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Jutta M. Ellermann Joel D. Siegal John P. Strupp Timothy J. Ebner Kâmil Ugurbil 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,131(2):272-285
The dorsal stream is a dominant visuomotor pathway that connects the striate and extrastriate cortices to posterior parietal areas. In turn, the posterior parietal areas send projections to the frontal primary motor and premotor areas. This cortical pathway is hypothesized to be involved in the transformation of a visual input into the appropriate motor output. In this study we used functional magnetic resonance imaging (fMRI) of the entire brain to determine the patterns of activation that occurred while subjects performed a visually guided motor task. In nine human subjects, fMRI data were acquired on a 4-T whole-body MR system equipped with a head gradient coil and a birdcage RF coil using aT*2-weighted EPI sequence. Functional activation was determined for three different tasks: (1) a visuomotor task consisting of moving a cursor on a screen with a joystick in relation to various targets, (2) a hand movement task consisting of moving the joystick without visual input, and (3) a eye movement task consisting of moving the eyes alone without visual input. Blood oxygenation level-dependent (BOLD) contrast-based activation maps of each subject were generated using period cross-correlation statistics. Subsequently, each subject's brain was normalized to Talairach coordinates, and the individual maps were compared on a pixel by pixel basis. Significantly activated pixels common to at least four out of six subjects were retained to construct the final functional image. The pattern of activation during visually guided movements was consistent with the flow of information from striate and extrastriate visual areas, to the posterior parietal complex, and then to frontal motor areas. The extensive activation of this network and the reproducibility among subjects is consistent with a role for the dorsal stream in transforming visual information into motor behavior. Also extensively activated were the medial and lateral cerebellar structures, implicating the cortico–ponto–cerebellar pathway in visually guided movements. Thalamic activation, particularly of the pulvinar, suggests that this nucleus is an important subcortical target of the dorsal stream. 相似文献
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A. Tönnißen J. Wanner K. W. Rothe H. Walther 《Applied Physics A: Materials Science & Processing》1979,18(3):297-304
The measurement of the concentration of gaseous hydrogen fluoride in the exhaust of an aluminum plant by resonance absorption
of the radiation of a cw chemical HF laser is reported. The HF concentration has been evaluated taking into account the simultaneous
absorption of water vapor and carbon dioxide at the HF laser frequencies. The absorption cross sections for H2O and CO2 at the HF laser lines have been determined in laboratory experiments. 相似文献
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Electrons on the surface of liquid 4He become localized in macroscopic dimples when the electric field perpendicular to the surface exceeds a critical value. These dimples, each holding about 107 electrons, form a two-dimensional hexagonal lattice. 相似文献
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Rodenko B Koch M van der Burg AM Wanner MJ Koomen GJ 《Journal of the American Chemical Society》2005,127(16):5957-5963
Modified purine derivatives are of great importance in biomedical sciences, and substitution reactions on the purine skeleton are intensively studied. In our laboratory, an efficient and selective purine C2-nitration reaction was developed using a mixture of tetrabutylammonium nitrate and trifluoroacetic anhydride. The resulting 2-nitro moiety appeared to be a versatile handle to introduce a variety of pharmacophores onto the purine skeleton. Since the mechanism of this selective purine C2-nitration reaction has remained unclear, we now present an extensive NMR study leading to its elucidation, using N9-Boc-protected 6-chloropurine as a model compound. Direct electrophilic aromatic nitration of the highly electron-deficient C2 position was excluded, and we demonstrate that this reaction occurs in a three-step process. Electrophilic attack by trifluoroacetyl nitrate on the purine N7 position results in a nitrammonium species that is trapped by a trifluoroacetate anion furnishing N7-nitramine intermediate 11. This intermediate was characterized at -50 degrees C by (1)H, (13)C, (15)N, and (19)F NMR. At T > -40 degrees C, the N7-nitramine intermediate undergoes a nitramine rearrangement, which generates a C2-nitro species that immediately eliminates TFA to give 2-nitro-6-chloro-9-Boc purine 10. The involvement of radicals during the nitramine rearrangement was unequivocally established by (15)N-CIDNP. Moreover, the emission signal observed for the rearranged product, 2-nitropurine 10, showed that it is primarily formed in an intermolecular process. A quantitative radical trapping experiment finally disclosed that 65-70% of the nitramine rearrangement takes place intermolecularly. 相似文献