Controversies about the enhanced vulnerability of the adolescent brain to develop addiction.

Adolescence, defined as a transition phase toward autonomy and independence, is a natural time of learning and adjustment, particularly in the setting of long-term goals and personal aspirations. It also is a period of heightened sensation seeking, including risk taking and reckless behaviors, which is a major cause of morbidity and mortality among teenagers. Recent observations suggest that a relative immaturity in frontal cortical neural systems may underlie the adolescent propensity for uninhibited risk taking and hazardous behaviors. However, converging preclinical and clinical studies do not support a simple model of frontal cortical immaturity, and there is substantial evidence that adolescents engage in dangerous activities, including drug abuse, despite knowing and understanding the risks involved. Therefore, a current consensus considers that much brain development during adolescence occurs in brain regions and systems that are critically involved in the perception and evaluation of risk and reward, leading to important changes in social and affective processing. Hence, rather than naive, immature and vulnerable, the adolescent brain, particularly the prefrontal cortex, should be considered as prewired for expecting novel experiences. In this perspective, thrill seeking may not represent a danger but rather a window of opportunities permitting the development of cognitive control through multiple experiences. However, if the maturation of brain systems implicated in self-regulation is contextually dependent, it is important to understand which experiences matter most. In particular, it is essential to unveil the underpinning mechanisms by which recurrent adverse episodes of stress or unrestricted access to drugs can shape the adolescent brain and potentially trigger life-long maladaptive responses

Load fluctuations drive actin network growth

The growth of actin filament networks is a fundamental biological process that drives a variety of cellular and intracellular motions. During motility, eukaryotic cells and intracellular pathogens are propelled by actin networks organized by nucleation-promoting factors, which trigger the formation of nascent filaments off the side of existing filaments in the network. A Brownian ratchet (BR) mechanism has been proposed to couple actin polymerization to cellular movements, whereby thermal motions are rectified by the addition of actin monomers at the end of growing filaments. Here, by following actin--propelled microspheres using three--dimensional laser tracking, we find that beads adhered to the growing network move via an object--fluctuating BR. Velocity varies with the amplitude of thermal fluctuation and inversely with viscosity as predicted for a BR. In addition, motion is saltatory with a broad distribution of step sizes that is correlated in time. These data point to a model in which thermal fluctuations of the microsphere or entire actin network, and not individual filaments, govern motility. This conclusion is supported by Monte Carlo simulations of an adhesion--based BR and suggests an important role for membrane tension in the control of actin--based cellular protrusions.Comment: To be published in PNA

Proof-theoretic Analysis of Rationality for Strategic Games with Arbitrary Strategy Sets

In the context of strategic games, we provide an axiomatic proof of the statement Common knowledge of rationality implies that the players will choose only strategies that survive the iterated elimination of strictly dominated strategies. Rationality here means playing only strategies one believes to be best responses. This involves looking at two formal languages. One is first-order, and is used to formalise optimality conditions, like avoiding strictly dominated strategies, or playing a best response. The other is a modal fixpoint language with expressions for optimality, rationality and belief. Fixpoints are used to form expressions for common belief and for iterated elimination of non-optimal strategies.Comment: 16 pages, Proc. 11th International Workshop on Computational Logic in Multi-Agent Systems (CLIMA XI). To appea

Supplementary Motor Area Encodes Reward Expectancy in Eye-Movement Tasks

Neural activity signifying the expectation of reward has been found recently in many parts of the brain, including midbrain and cortical structures. These signals can facilitate goal-directed behavior or the learning of new skills based on reinforcements. Here we show that neurons in the supplementary motor area (SMA), an area concerned with movements of the body and limbs, also carry a reward expectancy signal in the postsaccadic period of oculomotor tasks. While the monkeys performed blocks of memory-guided and object-based saccades, the neurons discharged a burst after a ∼200-ms delay following the target-acquiring saccade in the memory task but often fired concurrently with the target-acquiring saccade in the object task. The hypothesis that this postsaccadic bursting activity reflects the expectation of a reward was tested with a series of manipulations to the memory-guided saccade task. It was found that although the timing of the bursting activity corresponds to a visual feedback stimulus, the visual feedback is not required for the neurons to discharge a burst. Second, blocks of no-reward trials reveal an extinction of the bursting activity as the monkeys come to understand that they would not be rewarded for properly generated saccades. Finally, the delivery of unexpected rewards confirmed that in many of the neurons, the activity is not related to a motor plan to acquire the reward (e.g., licking). Thus we conclude that reward expectancy is represented by the activity of SMA neurons, even in the context of an oculomotor task. These results suggest that the reward expectancy signal is broadcast over a large extent of motor cortex, and may facilitate the learning of new, coordinated behavior between different body parts

The politics of alcohol policy in Nigeria: a critical analysis of how and why brewers use strategic ambiguity to supplant policy initiatives

The global call by the World Health Assembly (WHA) to control the rising alcohol-related problems caused by harmful consumption through policy became necessary in 2005 due to the recognition of the fact that many countries did not have alcohol policies. This gave rise to the adoption of a ten-point policy strategy by the World Health Organization (WHO) Member States in 2010. Against this backdrop, many countries adopted alcohol policies to reduce harmful alcohol consumption. Nigeria was one of the WHO Member Countries that adopted the resolution. Nigeria is among the 30 countries with the highest per capita consumption and alcohol-related problems, yet has not formulated alcohol policy to date. This paper draws on Eisenberg’s Strategic Ambiguity Model to explore the role of brewers in supplanting alcohol policy initiatives in Nigeria. It argues that the leading alcohol producers in Nigeria have been the main reason alcohol policies have not been formulated. The article focuses on why their campaigns for responsible drinking, promotions, sponsorships and ‘strategic social responsibilities’ may have increased since the WHA made the call and the WHO adopted the resolution in 2010. It concludes by arguing that there is an urgent need to formulate policies drawing from the WHO resolution to curtail the activities of these brewers and reduce harmful consumption