15,666 research outputs found
Existing motor state is favored at the expense of new movement during 13-35 Hz oscillatory synchrony in the human corticospinal system
Oscillations in local field potentials in the β-frequency band (13-35 Hz) are a pervasive feature of human and nonhuman primate motor cortical areas. However, the function of such synchronous activity across populations of neurons remains unknown. Here, we test the hypothesis that β activity may promote existing motor set and posture while compromising processing related to new movements. Three experiments were performed. First, healthy subjects were instructed to make reaction time movements of the outstretched index finger in response to imperative cues triggered by transient increases in corticospinal synchrony, as evidenced by phasic elevations of β-frequency band microtremor and intermuscular synchrony. Second, healthy subjects were instructed to resist a stretch to the index finger triggered in the same way. Finger acceleration in the reaction time task and transcortical components of the stretch reflex were measured and compared with those elicited by random cue or stretch presentation. Finally, we sought a correlation between finger acceleration in the reaction time task and cortical synchrony directly measured from the electrocorticogram in two patients undergoing functional neurosurgery. We demonstrate that movements are slowed and transcortical responses to stretch are potentiated during periods of elevated β-band cortical synchrony. The results suggest that physiological periods of β synchrony are associated with a cortical state in which postural set is reinforced, but the speed of new movements impaired. The findings are of relevance to Parkinson's disease, in which subcortical and cortical β-band synchronization is exaggerated in the setting of increased tone and slowed movements
GaAs(111)A and B in hydrazine sulfide solutions : extreme polarity dependence of surface adsorption processes
Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were
studied by synchrotron photoemission spectroscopy. At the B surface, the top
arsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by
sulfur, also bonded to underlying gallium, despite the sulfide molar
concentration being 103 times smaller than that of the hydrazine. This extreme
dependence on surface polarity is explained by competitive adsorption processes
of HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites,
which have distinct configurations on the A and B surfaces
Valence and magnetic instabilities in Sm compounds at high pressures
We report on the study of the response to high pressures of the electronic
and magnetic properties of several Sm-based compounds, which span at ambient
pressure the whole range of stable charge states between the divalent and the
trivalent. Our nuclear forward scattering of synchrotron radiation and specific
heat investigations show that in both golden SmS and SmB6 the pressure-induced
insulator to metal transitions (at 2 and about 4-7 GPa, respectively) are
associated with the onset of long-range magnetic order, stable up to at least
19 and 26 GPa, respectively. This long-range magnetic order, which is
characteristic of Sm(3+), appears already for a Sm valence near 2.7. Contrary
to these compounds, metallic Sm, which is trivalent at ambient pressure,
undergoes a series of pressure-induced structural phase transitions which are
associated with a progressive decrease of the ordered 4f moment.Comment: 15 pages (including 7 figures) submitted to J. Phys.: Condens. Matte
Simulation Studies of Nanomagnet-Based Architecture
We report a simulation study on interacting ensembles of Co nanomagnets that
can perform basic logic operations and propagate logic signals, where the state
variable is the magnetization direction. Dipole field coupling between
individual nanomagnets drives the logic functionality of the ensemble and
coordinated arrangements of the nanomagnets allow for the logic signal to
propagate in a predictable way. Problems with the integrity of the logic signal
arising from instabilities in the constituent magnetizations are solved by
introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each
nanomagnet. The enhanced stability allows for more complex components of a
logic architecture capable of random combinatorial logic, including horizontal
wires, vertical wires, junctions, fanout nodes, and a novel universal logic
gate. Our simulations define the focus of scaling trends in nanomagnet-based
logic and provide estimates of the energy dissipation and time per nanomagnet
reversal
Spitzer 3.6 micron and 4.5 micron full-orbit lightcurves of WASP-18
We present new lightcurves of the massive hot Jupiter system WASP-18 obtained
with the Spitzer spacecraft covering the entire orbit at 3.6 micron and 4.5
micron. These lightcurves are used to measure the amplitude, shape and phase of
the thermal phase effect for WASP-18b. We find that our results for the thermal
phase effect are limited to an accuracy of about 0.01% by systematic noise
sources of unknown origin. At this level of accuracy we find that the thermal
phase effect has a peak-to-peak amplitude approximately equal to the secondary
eclipse depth, has a sinusoidal shape and that the maximum brightness occurs at
the same phase as mid-occultation to within about 5 degrees at 3.6 micron and
to within about 10 degrees at 4.5 micron. The shape and amplitude of the
thermal phase curve imply very low levels of heat redistribution within the
atmosphere of the planet. We also perform a separate analysis to determine the
system geometry by fitting a lightcurve model to the data covering the
occultation and the transit. The secondary eclipse depths we measure at 3.6
micron and 4.5 micron are in good agreement with previous measurements and
imply a very low albedo for WASP-18b. The parameters of the system (masses,
radii, etc.) derived from our analysis are in also good agreement with those
from previous studies, but with improved precision. We use new high-resolution
imaging and published limits on the rate of change of the mean radial velocity
to check for the presence of any faint companion stars that may affect our
results. We find that there is unlikely to be any significant contribution to
the flux at Spitzer wavelengths from a stellar companion to WASP-18. We find
that there is no evidence for variations in the times of eclipse from a linear
ephemeris greater than about 100 seconds over 3 years.Comment: 17 pages, 10 figures. Accpeted for publication in MNRA
Electrostatics of ions inside the nanopores and trans-membrane channels
A model of a finite cylindrical ion channel through a phospholipid membrane
of width separating two electrolyte reservoirs is studied. Analytical
solution of the Poisson equation is obtained for an arbitrary distribution of
ions inside the trans-membrane pore. The solution is asymptotically exact in
the limit of large ionic strength of electrolyte on the two sides of membrane.
However, even for physiological concentrations of electrolyte, the
electrostatic barrier sizes found using the theory are in excellent agreement
with the numerical solution of the Poisson equation. The analytical solution is
used to calculate the electrostatic potential energy profiles for pores
containing charged protein residues. Availability of a semi-exact interionic
potential should greatly facilitate the study of ionic transport through
nanopores and ion channels
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