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Major theories of preattentive human texture perception due to Julesz [5,9] and to Beck [1,2] attribute preattentive texture discrimination to differences in first-order statistics of stimulus features such as orientation, size and brightness of constituent elements. These theories have typically been constructed for black and white dot or line patterns and are not directly applicable to grey scale images. An alternative approach [6,15, 4] has been to exploit the linear mechanisms (psychophysically observed spatial frequency channels and neurophysiologically observed blob, bar- and edge- sensitive neurons) which have been used to explain a range of phenomena in early spatial vision. Some experiments [4,3] suggest that this approach may explain texture perception better than the more symbolic, feature based approach of Beck and Julesz. However no scheme in this framework has been fully specified, implemented and successfully tested. The crucial experimental tests are the following: Does the model correctly predict the texture boundaries found preattentively by human observers, both in images of natural scenes and the synthetic stimuli from psychophysics literature? Even better, does it correctly predict the degree of discriminability for different texture pairs as measured by psychophysical experiments?
The intermolecular addition of the alpha-C-H bonds of unactivated dialkylamines to unactivated olefins in the presence of the chloro amido complex [TaCl3(NEt2)2]2 (2) is described. This process forms the branched insertion products in high yields (up to 96%) and selectivities, and represents a rare example of an intermolecular amine-olefin coupling reaction that does not require preactivation of either substrate. The reaction is shown to encompass the addition of the primary C-H bonds in linear- and branched-methylamines, as well as secondary C-H bonds in higher dialkylamines. The related chloroanilido complex [TaCl3(NMePh)2]2 (4) is also shown to catalyze the addition of N-alkyl-arylamines to olefins at temperatures as low as 90 degreesC. 1H NMR spectroscopy, identification of the catalyst structure, and deuterium-labeling experiments all suggest that reactions catalyzed by 2 and 4 occur by turnover-limiting generation of an eta2-imine complex. These labeling studies also imply that more favorable partitioning of the eta2-imine complex toward reaction with alkene versus regeneration of the starting bis-amido complex accounts for the higher reactivity of the mixed halide amido catalyst versus a homoleptic amido complex.
A unified black box approach is presented for synthesizing nonlinear dc circuit models of 3-terminal devices characterized by two families of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon-i</tex> curves in piecewise-linear form. In order to model arbitrary curves with nonuniform spacings and slopes, three simple but versatile building blocks-a controlled linear resistance, a controlled concave resistor, and a controlled convex resistor-are introduced and shown to be essential ingredients. The circuit parameters and functions characterizing each element in the black box model can be determined easily from the slopes and breakpoints associated with the segments of the prescribed characteristic curves. The paper concludes with the presentation of several nonlinear dc circuit models for four widely used devices; namely, bipolar transistors, field-effect transistors (FET's), unijunction transistors, and triacs.
We have placed the genes encoding ribulose-bisphosphate carboxylase/oxygenase from the Anabaena 7120 operon under transcriptional control of the lac promoter carried on the Escherichia coli plasmid pUC19. The genes encoding both the large and small subunit polypeptides ( rbcL and rbcS ) are transcribed and translated so that ≈0.6% of the soluble protein in E. coli extracts is a fully functional holoenzyme with a sedimentation coefficient of approximately 18S, which contains stoichiometric amounts of the two subunits. However, expression of the large subunit polypeptide vastly exceeds that of the small subunit because the majority of transcripts terminate in the intergenic region between the rbcL and rbcS genes. As a result, excess large subunit is synthesized and accumulates in E. coli as an insoluble and catalytically inactive form. Because small subunit is found only in the high molecular weight soluble form of ribulosebisphosphate carboxylase/oxygenase, we propose that the small subunit promotes assembly of the hexadecameric form of the enzyme via heterodimers of large and small subunits.
Abstract The electron‐rich title complex (II) is formed by a process typical of other low‐valent late transition‐metal complexes but its reactions are either unprecedented or similar to those seen with electrophilic d0 benzyne complexes (see scheme).
ABSTRACT Fluxes and concentrations of carbon dioxide and 13 CO 2 provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of 13 CO 2 within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model ( CANISOTOPE ) to simulate concentration profiles of 13 CO 2 , its flux density across the canopy–atmosphere interface and leaf‐level photosynthetic discrimination against 13 CO 2 is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against 13 CO 2 , by the deciduous forest canopy (Δ canopy ), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δ canopy occurred early and late in the growing season. Diurnal and seasonal variations in Δ canopy resulted from a strong dependence of Δ canopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy‐scale water use efficiency (CO 2 assimilation/transpiration) and Δ canopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well‐mixed environments.
