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321.
Hutchinson M Schiffer W Joseffer S Liu A Schlosser R Dikshit S Goldberg E Brodie JD 《Magnetic resonance imaging》1999,17(10):1808-1436
In general, image analysis of cognitive experiments using functional magnetic resonance imaging techniques has emphasized those regions of the brain where increases in signal intensity, with regard to the reference state, are associated with activation. Nevertheless, a number of recent papers have shown that there are areas of deactivation as well. In this study, we have used a univariate analysis and echo-planar functional magnetic resonance imaging to address the relationship of the reference state to the deactivations. We employed two dichotomous covert tasks, orthographic lexical retrieval and pure visual retrieval, to contrast with the reference state (baseline) of silent counting. Our analysis yielded extensive, task-specific landscapes of regional incremental and decremental responses. We have specifically demonstrated that the decremental responses are not due to activation in the reference state. We have also demonstrated that they are not an artifact of a specific part of the image analysis, and propose that they represent a physiological, task specific signal that should be considered an integral component of neural networks representing brain function. 相似文献
322.
323.
We prove C1, almost everywhere regularity for minimisers of functionals of the form F(x,u,Du), where F is uniformly strictly quasiconvex. This extends a recent result of Evans in which F is allowed only to depend on Du. 相似文献
324.
N.J. McCorkindale C.H. Calzadilla S.A. Hutchinson D.H. Kitson G. Ferguson I.M. Campbell 《Tetrahedron》1981,37(3):649-653
The structure of three new drimane sesquiterpenes (1–3) has been established from chemical and spectroscopic evidence and by single crystal X-ray crystallographic analysis of 7. Ring B in the crystal of 7 is in a chair conformation, slightly distorted because of cis fusion to the lactone ring and because three β axial substituents are present. NMR evidence suggests that the preferred conformation in solution is similar. 相似文献
325.
Iria Fraga S. Montserrat J. M. Hutchinson 《Journal of Thermal Analysis and Calorimetry》2007,87(1):119-124
TOPEM is a new temperature modulated DSC technique, introduced by Mettler-Toledo
in late 2005, in which stochastic temperature modulations are superimposed
on the underlying rate of a conventional DSC scan. These modulations consist
of temperature pulses, of fixed magnitude and alternating sign, with random
durations within limits specified by the user. The resulting heat flow signal
is analysed by a parameter estimation method which yields a so-called ‘quasi-static’
specific heat capacity and a ‘dynamic’ specific heat capacity
over a range of frequencies. In a single scan it is thus possible to distinguish
frequency-dependent phenomena from frequency-independent phenomena. Its application
to the glass transition is examined here. 相似文献
326.
Studying the Glass Transition by DSC and TMDSC 总被引:5,自引:0,他引:5
First, the principal features of the glass transformation process in polymers are reviewed, and then it is shown how they
are manifest in conventional DSC, and the quantitative analysis of typical DSC data is discussed in terms of the Tool-Narayanaswamy-Moynihan
(TNM) model. Subsequently, the way in which the glass transition is manifest in Temperature Modulated DSC is presented, and
the effects of both experimental and material parameters are discussed. In conclusion, the two techniques are compared in
terms of the information they provide about the glass transformation process.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
327.
1-Chloro-1-(2′,3′,5′,6′-tetrafluoropyridyl)tetrafluoroethane, obtained by reaction of chlorotrifluoroethylene with pentafluoropyridine in the presence of caesium fluoride, was dehalogenated over iron at 310°, giving perfluoro-4-vinylpyridine (IV). Reactions of (IV) have been surveyed; nucleophilic attack occurs at both the ring and the difluoromethylene group using MeO- but with PhO-, selective attack at the difluoromethylene group occurred. A dimer is produced by reaction of (IV) with CsF and the intermediate carbanion was trapped with tetrafluoropyridazine. Thermal cyclodimerisation of (IV) gave a single isomer whose structure was established by n.m.r. and by defluorination to a cyclobutene. Cyclobutanes were formed in thermal cycloaddition reactions with chlorotrifluoroethylene and with hexafluoropropene. Photochemical reaction of (IV) with hexafluoroacetone gave an oxetane. 相似文献
328.
The small magnitude and positive sign of the sodium atom hyperfine splitting constant of the Na+furil? radical triple ion is interpreted by elementary MO arguments. The model gives some insight into the nature of the Na—O coordination in such species. 相似文献
329.
330.
Sirtori C Gmachl C Capasso F Faist J Sivco DL Hutchinson AL Cho AY 《Optics letters》1998,23(17):1366-1368
Laser waveguides based on surface plasmons at a metal-semiconductor interface have been demonstrated by use of quantum cascade (QC) lasers emitting in the 8-11.5-microm wavelength range. The guided modes are transverse magnetic polarized surface waves that propagate at the metal (Pd or Ti-Au)-semiconductor interface between the laser top contact and the active region without the necessity for waveguide cladding layers. The resultant structure has the advantages of a strong decrease in the total layer thickness and a higher confinement factor of the laser-active region compared with those of a conventional layered semiconductor waveguide, and strong coupling to the active material, which could be used in devices such as distributed-feedback lasers. These advantages have to be traded against the disadvantage of increased absorption losses. A peak output power exceeding 25 mW at 90 K and a maximum operating temperature of 150 K were measured for a QC laser with an emission wavelength lambda approximately 8 microm . At lambda approximately 11.5 microm the peak power levels are several milliwatts and the maximum operating temperature is 110 K. 相似文献