Note on Shape Moduli Stabilization, String Gas Cosmology and the Swampland Criteria

In String Gas Cosmology, the simplest shape modulus fields are naturally stabilized by taking into account the presence of string winding and momentum modes. We determine the resulting effective potential for these fields and show that it obeys the de Sitter conjecture, one of the swampland criteria for effective field theories to be consistent with superstring theory.Comment: 4 page

REGULATION PHYSIOLOGIQUE DE LA VIGILANCE (RELATIONS ENTRE LES ACTIVITES ELECTROENCEPHALOGRAPHIQUES ET CORTICOTROPE AU COURS DU SOMMEIL ET DE LA VEILLE)

LES MECANISMES PHYSIOLOGIQUES DE REGULATION DE LA VIGILANCE, CONCEPT OPERATIONNEL D'EFFICIENCE COMPORTEMENTALE ET DE PERFORMANCE, NE SONT PAS ENTIEREMENT ELUCIDES. SI CERTAINES INVESTIGATIONS ONT PU ETABLIR L'EXISTENCE DE RELATIONS ENTRE LE SYSTEME NEUROENDOCRINIEN ET LES DIFFERENTS ETATS DE VIGILANCE, LE ROLE DES RYTHMES HORMONAUX ET LEUR IMPLICATION DANS LA REGULATION DE LA VIGILANCE RESTENT OBSCURS. L'OBJECTIF DE CE TRAVAIL, CONDUIT CHEZ L'HOMME A L'AIDE DE NOUVELLES TECHNIQUES DE MESURE ET D'ANALYSE DE L'ELECTROENCEPHALOGRAMME (EEG) ET DES SECRETIONS HORMONALES, A ETE 1) D'IDENTIFIER DES INDICES D'ACTIVATION EEG QUI REFLETERAIENT LES FLUCTUATIONS ENDOGENES DE LA VIGILANCE ET 2) D'ETUDIER LES RELATIONS DE CES INDICES AVEC L'ACTIVITE DE L'AXE HYPOTHALAMO-HYPOPHYSO-CORTICOSURRENALIEN. LA DYNAMIQUE TEMPORELLE DU SPECTRE DE L'EEG DE VEILLE, MANIPULEE EXPERIMENTALEMENT PAR LA PRIVATION DE SOMMEIL ET PAR L'ADMINISTRATION DE STIMULANTS, MONTRE UNE RYTHMICITE CIRCADIENNE ET UNE REPONSE HOMEOSTASIQUE A LA PRIVATION DE SOMMEIL, QUI DEPENDENT DE LA FREQUENCE EEG, AINSI QU'UNE PERIODICITE ULTRADIENNE LENTE (3-4 HEURES) DE TOUT LE SPECTRE. LES RYTHMES CIRCADIENS DES ACTIVITES (13,5-20 HZ) ET (20-30 HZ) SEMBLENT REFLETER DEUX MODES DISTINCTS D'ACTIVATION CEREBRALE EN DEPHASAGE DE 6-7 HEURES ET POURRAIENT SERVIR DE BASE A L'ELABORATION D'UN NOUVEAU MODELE CAPABLE DE REPRODUIRE, A PARTIR DE MESURES DIRECTES DE L'EEG, L'ASPECT BIPHASIQUE DU PROFIL NYCTHEMERAL DE LA VIGILANCE. LES RYTHMES ULTRADIENS D'ACTIVATION CEREBRALE ET CORTICOTROPE MONTRENT UN FORT COUPLAGE TEMPOREL, LA LIBERATION DE CORTISOL ETANT ASSOCIEE EN OPPOSITION DE PHASE AVEC L'ACTIVITE (0,5-3,5 HZ) PENDANT LE SOMMEIL ET EN PHASE AVEC L'ACTIVITE (20-45 HZ) PENDANT LA PENDANT VEILLE. LORS DE LA PRIVATION DE SOMMEIL, L'ACTIVITE ET LA SECRETION DE CORTISOL SONT AUGMENTEES PROPORTIONNELLEMENT, LE COUPLAGE TEMPOREL DE LEURS FLUCTUATIONS ETANT AFFAIBLI. CES RESULTATS SUGGERENT L'EXISTENCE D'UN MECANISME COMMUN DE CONTROLE DE L'EEG ET DE LA SECRETION DE CORTISOL. LE COUPLAGE DES ACTIVITES EEG ET CORTICOTROPE AU COURS DU CYCLE VEILLE-SOMMEIL CONTRIBUE PROBABLEMENT A LA REGULATION DU METABOLISME ENERGETIQUE CEREBRAL. EN SITUATION PATHOLOGIQUE OU DE STRESS, LE DECOUPLAGE DE CES ACTIVITES POURRAIT ETRE A L'ORIGINE DE TROUBLES DE LA VIGILANCE ET DES RYTHMES BIOLOGIQUES.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

LE SYSTEME RENINE-ANGIOTENSINE-ALDOSTERONE AU COURS DU CYCLE VEILLE-SOMMEIL

STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

A bouncing cosmology from VECROs

We argue that, in the same way that in a black hole space-time VECROs will form in order to cancel the gravitational effects of a collapsing mass shell and prevent the formation of a singularity, in a contracting universe a gas of VECROs will form to hold up the contraction, prevent a Big Crunch singularity, and lead to a nonsingular cosmological bounce

Répercussions de l'activité physique sur la variabilité cardiaque au cours de la veille et du sommeil

Par la mesure des indices temporels et spectraux de la variabilité cardiaque, nous avons appréhendé l'activité du système nerveux autonome en réponse à la pratique régulière d'une activité physique, évaluée à l'aide d'accéléromètres et de questionnaires validés. Nous avons tout d'abord démontré l'importance des conditions d'enregistrement électroencéphalographique pour une détermination optimale de la balance sympathovagale, écarté les mesures durant l'exercice et proposé la veille calme rigoureusement contrôlée et le sommeil à ondes lentes comme des moments appropriés permettant de limiter l'effet de facteurs confondants pouvant perturber l'estimation de l'activité autonome. Ensuite, l'étude de l'influence relative de la quantité et de l'intensité de l'activité physique sur la variabilité cardiaque auprès d'enfants, de jeunes adultes et des personnes âgées, a permis de montrer qu'une charge d'entraînement ou une dépense énergétique modérée est suffisante pour observer des adaptations neurovégétatives significatives, mais qu'une intensité au minimum modérée est nécessaire. En revanche, nos études montrent qu'une dépense énergétique très importante, même en l'absence de tout stress ou de surcharge d'entraînement, est associée à des indices liés à l'activité parasympathique comparables à ceux des sujets sédentaires. Bien que des études restent à mener pour expliquer ces résultats et observer les effets de toutes les plages d'intensités, nos résultats permettent d'apporter des éléments complémentaires aux recommandations concernant les caractéristiques de l'activité physique à entreprendre pour la promotion de la santé et la prévention des facteurs de risque.L'utilisation pratique et clinique des indices de variabilité, leur signification et leur intérêt devront être approfondis, et des investigations restent à entreprendre pour envisager les nouvelles perspectives offertes par les enregistrements au cours du sommeil dont nous avons avancé la pertinence.We have investigated the effect on cardiac autonomic control, as inferred from heart rate variability (HRV) analysis, of regular physical activity, evaluated by accelerometry and questionnaires. First, we have demonstrated the importance of appropriate electrocardiophic recording conditions. We rejected measurements during exercise, proposed standardized quiet wake periods, and underlined the interest of quiet sleep periods, which offer optimal condition required for HRV analysis, as they are associated with regular respiratory patterns and the absence of any body movement that could confound autonomic control evaluation. Secondly, we have evaluated the associations between physical activity quantity and intensity and HRV in children, young adults and elderly. Our results showed that moderate energy expenditure is sufficient to observe higher vagal-related indexes, but that at least moderate intensities are required. However, very high energy expenditure, even in the absence of stress or any stressful stimuli, is associated with similar HRV indexes than those displayed by sedentary subjects. Although further investigations are warranted to explain these results and to objective the specific effects of each physical activity intensity range, our studies lend support to guidelines that propose moderate but not too high training load for promoting health and preventing cardiovascular .The pertinence of the use of HRV indexes should be further documented in field studies and in pathological conditions, and the relevance of quiet sleep as an optimal time of observation for discriminating sympathovagal balance has to be confirmed in various conditions and populations.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Cortisol secretion is related to electroencephalographic alertness in human subjects during daytime wakefulness

ABSTRACT To determine whether human hypothalamo-pituitary-adrenal axis activity is related to the alertness level during wakefulness, 10 healthy young men were studied under resting conditions in the daytime (0900 -1800 h) after an 8-h nighttime sleep (2300 -0700 h). A serial 70-sec gaze fixation task was required every 10 min throughout the daytime experimental session. The corresponding waking electroencephalographic (EEG) segments were submitted to quantitative spectral analysis, from which EEG ␤ activity (absolute power density in the 13-35 Hz frequency band), an index of central alertness, was computed. Blood was collected continuously through an indwelling venous catheter and sampled at 10-min intervals. Plasma cortisol concentrations were measured by RIA, and the corresponding secretory rates were determined by a deconvolution procedure. Analysis of individual profiles demonstrated a declining tendency for EEG ␤ activity and cortisol secretory rate, with an overall temporal relationship indicated by positive and significant cross-correlation coefficients between the two variables in all subjects (average r ϭ 0.565, P Ͻ 0.001). Changes in cortisol secretion lagged behind fluctuations in EEG ␤ activity, with an average delay of 10 min for all the subjects. On the average, 4.6 Ϯ 0.4 cortisol secretory pulses and 4.9 Ϯ 0.5 peaks in EEG ␤ activity were identified by a detection algorithm. A significant, although not systematic, association between the episodes in the two variables was found: 44% of the peaks in EEG ␤ activity (relative amplitude, near 125%; P Ͻ 0.001) occurred during an ascending phase of cortisol secretion, cortisol secretory rates increasing by 40% (P Ͻ 0.01) 10-min after peaks in EEG ␤ activity. However, no significant change in EEG ␤ activity was observed during the period from 50 min before to 50 min after pulses in cortisol secretion. In conclusion, the present study describes a temporal coupling between cortisol release and central alertness, as reflected in the waking EEG ␤ activity. These findings suggest the existence of connections between the mechanisms involved in the control of hypothalamo-pituitary-adrenal activity and the activation processes of the brain, which undergoes varying degrees of alertness throughout daytime wakefulness. (J Clin Endocrinol Metab 83: [4263][4264][4265][4266][4267][4268] 1998) C ORTISOL is released in pulses by adrenocortical glands under pituitary ACTH control, with a periodicity of 80 -110 min in man (1). The 24-h pattern of cortisol levels shows an early-morning acrophase and an evening nadir (2). This is the consequence of an amplitude modulation of ACTH secretory bursts (3), probably driven by the suprachiasmatic nucleus of the hypothalamus, which controls CRH and arginine vasopressin cells of the paraventricular nucleus (4). Although cortisol secretion is known to be primarily under a circadian influence (5), independent of sleeping and waking, several studies in humans have suggested that sleep, especially slow-wave sleep, may exert an inhibitory influence on cortisol secretion (6, 7). Other authors argue that an underlying mechanism decreases cortisol secretion and facilitates sleep onset and slow-wave sleep installation (8 -10). Dynamic relationships have been described between human sleep electroencephalographic (EEG) activity, which reflects central nervous sleep processes, and cortisol secretory activity Although sleep EEG has been extensively studied (14), the time course of the waking EEG activity has been studied far less, because of artifacts contaminating EEG recordings. However, diurnal fluctuations of the human background EEG, a neurophysiological indicator of the brain's functional state (15), have been shown to occur spontaneously, with patterns depending on the EEG spectrum frequency ban