Boston University

About: Boston University is a education organization based out in Boston, Massachusetts, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 48688 authors who have published 119622 publications receiving 6276020 citations. The organization is also known as: BU & Boston U.

Neural dynamics of planned arm movements: emergent invariants and speed-accuracy properties during trajectory formation

Abstract: A real-time neural network model, called the vector-integration-to-endpoint (VITE) model is developed and used to simulate quantitatively behavioral and neural data about planned and passive arm movements. Invariants o farm movements emerge through network interactions rather than through an explicitly precomputed trajectory. Motor planning occurs in the form of a target position command (TPC), which specifies where the arm intends to move, and an independently controlled GO command, which specifies the movement's overall speed. Automatic processes convert this information into an arm trajectory with invariant properties. These automatic processes include computation of a present position command (PPC) and a difference vector (DV). The DV is the difference between the PPC and the TPC at any time. The PPC is gradually updated by integrating the DV through time. The GO signal multiplies the DV before it is integrated by the PPC. The PPC generates an outflow movement command to its target muscle groups. Opponent interactions regulate the PPCs to agonist and antagonist muscle groups. This system generates synchronous movements across synergetic muscles by automatically compensating for the different total contractions that each muscle group must undergo. Quantitative simulations are provided of Woodworth's law, of the speed-accuracy trade-offknown as Fitts's law, of isotonic arm-movement properties before and after deafferentation, of synchronous and compensatory "central-error-correction" properties of isometric contractions, of velocity amplification during target switching, of velocity profile invariance and asymmetry, of the changes in velocity profile asymmetry at higher movement speeds, of the automarie compensation for staggered onset times of synergetic muscles, of vector cell properties in precentral motor cortex, of the inverse relation between movement duration and peak velocity, and of peak acceleration as a function of movement amplitude and duration. It is shown that TPC, PPC, and DV computations are needed to actively modulate, or gate, the learning of associative maps between TPCs of different modalities, such as between the eye-head system and the hand-arm system. By using such an associative map, looking at an object can activate a TPC of the hand-arm system, as Piaget noted. Then a VITE circuit can translate this TPC into an invariant movement trajectory. An auxiliary circuit, called the Passive Update of Position (PUP) model is described for using inflow signals to update the PPC during passive arm movements owing to external forces. Other uses of outflow and inflow signals are also noted, such as for adaptive linearization of a nonlinear muscle plant, and sequential readout of TPCs during a serial plan, as in reaching and grasping. Comparisons are made with other models of motor control, such as the mass-spring and minimumjerk models.

Critical values for multiple structural change tests

Abstract: Bai and Perron (1998), henceforth BP, considered estimating multiple structural changes in a linear model. The results are obtained under a general framework of partial structural changes which allows a subset of the parameters not to change.1 Methods to efficiently compute estimates are discussed in Bai and Perron (2003). BP also addressed the problem of testing for multiple structural changes under very general conditions on the data and the errors: they considered a type test for the null hypothesis of no change vs. a pre-specified number of changes and also vs. an alternative of an arbitrary number of changes (up to some maximum), as well as a procedure that allows one to test the null hypothesis of, say, I changes, vs. the alternative hypothesis of I + 1 changes. The latter is particularly useful in that it allows a specific to general modeling strategy to consistently determine the appropriate number of changes in the data. The tests can be constructed allowing different serial correlation in the errors, different distribution for the data and the errors across segments or imposing a common structure.

Cutting Edge: Different Toll-Like Receptor Agonists Instruct Dendritic Cells to Induce Distinct Th Responses via Differential Modulation of Extracellular Signal-Regulated Kinase-Mitogen-Activated Protein Kinase and c-Fos

Abstract: Dendritic cells (DCs) are pivotal in determining the class of an adaptive immune response. However, the molecular mechanisms within DCs that determine this decision-making process are unknown. Here, we demonstrate that distinct Toll-like receptor (TLR) ligands instruct human DCs to induce distinct Th cell responses by differentially modulating mitogen-activated protein kinase signaling. Thus, Escherichia coli LPS and flagellin, which trigger TLR4 and TLR5, respectively, instruct DCs to stimulate Th1 responses via IL-12p70 production, which depends on the phosphorylation of p38 and c-Jun N-terminal kinase 1/2. In contrast, the TLR2 agonist, Pam3cys, and the Th2 stimulus, schistosome egg Ags: 1) barely induce IL-12p70; 2) stimulate sustained duration and magnitude of extracellular signal-regulated kinase 1/2 phosphorylation, which results in stabilization of the transcription factor c-Fos, a suppressor of IL-12; and 3) yield a Th2 bias. Thus, distinct TLR agonists differentially modulate extracellular signal-regulated kinase signaling, c-Fos activity, and cytokine responses in DCs to stimulate different Th responses.

Knowledge relatedness and the performance of multibusiness firms

Abstract: This study examines corporate performance effects of cross-business knowledge synergies in multibusiness firms. It synthesizes the resource-based view of diversification and the economic theory of complementarities to conceptualize cross-business knowledge synergies in terms of the relatedness and the complementarity of knowledge resources across business units of the multibusiness firm. The study hypothesizes that corporate performance is improved when the firm simultaneously exploits a complementary set of related knowledge resources across its business units. In a sample of 303 multibusiness firms, the study finds that synergies arising from product knowledge relatedness, customer knowledge relatedness, or managerial knowledge relatedness do not improve corporate performance on their own. Synergies arising from the complementarity of the three types of knowledge relatedness significantly improve both market-based and accounting-based performance of the multibusiness corporation. Copyright © 2004 John Wiley & Sons, Ltd.