Pierre-Alain Barraud

Effects of fluid ingestion on cognitive function after heat stress or exercise-induced dehydration.

This study investigated the effects of heat exposure, exercise-induced dehydration and fluid ingestion on cognitive performance. Seven healthy men, unacclimatized to heat, were kept euhydrated or were dehydrated by controlled passive exposure to heat (H, two sessions) or by treadmill exercise (E, two sessions) up to a weight loss of 2.8%. On completion of a 1-h recovery period, the subjects drank a solution containing 50 g l(-1) glucose and 1.34 g l(-1) NaCl in a volume of water corresponding to 100% of his body weight loss induced by dehydration. (H1 and E1) or levels of fluid deficit were maintained (H0, E0). In the E0, H0 and control conditions, the subject drank a solution containing the same quantity of glucose diluted in 100 ml of water. Psychological tests were administered 30 min after the dehydration phase and 2 h after fluid ingestion. Both dehydration conditions impaired cognitive abilities (i.e. perceptive discrimination, short-term memory), as well as subjective estimates of fatigue, without any relevant differences between the methods. By 3.5 h after fluid deficit, dehydration (H0 and E0) no longer had any adverse effect, although the subjects felt increasingly tired. Thus, there was no beneficial effect of fluid ingestion (H1 and E1) on the cognitive variables. However, long-term memory retrieval was impaired in both control and dehydration situations, whereas there was no decrement in performance in the fluid ingestion condition (H1, E1).

Subjective vertical and postural activity.

Three experiments were conducted to test the effect of postural information, resulting from the active control of balance, on the perception of the vertical. Subjects were required to adjust a luminous rod in two different visual contexts: in the dark or within a tilted visual frame. In these experiments, postural activity was manipulated by placing observers either in a situation of slight postural imbalance (Experiment 1) or in a situation of postural immobilization (Experiment 2). In both situations performance was compared with a control condition in which subjects were standing upright freely (Experiment 1) or sitting unconstrained (Experiment 2). Results showed no main effect of active posture or of immobilization on the visual perception of the vertical. In the third experiment, subjects were supine with their Z body axis perpendicular to the plane of the luminous rod. Thus, body orientation relative to gravity was modified and motor activity reduced. In this position, the physical vertical was perceived quite accurately in a dark environment. Moreover, in the titled frame condition, the supine body position clearly improved vertical judgements. These results are discussed in relation to the ecological theory of orientation.

Effect of Achilles tendon vibration on postural orientation

Vibration applied to the Achilles tendon is well known to induce in freely standing subjects a backward body displacement and in restrained subjects an illusory forward body tilt. The purpose of the present experiment was to evaluate the effect of Achilles tendon vibration (90Hz) on postural orientation in subjects free of equilibrium constraints. Subjects (n=12) were strapped on a backboard that could be rotated in the antero-posterior direction with the axis of rotation at the level of the ankles. They stood on a rigid horizontal floor with the soles of their feet parallel to the ground. They were initially positioned 7 degrees backward or forward or vertical and were required to adjust their body (the backboard) to the vertical orientation via a joystick. Firstly, results showed that in response to Achilles tendon vibration, subjects adjusted their body backward compared to the condition without vibration. This backward body adjustment likely cancel the appearance of an illusory forward body tilt. It was also observed that the vibratory stimulus applied to the Achilles tendon elicited in restrained standing subjects an increased EMG activity in both the gastrocnemius lateralis and the soleus muscles. Secondly, this vibration effect was more pronounced when passive displacement during the adjustment phase was congruent with the simulated elongation of calf muscles. These results indicated that the perception of body orientation is coherent with the postural response classically observed in freely standing subjects although the relationship between these two responses remains to be elucidated.

Effects of modafinil on attentional processes during 60 hours of sleep deprivation

The present study investigates the effects of modafinil (300 mg/ 24 h) versus a placebo on the performance of a visual search task during 60 h of sleep deprivation. Modafinil was administrated in doses of 100 mg three times per day during sleep deprivation. Six healthy volunteers participated in a double‐blind experiment including two experimental sessions of 7 days each. The experiment used the visual search paradigm for an ‘O’ target among ‘Q’ distractors and the reverse. The speed and accuracy in detecting the target were measured by RTs slopes (i.e. search rates) and the number of errors (i.e. error rates), respectively. Many authors attribute rapid search rates obtained for ‘Q’ targets (low RTs slopes) to parallel/ automatic processes and slow search rates obtained for ‘O’ targets (high RTs slope) to serial/ attentional processes. The results revealed an asymmetrical search pattern for the detection of ‘Q’ versus ‘O’ targets across the sleep deprivation period (i.e. parallel versus serial search, respectively). Rapid search rates for ‘Q’ targets remained unchanged between placebo and modafinil conditions during sleep deprivation. However, slow search rates for ‘O’ targets increased linearly in placebo condition, but remained at the same level as the control‐test in modafinil condition. Error rates and search rates also increased. For ‘O’ and ‘Q’ targets, the number of errors increased in the placebo condition, but remained stable in the modafinil condition. In summary, we can conclude that the administration of modafinil (300 mg/ 24 h) during sleep deprivation prevents the slowing of serial processes (attentional shifts) and the increasing of errors.

Contribution of ankle, knee, and hip joints to the perception threshold for support surface rotation

The purpose of the present experiment was to investigate the extent to which subjects can perceive, at very slow velocities, an angular rotation of the support surface about the medio-lateral axis of the ankle, knee, hip, or neck joint when visual cues are not available. Subjects were passively displaced on a slowly rotating platform at .01, .03, and .05 deg/sec. The subjects’ task was to detect movements of the platform in four different postural conditions allowing body oscillations about the ankle, knee, hip, or neck joint. In Experiment 1, subjects had to detect backward and forward rotation (pitching). In Experiment 2, they had to detect left and right rotations of the platform (rolling). In Experiment 3, subjects had to detect both backward/forward and left/right rotations of the platform, with the body fixed and the head either fixed or free to move. Overall, when the body was free to oscillate about the ankle, knee, or hip joints, a similar threshold for movement perception was observed. This threshold was lower for rolling than for pitching. Interestingly, in these postural conditions, an unconscious compensation in the direction opposite to the platform rotation was observed on most trials. The threshold for movement perception was much higher when the head was the only segment free to oscillate about the neck joint. These results suggest that, in static conditions, the otoliths are poor detectors of the direction of gravity forces. They also suggest that accurate perception of body orientation is improved when proprioceptive information can be dynamically integrated.

Contribution of somesthetic information to the perception of body orientation in the pitch dimension

This study investigated the contribution of otolithic and somesthetic inputs in the perception of body orientation when pitching at very slow velocities. In Experiment 1, the subjects’ task was to indicate their subjective postural vertical, in two different conditions of body restriction, starting from different angles of body tilt. In the “strapped” condition, subjects were attached onto a platform by means of large straps. In the “body cast” condition, subjects were completely immobilized in a depressurized system, which attenuates gravity-based somesthetic cues. Results showed that the condition of body restriction and the initial tilt largely influenced the subjective postural vertical. In Experiment 2, subjects were displaced from a vertical position and had to detect the direction of body tilts. Results showed that the threshold for the perception of body tilt was higher when subjects were immobilized in the body cast and when they were tilted backward. Experiment 3 replicated the same protocol from a supine starting position. Compared to results of Experiment 2, the threshold for the perception of body tilt decreased significantly. Overall, these data suggested that gravity-based somesthetic cues are more informative than otolithic cues for the perception of a quasi-static body orientation.

Difference in the perception of the horizon during true and simulated tilt in the absence of semicircular canal cues

Perception of tilt (somatogravic illusion) in response to sustained linear acceleration is generally attributed to the otolithic system which reflects either a translation of the head or a reorientation of the head with respect to gravity (tilt/translation ambiguity). The main aim of this study was to compare the tilt perception during prolonged static tilt and translation between 8 and 20° of tilt relative to the gravitoinertial forces (i.e., G and GIF, respectively) when the semicircular cues were no more available. An indirect measure of tilt perception was estimated by means of a visual and kinesthetic judgment of the gravitational horizon. The main results contrast with the interpretation regarding the tilt/translation ambiguity as the same orientation relative to the shear forces G for the true tilt or GIF in the centrifuge did not induce the same horizon perception. Visual adjustment and arm pointing in the centrifuge were always above the ones observed in a G environment. Part of the lowering of the judgment in the centrifuge may be related to the mechanical effect of GIF on the effectors as shown by the shift of the egocentric coordinates in the direction of GIF. The role of the extravestibular graviceptors in the judgment of the degree of tilt of one’s own body relative to G or GIF was discussed.

Physiological and Psychological Effects of Escape from a Sunken Submarine on Shore and at Sea

INTRODUCTION The stress effects induced by diverse military scenarios are usually studied under tightly controlled conditions, while only limited research has addressed realistic scenarios. This study was designed to compare the effects of two levels of realism in stressful training for escape from a sunken submarine. METHODS Thirteen qualified submariners served as subjects. All had previously participated in underwater escape training using a simulated submarine in a land-based tank submerged at a depth of 6 m; for this study, they repeated the simulator escape, following which six of them executed escape from an actual submarine lying at a depth of 30 m on the sea floor. The men were studied before the exercises, immediately after surfacing, and 2 h later. Measured variables included sympathovagal balance, salivary cortisol, perceived mood, and sleep, as well as short-term and declarative memory. RESULTS Compared to the simulator exercise in the tank, the escape at sea showed the following significant differences: 1) higher salivary cortisol values (6.33 +/- 3.9 nmol x L(-1) on shore and 13.38 +/- 7.5 nmol x L(-1) at sea); 2) greater adverse changes in mood, including vigor, tension, and ability to fall asleep; and 3) impairment in declarative memory. Responses were found to differ further for the five submariners who had prior experience of accident or injury while at sea. CONCLUSION The psychophysiological and cognitive effects of military exercises may be influenced by the realism of conditions and by prior exposure to life-threatening situations.

Effects of a visual frame and of low radial accelerations on the visually perceived eye level

The purpose of this study was to determine how the combined effects of a reference frame and of very low gravito-inertial forces produced by centrifugation affect the visually perceived eye level (VPEL). Twenty subjects were instructed to set a luminous target to the VPEL under various experimental conditions involving two main factors: (1) visual context (frameless, frame centered, frame moved down 50 mm, and frame moved up 50 mm) and (2) gravito-inertial context (motionless, Gi1=9.81001 m/sec2 and Gi2 = 9.95 m/sec2). The visual context significantly reduced the lowering of VPEL in darkness as caused by radial acceleration; this confirms the prevailing role of vision versus propriosomesthesis. However, under condition Gi2, there was a significant effect on the VPEL in spite of the presence of the luminous frame; this demonstrates that VPEL processing involves both visual and propriosomesthesic information. Furthermore, the VPEL varied linearly with the vertical shift of the luminous frame for any of the gravito-inertial conditions used in this study, but, under condition Gi2, the VPEL was shifted downward.

Perceived head-trunk angle during microgravity produced by parabolic flight.

INTRODUCTION Neck proprioceptors are essential for orienting the head relative to the trunk. However, it has been shown that the available information about the relationship of gravity to different body parts would augment the clues about their relative orientation. In weightlessness, the absence of relevant body position signals from the otoliths and other inertial graviceptors requires the substitution of other sensory information. The aim of the present study was to investigate the ability of humans to accurately locate the head relative to the trunk in microgravity. METHODS Experiments were conducted during two separate sessions: on Earth and during parabolic flights. Volunteers were asked to adjust a visual rod until it looked parallel to their head or trunk axis in two different segmental configurations: head and trunk aligned or head tilted. RESULTS There was no effect of microgravity when the head and trunk were aligned. However, when the head was tilted with respect to the trunk, the orientation of the visual rod relative to the head or the trunk (visual egocentric coordinates) was deviated toward the head tilt, although the orientation between the body parts themselves (head-trunk angle) was correctly estimated. DISCUSSION These results suggested that, in microgravity, the proprioceptive signals from neck muscles seem sufficient to provide accurate head on trunk information. However, the representation of orientation in visual space was modified. This experiment provides evidence for the role of gravity on the visual perception of head- and trunk-based egocentric coordinates.