Hierarchical control over effortful behavior by rodent medial frontal cortex : a computational model

The anterior cingulate cortex (ACC) has been the focus of intense research interest in recent years. Although separate theories relate ACC function variously to conflict monitoring, reward processing, action selection, decision making, and more, damage to the ACC mostly spares performance on tasks that exercise these functions, indicating that they are not in fact unique to the ACC. Further, most theories do not address the most salient consequence of ACC damage: impoverished action generation in the presence of normal motor ability. In this study we develop a computational model of the rodent medial prefrontal cortex that accounts for the behavioral sequelae of ACC damage, unifies many of the cognitive functions attributed to it, and provides a solution to an outstanding question in cognitive control research-how the control system determines and motivates what tasks to perform. The theory derives from recent developments in the formal study of hierarchical control and learning that highlight computational efficiencies afforded when collections of actions are represented based on their conjoint goals. According to this position, the ACC utilizes reward information to select tasks that are then accomplished through top-down control over action selection by the striatum. Computational simulations capture animal lesion data that implicate the medial prefrontal cortex in regulating physical and cognitive effort. Overall, this theory provides a unifying theoretical framework for understanding the ACC in terms of the pivotal role it plays in the hierarchical organization of effortful behavior

Initial Conditions, Institutional Dynamics and Economic Performance: Evidence from the American States

Using state-level data from the United States, we find that differences in colonial legal institutions affect the current quality of state legal institutions. These differences in colonial legal institutions arose because some states were settled by Great Britain, a common law country, and other states were settled by France, Spain, and Mexico, all civil law countries. To explain these findings, we develop a transplant-civil law hypothesis that highlights the disruption associated with large-scale legal transplantation and the possible relative inefficiencies of colonial civil law. We find strong support for the transplant-civil law hypothesis. Our results are robust to inclusion of additional variables capturing climate, geography, initial population and resource endowments. Given the 150-200 year gap between the initial conditions and the measures of the current quality of legal institutions, we provide indirect evidence on the persistence of legal institutions. We then use initial legal systems and climate to quantify the substantial impact of current institutions on current economic performance.http://deepblue.lib.umich.edu/bitstream/2027.42/40001/3/wp615.pd

Forms of early walking

Children in the first weeks of independent locomotion display a wide variety of walking forms. The walking forms differ in mechanical strategy and concern with balance. Three extreme walking forms are presented: the Twister, who uses trunk twist, the Faller, who uses gravity, and the Stepper, who remains balanced as much as possible. Each walking form is presented as a ''d-space'', a mathematical format combining continuous and discrete aspects, developed to express the sequence and pattern of a movement without the inappropriate precision of a physical trajectory. The three d-spaces represent analyses of three extreme modes of early walking. They are used to generate the variety of early walking forms and to predict mixtures of mechanical strategies as children mature and converge to more similar walking forms over the first few months of independent locomotion. (C) 1995 Academic Press Limite

Absence of long-range superconducting correlations in the frustrated 1/2-filled band Hubbard model

We present many-body calculations of superconducting pair-pair correlations in the ground state of the half-filled band Hubbard model on large anisotropic triangular lattices. Our calculations cover nearly the complete range of anisotropies between the square and isotropic triangular lattice limits. We find that the superconducting pair-pair correlations decrease monotonically with increasing onsite Hubbard interaction U for inter-pair distances greater than nearest neighbor. For the large lattices of interest here the distance dependence of the correlations approaches that for noninteracting electrons. Both these results are consistent with the absence of superconductivity in this model in the thermodynamic limit. We conclude that the effective 1/2-filled band Hubbard model, suggested by many authors to be appropriate for the kappa-(BEDT-TTF)-based organic charge-transfer solids, does not explain the superconducting transition in these materials.Comment: 9 pages, 7 figures. Revised version to appear in Phys. Rev.

Relationship between Aphid Infestations, Aphid Management Regimes, and the Incidence of Barley Yellow Dwarf Virus in Soft Red Winter Wheat

Aphid (Hemiptera: Aphididae) feeding may cause substantial loss of yield and grain quality by transmitting barley yellow dwarf (BYD) in wheat, Triticum aestivum L. Neonicotinoid seed treatments and foliar-applied insecticides are the two most common methods to manage aphid infestations and BYD. The overall goal for my research was to refine recommendations on the management of aphids in wheat to provide a consistent return on investment. An analysis was done across 33 insecticide efficacy trials in west Tennessee during the last eleven years to determine how neonicotinoid seed treatments and a late-winter foliar insecticide application affected aphid populations, incidence of BYD, and yield. A significant decrease in aphid populations and incidence of BYD was observed where an insecticide seed treatment, foliar insecticide, or both were used. Average wheat yields were increased by 280 – 381 kg/ha if an insecticide seed treatment or when a foliar insecticide application was made. A factorial experiment was repeated at four locations to examine how variety tolerance to BYD, a neonicotinoid seed treatment, and a foliar insecticide application affected populations of aphids, the occurrence of BYD, and yield. Similarly, experiments with various neonicotinoid seed treatments and foliar insecticide spray regimens were repeated to evaluate treatment effects on aphids, BYD, and yield. Aphid populations and BYD symptomology were consistently reduced by the use of insecticides, particularly foliar insecticides. Quantitative ELISA assays confirmed the presence of BYD in leaf tissue and was also able to detect some treatment effects on the concentration of BYD. Varieties considered tolerant to BYD had reduced symptoms of BYD, but this difference was not reflected in yield responses, and ELISA assays detected no difference between varieties in the concentration of BYD in leaf tissue. In these tests, foliar applied insecticides provided more benefit than neonicotinoid seed treatments. Collectively, my data show that insecticides should be used to manage aphids and BYD, and foliar-applied insecticides appear to provide more benefit and flexibility than insecticide seed treatments