The Hippocampus is Preferentially Associated with Memory for Spatial Context

The existence of a functional-anatomic dissociation for retrieving item versus contextual information within subregions of the medial temporal lobe (MTL) is currently under debate. We used a spatial source memory paradigm during event-related functional magnetic resonance imaging to investigate this issue. At study, abstract shapes were presented to the left or right of fixation. During test, old and new shapes were presented at fixation. Participants responded whether each shape had been previously presented on the “left,” the “right,” or was “new.” Activity associated with contextual memory (i.e., source memory) was isolated by contrasting accurate versus inaccurate memory for spatial location. Item-memory-related activity was isolated by contrasting accurate item recognition without contextual memory with forgotten items. Source memory was associated with activity in the hippocampus and parahippocampal cortex. Although item memory was not associated with unique MTL activity at our original threshold, a region-of-interest (ROI) analysis revealed item-memory-related activity in the perirhinal cortex. Furthermore, a functional-anatomic dissociation within the parietal cortex for retrieving item and contextual information was not found in any of three ROIs. These results support the hypothesis that specific subregions in the MTL are associated with item memory and memory for context

WAGE DIFFERENTIALS AND TRADE RELATIONSHIPS IN JAMAICA: APPLICATIONS OF TRUNCATED REGRESSION MODELS AND REPEATED CROSS-SECTION DATA

Labor and skill premiums and their relationships with trade measures are simultaneously estimated within the framework of an individual level labor supply model for Jamaica using truncated regression models. Increased imports from the US were found to reduce the return to unskilled labor, and increase the skill premium in Jamaica. Increased exports to the US had the opposite effects on returns to Jamaican skilled and unskilled labor.Labor and Human Capital,

Using a multifrontal sparse solver in a high performance, finite element code

We consider the performance of the finite element method on a vector supercomputer. The computationally intensive parts of the finite element method are typically the individual element forms and the solution of the global stiffness matrix both of which are vectorized in high performance codes. To further increase throughput, new algorithms are needed. We compare a multifrontal sparse solver to a traditional skyline solver in a finite element code on a vector supercomputer. The multifrontal solver uses the Multiple-Minimum Degree reordering heuristic to reduce the number of operations required to factor a sparse matrix and full matrix computational kernels (e.g., BLAS3) to enhance vector performance. The net result in an order-of-magnitude reduction in run time for a finite element application on one processor of a Cray X-MP

6-fluoropyridoxal phosphate and pyridoxal sulfate in aspartate aminotransferase

The chemical and spectroscopic properties of 6-fluoropyridoxal 5\u27-phosphate, of its Schiff base with valine and of 6-fluoropyridoxamine 5\u27-phosphate have been investigated. The modified coenzymes have also been combined with the apo form of cytosolic aspartate aminotransferase and the properties of the resulting enzymes and of their complexes with substrates and inhibitors have been recorded. Although the presence of the 6-fluoro substituent reduces the basicity of the ring nitrogen over 10,000-fold, the modified coenzymes bind predominately in their dipolar ionic forms as do the natural coenzymes. Enzyme containing the modified coenzymes binds substrates and dicarboxylate substrates normally, and has about 42% of the catalytic activity of the native enzyme. Fluorine-19 NMR measurements show that the ring nitrogen of bound 6-fluoropyridoxamine phosphate is protonated at pH 7 or below but becomes deprotonated at high pH around a pKa of 8.2. The bound 6-fluoropyridoxal phosphate, which exists as a Schiff base with a dipolar ionic ring at high pH beomes protonated with a pKa of 7.1, corresponding to the pKa of 6.4 in the native enzyme. Below this pKa a single (\u2719)F resonance is seen corresponding to keoenamine and enolimine tautomers of the Schiff base. The tautomeric ratio and chemical shift is altered upon binding of dicarboxylate inhibitors indicating that during the rapid tautomerization a proton is synchronously moved from the ring nitrogen (in the ketoenamine tautomer) onto the aspartate 222 carboxylate (in the enolimine tautomer). Titration of the (\u2719)F NMR resonance with substrates and inhibitors can be related to exchange rates and equilibrium constants;A fluorescent derivative of lysine 258 isolated from the active site of aspartate aminotransferase modified by treatment of the apoenzyme with pyridoxal 5\u27-sulfate has been characterized as a substituted 2H-pyrrolo 3,4-c pyridine. Similar pyrrolopyridines are produced in up to 20% yield by reaction of pyridoxal sulfate with simple alkylamines or with amino acids. The reaction with lysine forms two products, one of which is identical to the isolated chromophore. The pyrrolopyridine derived from ethylamine has been characterized by (\u271)H and (\u2713)C NMR, UV-visible, and mass spectroscopy