共查询到20条相似文献，搜索用时 46 毫秒
1.
Real-time synchrotron diffraction has been used to monitor the phase transformations of highly exothermic, fast self-propagating solid combustion reactions on a subsecond time scale down to 100 milliseconds and in some instances to 10 milliseconds. Three systems were investigated: Ti + C --> TiC; Ti + C + xNi --> TiC + Ni-Ti alloy; and Al + Ni --> AlNi. In all three reactions, the first step was the melting of the metal reactants. Formation of TiC in the first two reactions was completed within 400 milliseconds of the melting of the Ti metal, indicating that the formation of TiC took place during the passage of the combustion wave front. In the Al + Ni reaction, however, passage of the wave front was followed by the appearance and disappearance of at least one intermediate in the afterburn region. The final AlNi was formed some 5 seconds later and exhibited a delayed appearance of the (210) reflection, which tends to support a phase transformation from a disordered AlNi phase at high temperature to an ordered CsCl structure some 20 seconds later. This new experimental approach can be used to study the chemical dynamics of high-temperature solid-state phenomena and to provide the needed database to test various models for solid combustion. 相似文献
2.
Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events 总被引：16，自引：0，他引：16
Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity. 相似文献
3.
Electrical signs of selective attention in the human brain 总被引：32，自引：0，他引：32
Auditory evoked potentials were recorded from the vertex of subjects who listened selectively to a series of tone pips in one ear and ignored concurrent tone pips in the other ear. The negative component of the evoked potential peaking at 80 to 110 milliseconds was substantially larger for the attended tones. This negative component indexed a stimulus set mode of selective attention toward the tone pips in one ear. A late positive component peaking at 250 to 400 milliseconds reflected the response set established to recognize infrequent, higher pitched tone pips in the attended series. 相似文献
4.
The pulse structure of the four known pulsars is given. The pulse is about 38 milliseconds for the two pulsars of longest period, and within the pulsewidth three subpulses typically appear. The pulsar of next longest period typically radiates two pulses separated about 23 milliseconds in time. The one short-period pulsar emits single pulses of constant shape. The first subpulses of all pulsars have nearly the same shape. The shape of the first subpulse agrees well with the pulse shape expected from a radio-emitting sphere which is excited by a spherically expanding disturbance, and in which the radio emission, once excited, decays exponentially. 相似文献
5.
The temporal resolution of neuronal integration depends on the time window within which excitatory inputs summate to reach the threshold for spike generation. Here, we show that in rat hippocampal pyramidal cells this window is very narrow (less than 2 milliseconds). This narrowness results from the short delay with which disynaptic feed-forward inhibition follows monosynaptic excitation. Simultaneous somatic and dendritic recordings indicate that feed-forward inhibition is much stronger in the soma than in the dendrites, resulting in a broader integration window in the latter compartment. Thus, the subcellular partitioning of feed-forward inhibition enforces precise coincidence detection in the soma, while allowing dendrites to sum incoming activity over broader time windows. 相似文献
6.
Electrical reexcitation of rat papillary muscle after a short interval (50 to 80 milliseconds) results in action potentials with no significant mechanical counterpart. The mechanical response recovers as the interval is increased beyond 80 milliseconds. The rate of recovery is slowed at low external calcium levels. It appears that the coupling mechanism passes through a refractory stage owing to the depletion of an intracellular "releasable calcium" fraction. 相似文献
7.
An image of an unusual luminous electrical discharge over a thunderstorm 250 kilometers from the observing site has been obtained with a low-light-level television camera. The discharge began at the cloud tops at 14 kilometers and extended into the clear air 20 kilometers higher. The image, which had a duration of less than 30 milliseconds,resembled two jets or fountains and was probably caused by two localizd electric charge concentrations at the cloud tops. Large upward discharges may create a hazard for aircraft and rocket launches and, by penetrating into the ionosphere, may initiate whistler waves and other effects on a magnetospheric scale. Such upward electrical discharges may account for unexplained photometric observations of distant lightning events that showed a low rise rate of the luminous pulse and no electromagnetic sferic pulse of the type that accompanies cloud-to-earth lightning strokes. An unusually high rate of such photometric events was recorded during the night of 22 to 23 September 1989 during a storm associated with hurricane Hugo. 相似文献
8.
It has been hypothesized that rats and other mammals can use stereo cues to localize odor sources, but there is limited behavioral evidence to support this hypothesis. We found that rats trained on an odor-localization task can localize odors accurately in one or two sniffs. Bilateral sampling was essential for accurate odor localization, with internasal intensity and timing differences as directional cues. If the stimulus arrived at the correct point of the respiration cycle, internasal timing differences as short as 50 milliseconds sufficed. Neuronal recordings show that bulbar neurons responded differentially to stimuli from the left and stimuli from the right. 相似文献
9.
The latency of pathways containing the site of motor learning in the monkey vestibulo-ocular reflex 总被引：1，自引：0，他引：1
S G Lisberger 《Science (New York, N.Y.)》1984,225(4657):74-76
The vestibulo-ocular reflex helps to stabilize retinal images by generating smooth eye movements that are equal to and opposite each rotatory head movement. It is well known that the reflex undergoes adaptive plasticity or "motor learning" whenever there is persistent image motion during head turns: the resulting changes in the reflex occur gradually and help to restore image stability. A new approach makes it possible to identify the pathways containing the site of motor learning according to their total latency in response to natural vestibular stimuli. The fastest pathways required 14 milliseconds to initiate a vestibulo-ocular reflex, but the site of motor learning was in pathways having latencies of at least 19 milliseconds. 相似文献
10.
Localized temporal change of the Earth's inner core boundary 总被引：1，自引：0，他引：1
Wen L 《Science (New York, N.Y.)》2006,314(5801):967-970
Compressional waves of an earthquake doublet (two events occurring in the South Sandwich Islands on 1 December 1993 and 6 September 2003), recorded at three seismic stations in Russia and Kyrgyzstan and reflected off Earth's inner core boundary, arrived at least from 39 to 70 milliseconds earlier in the 2003 event than in the 1993 event. Such changes indicate that Earth's inner core radius enlarged locally beneath middle Africa by 0.98 to 1.75 kilometers between the times of these two events. Changes of the inner core radius may be explained by either a differential motion of the inner core, assuming that irregularities are present at the inner core boundary and fixed to the inner core, or a rapid growth of the inner core by this amount. 相似文献
11.
Palmer LM Schulz JM Murphy SC Ledergerber D Murayama M Larkum ME 《Science (New York, N.Y.)》2012,335(6071):989-993
Interhemispheric inhibition is thought to mediate cortical rivalry between the two hemispheres through callosal input. The long-lasting form of this inhibition is believed to operate via γ-aminobutyric acid type B (GABA(B)) receptors, but the process is poorly understood at the cellular level. We found that the firing of layer 5 pyramidal neurons in rat somatosensory cortex due to contralateral sensory stimulation was inhibited for hundreds of milliseconds when paired with ipsilateral stimulation. The inhibition acted directly on apical dendrites via layer 1 interneurons but was silent in the absence of pyramidal cell firing, relying on metabotropic inhibition of active dendritic currents recruited during neuronal activity. The results not only reveal the microcircuitry underlying interhemispheric inhibition but also demonstrate the importance of active dendritic properties for cortical output. 相似文献
12.
Brill TM Ryu S Gaylor R Jundt J Griffin DD Song YQ Sen PN Hürlimann MD 《Science (New York, N.Y.)》2002,297(5580):369-372
Nonresonant manipulation of nuclear spins can probe large volumes of sample situated in inhomogeneous fields outside a magnet, a geometry suitable for mobile sensors for the inspection of roads, buildings, and geological formations. However, the interference by Earth's magnetic field causes rapid decay of the signal within a few milliseconds for protons and is detrimental to this method. Here we describe a technique to suppress the effects of Earth's field by using adiabatic rotations and sudden switching of the applied fields. We observed hundreds of spin echo signals lasting for more than 600 milliseconds and accurately measured the relaxation times of a liquid sample. 相似文献
13.
The dynamics of free calcium in dendritic spines in response to repetitive synaptic input 总被引：10，自引：0，他引：10
Increased levels of intracellular calcium at either pre- or postsynaptic sites are thought to precede changes in synaptic strength. Thus, to induce long-term potentiation in the hippocampus, periods of intense synaptic stimulation would have to transiently raise the levels of cytosolic calcium at postsynaptic sites--dendritic spines in the majority of cases. Since direct experimental verification of this hypothesis is not possible at present, calcium levels have been studied by numerically solving the appropriate electro-diffusion equations for two different postsynaptic structures. Under the assumption that voltage-dependent calcium channels are present on dendritic spines, free intracellular calcium in spines can reach micromolar levels after as few as seven spikes in 20 milliseconds. Moreover, a short, but high-frequency, burst of presynaptic activity is more effective in raising levels of calcium and especially of the calcium-calmodulin complex than sustained low-frequency activity. This behavior is different from that seen at the soma of a typical vertebrate neuron. 相似文献
14.
《Science (New York, N.Y.)》1983,221(4608):320
In the article "the 0.001557806449023-Second Pulsar" by M. Mitchell Waldrop (Research News, 18 Feb., p. 831), the last sentence of the last full paragraph in the second column should have read: "Within a few weeks he was in Aricebo himself, and on 14 November he verified pulses at an astonishing 642 hertz, equivalent to a period of 1.558 milliseconds." 相似文献
15.
N Y Alexeenko 《Science (New York, N.Y.)》1970,167(920):1009-1010
Monaural change of the duration of a binaural acoustic stimulus within the range of 0.5 to 1.5 milliseconds caused a considerable shift of the sound image. This could be counterbalanced by a unilateral change of the signal intensity. Further unilateral lengthening of the stimulus did not affect the sound localization, but it revealed a masking aftereffect (4 to 10 milliseconds) of the binaural signal. The data define the temporal characteristics of binaural effect influenced the binaural interaction. 相似文献
16.
Robertson DS Carter WE Tapley BD Schutz BE Eanes RJ 《Science (New York, N.Y.)》1985,229(4719):1259-1261
An important bound on the accuracy of modern techniques for monitoring polar motion is established by intercomparison of measurement series from two different observing techniques, very long baseline interferometry and satellite laser ranging. The root-mean-square differences between the estimates of the pole position from both techniques are shown to be only 2 milliseconds of arc (about 6 centimeters at one Earth radius). In the absence of common systematic errors, these differences bound the total errors in both sets of estimates. An initial investigation did not reveal any clear signature in the pole position that seems to be associated with major earthquakes. Continued measurements at this level of accuracy hold promise for resolving long-standing arguments over such questions as the nature of the excitation mechanism required to maintain the motion of the pole. 相似文献
17.
Single-shot channel activation accounts for duration of inhibitory postsynaptic potentials in a central neuron 总被引：4，自引：0，他引：4
In the goldfish Mauthner cell, inhibitory postsynaptic currents evoked by intracellular stimulation of presynaptic neurons decay exponentially, with a mean time constant of 6.65 milliseconds. Analysis of membrane conductance fluctuations induced by iontophoresis of glycine and gamma-aminobutyric acid indicates a mean inhibitory channel lifetime of 7.15 milliseconds. The results thus suggest that the relaxation kinetics of activated inhibitory channels are rate-limiting during decay of the inhibitory postsynaptic current. 相似文献
18.
The human brain is found to produce a magnetic field near the scalp which varies in synchrony with periodic electrical stimulation applied to a finger. Use of a highly sensitive superconducting quantum interference device as a magnetic field detector reveals that the brain's field is sharply localized over the primary projection area of the sensory cortex contralateral to the digit being stimulated. The phase of the response at the stimulus frequency varies monotonically with the repetition rate and at intermediate frequencies yields a latency of approximately 70 milliseconds for cortical response. 相似文献
19.
Tobacco and evoked potential 总被引：2，自引：0，他引：2
Significant changes were found in two indices of the averaged visual evoked potentials in nine smokers after 12 and 36 hours of abstinence and after resumption of smoking. There was a decrease of the amplitude envelope accompanying withdrawal and an increase with resumption of smoking. These changes are consistent with the contention that tobacco increases arousal. Amplitude changes were found in a specific component of the evoked potential occurring between 100 and 125 milliseconds after the onset of the flash. The latter changes suggest the possibility that smoking selectively enhances the perceptiont of weak stimuli. 相似文献
20.
MR Andrews CG Townsend H Miesner DS Durfee DM Kurn W Ketterle 《Science (New York, N.Y.)》1997,275(5300):637-641
Interference between two freely expanding Bose-Einstein condensates has been observed. Two condensates separated by approximately 40 micrometers were created by evaporatively cooling sodium atoms in a double-well potential formed by magnetic and optical forces. High-contrast matter-wave interference fringes with a period of approximately 15 micrometers were observed after switching off the potential and letting the condensates expand for 40 milliseconds and overlap. This demonstrates that Bose condensed atoms are "laser-like"; that is, they are coherent and show long-range correlations. These results have direct implications for the atom laser and the Josephson effect for atoms. 相似文献