Resolution Of Current Methods Research Articles

Examining the initial stages of shear localization in amorphous metals

A mesoscale simulation by shear transformation zone (STZ) dynamics is used to analyze the initial stages of shear localization in amorphous metals, of which many details remain unresolved due to the difficulty in accessing the appropriate time- and length-scales. Examination of a constant strain-rate tensile test of a model amorphous metal reveals four different stages in the microscopic processes that lead to formation of a shear band. These stages are identified as: (i) STZ clustering, where potential nucleation sites form as STZ activations cluster in space; (ii) growth following nucleation, where a nascent shear band exhibits a propagating front that defines the shear band path; (iii) relaxation thickening, where the shear band thickens and relaxes the system down to the flow stress; and (iv) flow thickening, where the shear band exhibits prolonged thickening at the flow stress. The results are consistent with the literature but suggest that a shear band is defined by a front that propagates very quickly with negligible accumulated strain. This propagating front likely occurs below the resolution of current methods that only observe simultaneous slip in a shear band where significant strain can be accumulated at much slower rates. Analysis of a thermodynamic model also suggests a specific critical stress that is required to nucleate a shear band, after which the shear band is allowed to grow unconstrained.

Postmortem findings in suicide victims. Implications for in vivo imaging studies.

Alterations in serotonergic and noradrenergic receptor binding in membrane homogenates from the brain of suicide victims suggest a biological substrate for the vulnerability to commit suicide. We and others have employed high-resolution quantitative autoradiography of full coronal sections of the prefrontal cortex to map the locus of maximal change in receptor binding. We found alterations in binding to the serotonin transporter, the 5-HT1A, and the 5-HT2A receptors primarily in the ventral and ventrolateral prefrontal cortex of suicide victims. Importantly, these changes are often modest in magnitude and anatomically restricted to one or two Brodmann areas. Furthermore, we have found that care in case selection is essential, because sex, age, drugs, and comorbid diagnoses contribute to receptor binding. The implications for in vivo imaging are considerable, directing the focus of such studies toward the ventrolateral prefrontal cortex. However, because ligands are limited, as is the resolution of current methods, including PET, automated analyses that produce statistical images, rather than manual selection of individual slices, will likely lack the ability to detect the discrete receptor changes found postmortem. Alternatively, the advantages of examining large numbers of subjects, imaging the entire brain, obtaining detailed clinical information in the living patient, and magnifying the changes with neuropharmacological challenges present a promising outlook for making major advances into the identification of brain abnormalities associated with suicide risk.