Scaling properties and universality of first-passage time probabilities in financial markets

Financial markets provide an ideal frame for the study of crossing or first-passage time events of non-Gaussian correlated dynamics mainly because large data sets are available. Tick-by-tick data of six futures markets are herein considered resulting in fat tailed first-passage time probabilities. The scaling of the return with the standard deviation collapses the probabilities of all markets examined, and also for different time horizons, into single curves, suggesting that first-passage statistics is market independent (at least for high-frequency data). On the other hand, a very closely related quantity, the survival probability, shows, away from the center and tails of the distribution, a hyperbolic $t^{-1/2}$ decay typical of a Markovian dynamics albeit the existence of memory in markets. Modifications of the Weibull and Student distributions are good candidates for the phenomenological description of first-passage time properties under certain regimes. The scaling strategies shown may be useful for risk control and algorithmic trading.Comment: 7 pages, 5 figure

The Role of Ghrelin in Reward-Based Eating

The peptide hormone ghrelin acts in the central nervous system as a potent orexigenic signal. Not only is ghrelin recognized as playing an important role in feeding circuits traditionally thought of as affecting body weight homeostasis, but also an accumulating number of scientific studies have identified ghrelin as being a key regulator of reward-based, hedonic eating behaviors. In the current article, we review ghrelin's orexigenic actions, the evidence linking ghrelin to food reward behavior, potential mechanisms by which ghrelin mediates reward-based eating behavior, and those studies suggesting an obligatory role for ghrelin in the changed eating behaviors induced by stress.Instituto Multidisciplinario de Biología Celula

The Role of Ghrelin in Reward-Based Eating

The peptide hormone ghrelin acts in the central nervous system as a potent orexigenic signal. Not only is ghrelin recognized as playing an important role in feeding circuits traditionally thought of as affecting body weight homeostasis, but also an accumulating number of scientific studies have identified ghrelin as being a key regulator of reward-based, hedonic eating behaviors. In the current article, we review ghrelin's orexigenic actions, the evidence linking ghrelin to food reward behavior, potential mechanisms by which ghrelin mediates reward-based eating behavior, and those studies suggesting an obligatory role for ghrelin in the changed eating behaviors induced by stress.Instituto Multidisciplinario de Biología Celula

Brain circuits mediating the orexigenic action of peripheral ghrelin: narrow gates for a vast kingdom

The nervous and endocrine systems act together to regulate all physiological processes essential for the body homeostasis control. Given the strict communication restrictions that the brain–blood barrier (BBB) imposes, the interplay between these two systems requires a variety of delicate anatomical interfaces and physiological mechanisms that guarantee the precise function of the neuroendocrine system as a whole. The study of the mechanisms by which hormones act in the brain in order to regulate specific neuronal populations is a research topic rather neglected. Our group studies the neuronal circuitries and molecular mechanisms by which the stomach-produced hormone ghrelin regulates appetite and other physiological functions. A clear notion of the brain targets of peripheral ghrelin is essential for the comprehensive understanding of the physiological role of this hormone. Ghrelin is called “the hunger hormone” since it is the only known orexigenic peptide hormone. The target for ghrelin orexigenic actions is the brain, which contains a variety of ghrelin-responsive nuclei; however, several evidences suggest that the accessibility of peripheral ghrelin to the brain is strikingly low. Here, we briefly summarize the current knowledge in this topic and discuss this intriguing neuroendocrinological issue

Brain circuits mediating the orexigenic action of peripheral ghrelin: narrow gates for a vast kingdom

The nervous and endocrine systems act together to regulate all physiological processes essential for the body homeostasis control. Given the strict communication restrictions that the brain-blood barrier (BBB) imposes, the interplay between these two systems requires a variety of delicate anatomical interfaces and physiological mechanisms that guarantee the precise function of the neuroendocrine system as a whole. The study of the mechanisms by which hormones act in the brain in order to regulate specific neuronal populations is a research topic rather neglected. Our group studies the neuronal circuitries and molecular mechanisms by which the stomach-produced hormone ghrelin regulates appetite and other physiological functions. A clear notion of the brain targets of peripheral ghrelin is essential for the comprehensive understanding of the physiological role of this hormone. Ghrelin is called "the hunger hormone" since it is the only known orexigenic peptide hormone. The target for ghrelin orexigenic actions is the brain, which contains a variety of ghrelin-responsive nuclei; however, several evidences suggest that the accessibility of peripheral ghrelin to the brain is strikingly low. Here, we briefly summarize the current knowledge in this topic and discuss this intriguing neuroendocrinological issue.Instituto Multidisciplinario de Biología Celula

Mapping individual behavior in financial markets: synchronization and anticipation

In this paper we develop a methodology, based on Mutual Information and Transfer of Entropy, that allows to identify, quantify and map on a network the synchronization and anticipation relationships between financial traders. We apply this methodology to a dataset containing 410,612 real buy and sell operations, made by 566 non-professional investors from a private investment firm on 8 different assets from the Spanish IBEX market during a period of time from 2000 to 2008. These networks present a peculiar topology significantly different from the random networks. We seek alternative features based on human behavior that might explain part of those 12,158 synchronization links and 1031 anticipation links. Thus, we detect that daily synchronization with price (present in 64.90% of investors) and the one-day delay with respect to price (present in 4.38% of investors) play a significant role in the network structure. We find that individuals reaction to daily price changes explains around 20% of the links in the Synchronization Network, and has significant effects on the Anticipation Network. Finally, we show how using these networks we substantially improve the prediction accuracy when Random Forest models are used to nowcast and predict the activity of individual investors

ESTUDIO DE LA REMODELACIÓN DE LOS CIRCUITOS NEURONALES HIPOTALÁMICOS QUE CONTROLAN EL APETITO ANTE SITUACIONES DE DÉFICIT ENERGÉTICO: ROL DE LA HORMONA GHRELINA

La ghrelina, producida principalmente por el tracto digestivo, es un péptido de 28 residuos y esterificado con un ácido n-octanoico en la ser3. La ghrelina se destaca por sus efectos estimulantes sobre la secreción de la hormona de crecimiento, la regulación de la respuesta al estrés y la ingesta de alimento. Los valores plasmáticos de ghrelina aumentan en condiciones de balance energético negativo, como ayuno o restricción calórica, donde la hormona es esencial para aumentar el apetito y activar mecanismos neuroendocrinos hiper-glucemiantes. La ghrelina actúa a través del receptor secretagogo de la hormona de crecimiento (GHSR, por sus siglas en inglés), el cual se expresa en la mayoría de los núcleos importantes en el control del apetito y de los ejes neuroendocrinos. Una de las características más importantes de GHSR es su alta actividad constitutiva, la cual causa señalización intracelular en ausencia de ghrelina. Dicha actividad constitutiva de GHSR podría también cumplir un rol en situaciones de balance energético negativo, cuando la expresión del receptor en el cerebro aumenta. Recientemente el grupo de Neurofisiología del IMBICE describió que la ghrelina posee, además, un efecto neurotrófico, es decir que controlaría el crecimiento, desarrollo y diferenciación neuronal. Uno de los principales blancos neuronales de la ghrelina son las neuronas del núcleo arcuato (ARC) hipotalámico que producen el péptido relacionado al gen agouti (AgRP) (o neuronas ARC-AgRP), las cuales expresan elevados niveles de GHSR y son necesarias para que la ghrelina aumente el apetito. El grupo de Neurofisiología descubrió que las proyecciones de ARC-AgRP al núcleo paraventricular hipotalámico (PVH) se remodelan en ratones adultos ayunados de una manera dependiente de GHSR. Dentro del hipotálamo, las neuronas ARC-AgRP no solo inervan el PVH, sino también el área hipotalámica lateral (AHL), la cual cumple un rol importante articulando los componentes homeostáticos y hedónicos de la ingesta de alimento. Estudios recientes de resonancia magnética de difusión combinada con tractografía probabilística mostraron que la conectividad del ARC y del AHL se altera en pacientes que sufren de anorexia nerviosa, quienes se encuentran en un déficit energético crónico. Por este motivo, el objetivo general de este proyecto es estudiar el proceso de remodelación de los circuitos neuronales hipotalámicos que controlan el apetito en ratones adultos sometidos a situaciones de déficit energético y el rol de la hormona ghrelina sobre este fenómeno. Para esto estudiaremos si la acción de ghrelina es requerida, utilizando modelos con bloqueo farmacológico o genético de la señalización de GHSR y también investigaremos si la acción de ghrelina es suficiente para que ocurra esta remodelación. A su vez evaluaremos si la activación mediante fármaco-genética de las neuronas ARC-AgRP es suficiente para que ocurran estos cambios