-ocular reflexes, eye movements were recorded with videonystagmography while subjects were rotated on a rotatory chair with the axis of rotation being vertical (canal-ocular reflex) or inclined to 17° (otolith-ocular reflex). Motion sickness was evaluated after the rotatory test using the Graybiel diagnostic criteria. General motion sickness susceptibility and visual field dependence were also evaluated.RESULTS: Averaged data did not show significant difference in canal-ocular reflex and otolith ocular-reflex between groups. However, a significant asymmetry in otolith-driven ocular responses was found in pilots (CW 0.50 ± 1.21° · s-1 vs. CCW 1.59 ± 1.12° · s-1), though visual vertical estimation was not altered in pilots and both groups were found field independent. Pilots were generally less susceptible to motion sickness (MSSQ scores: 2.52 ± 5.59 vs. 13.5 ± 11.36) and less affected by the nauseogenic stimulation (Graybiel diagnostic criteria 3.36 ± 3.81 vs. 8.39 ± 7.01).DISCUSSION: We did not observe the expected habituation in the group of aerobatic pilots. However, there was a significant asymmetry in the otolith-driven ocular responses in pilots, but not in the controls, which may result from the asymmetry in piloting protocols.Kuldavletova O, Tanguy S, Denise P, Quarck G. Vestibulo-ocular responses, visual field dependence, and motion sickness in aerobatic pilots. Aerosp Med Hum Perform. 2020; 91(4):326-331.

Effect of vestibular stimulation using a rotatory chair in human rest/activity rhythm.

The vestibular system is responsible for sensing every angular and linear head acceleration, mainly during periods of motor activity. Previous animal and human experiments have shown biological rhythm disruptions in small rodents exposed to a hypergravity environment, but also in patients with bilateral vestibular loss compared to a control population. This raised the hypothesis of the vestibular afferent influence on circadian rhythm synchronization. The present study aimed to test the impact of vestibular stimulation induced by a rotatory chair on the rest/activity rhythm in human subjects. Thirty-four healthy adults underwent both sham (SHAM) and vestibular stimulation (STIM) sessions scheduled at 18:00 h. An off-vertical axis rotation on a rotatory chair was used to ecologically stimulate the vestibular system by head accelerations. The rest/activity rhythm was continuously registered by actigraphy. The recording started one week before the first session (BASELINE), continued in the week between the two sessions and one week after the second session. Vestibular stimulation caused a significant decrease in the average activity level in the evening following the vestibular stimulation. A significant phase advance in the rest/activity rhythm occurred two days after the 18:00 h vestibular stimulation session. Moreover, the level of motion sickness symptoms increased significantly after vestibular stimulation. The present study confirms previous results on the effect of vestibular stimulation and the role of vestibular afferents on circadian biological rhythmicity. Our results support the hypothesis of the implication of vestibular afferents as non-photic stimuli acting on circadian rhythms.

Vestibular stimulation by 2G hypergravity modifies resynchronization in temperature rhythm in rats.

Input from the light/dark (LD) cycle constitutes the primary synchronizing stimulus for the suprachiasmatic nucleus (SCN) circadian clock. However, the SCN can also be synchronized by non-photic inputs. Here, we hypothesized that the vestibular system, which detects head motion and orientation relative to gravity, may provide sensory inputs to synchronize circadian rhythmicity. We investigated the resynchronization of core temperature (Tc) circadian rhythm to a six-hour phase advance of the LD cycle (LD + 6) using hypergravity (2 G) as a vestibular stimulation in control and bilateral vestibular loss (BVL) rats. Three conditions were tested: an LD + 6 exposure alone, a series of seven 2 G pulses without LD + 6, and a series of seven one-hour 2 G pulses (once a day) following LD + 6. First, following LD + 6, sham rats exposed to 2 G pulses resynchronized earlier than BVL rats (p = 0.01), and earlier than sham rats exposed to LD + 6 alone (p = 0.002). Each 2 G pulse caused an acute drop of Tc in sham rats (-2.8 ± 0.3 °C; p < 0.001), while BVL rats remained unaffected. This confirms that the vestibular system influences chronobiological regulation and supports the hypothesis that vestibular input, like physical activity, should be considered as a potent time cue for biological rhythm synchronization, acting in synergy with the visual system.

Spatial Updating Depends on Gravity.

As we move through an environment the positions of surrounding objects relative to our body constantly change. Maintaining orientation requires spatial updating, the continuous monitoring of self-motion cues to update external locations. This ability critically depends on the integration of visual, proprioceptive, kinesthetic, and vestibular information. During weightlessness gravity no longer acts as an essential reference, creating a discrepancy between vestibular, visual and sensorimotor signals. Here, we explore the effects of repeated bouts of microgravity and hypergravity on spatial updating performance during parabolic flight. Ten healthy participants (four women, six men) took part in a parabolic flight campaign that comprised a total of 31 parabolas. Each parabola created about 20-25 s of 0 g, preceded and followed by about 20 s of hypergravity (1.8 g). Participants performed a visual-spatial updating task in seated position during 15 parabolas. The task included two updating conditions simulating virtual forward movements of different lengths (short and long), and a static condition with no movement that served as a control condition. Two trials were performed during each phase of the parabola, i.e., at 1 g before the start of the parabola, at 1.8 g during the acceleration phase of the parabola, and during 0 g. Our data demonstrate that 0 g and 1.8 g impaired pointing performance for long updating trials as indicated by increased variability of pointing errors compared to 1 g. In contrast, we found no support for any changes for short updating and static conditions, suggesting that a certain degree of task complexity is required to affect pointing errors. These findings are important for operational requirements during spaceflight because spatial updating is pivotal for navigation when vision is poor or unreliable and objects go out of sight, for example during extravehicular activities in space or the exploration of unfamiliar environments. Future studies should compare the effects on spatial updating during seated and free-floating conditions, and determine at which g-threshold decrements in spatial updating performance emerge.

A combined neuropsychological and brain imaging study of obstructive sleep apnea.

Patients with obstructive sleep apnea (OSA) show neuropsychological impairments ranging from vigilance decrements, attentional lapses and memory gaps to decreased motor coordination, but their cognitive profile, and the origin of the impairments, remain unclear. We sought to establish the neuropsychological profile of 16 newly diagnosed apneics and to highlight both their morphological and functional brain abnormalities. We used an extensive neuropsychological test battery to investigate attention and vigilance, executive functions, episodic memory and motor domains. For brain imaging, we used the optimized voxel-based morphometry procedure for the MRI data, resting-state (18)F-fluoro-2-deoxy-D-Glucose positron emission tomography ((18)FDG-PET) with correction for partial volume effects (PVEs) and voxel-based analyses. In terms of neurobehavioral performance, our patients displayed objective daytime somnolence but little impairment in memory and motor domains. Cerebral data revealed gray matter loss in the frontal and temporo-parieto-occipital cortices, the thalamus, hippocampal region, some basal ganglia and cerebellar regions, mainly in the right hemisphere. The decrease in brain metabolism was also right-lateralized, but more restricted than the gray matter density changes, and involved the precuneus, the middle and posterior cingulate gyrus, and the parieto-occipital cortex, as well as the prefrontal cortex. To conclude, despite the presence of only minor memory and motor impairments, our patients displayed significant cerebral changes in terms of both gray matter density and metabolic levels, and may have benefited from cognitive reserve and compensatory mechanisms. Thus, cerebral changes in OSA patients may precede the onset of notable neuropsychological consequences.