6,187 research outputs found
Representational momentum in the motor system?
PURPOSE: If presented with a moving object which suddenly disappears observers usually misjudge the object's last seen position as being further forward along the path of motion. This effect, called representational momentum, can also be seen in objects that change size or shape. It has been argued that the effect is due to perceptual anticipation. We tested whether a similar effect is present in the motor system. METHODS: Using stereo computer graphics we presented cubes of different sizes on a CRT monitor. In each trial three cubes were successively presented for 200 msec with increasing or decreasing size (steps of 1 cm width difference). Ten participants either compared the last cube to a comparison cube (perceptual task) or grasped the cube using a virtual haptic setup (motor task). The setup consisted of two robot arms (Phantom TM) attached to index finger and thumb. The robot arms were controlled to create forces equivalent to the forces created by real objects. The CRT monitor was viewed via a mirror such that the visual position of the cubes matched the position of the virtual haptic objects. RESULTS: In the motor task participants opened their fingers by 1.1+/-0.4 mm wider if they grasped a cube that was preceded by smaller cubes than if they grasped a cube that was preceded by larger cubes. This is the well-known representational momentum effect. In the perceptual task the effect was reversed (-2.2+/-0.4 mm). The effects correlated between observers (r=.71, p=.02). CONCLUSIONS: It seems that a representational momentum occurs also in grasping tasks. The correlation between observers suggests that the motor effect is related to the perceptual effect. However, our perceptual task showed a reversed effect. Reasons for this discrepancy will be discussed
L\'evy Processes on as Infinitely Divisible Representations
L\'evy processes on bialgebras are families of infinitely divisible
representations. We classify the generators of L\'evy processes on the compact
forms of the quantum algebras , where is a simple Lie algebra. Then
we show how the processes themselves can be reconstructed from their generators
and study several classical stochastic processes that can be associated to
these processes.Comment: 13 pages, LATEX file, ASI-TPA/13/99 (TU Clausthal); 6/99
(Preprint-Reihe Mathmatik, Univ. Greifswald)
On chaos in mean field spin glasses
We study the correlations between two equilibrium states of SK spin glasses
at different temperatures or magnetic fields. The question, presiously
investigated by Kondor and Kondor and V\'egs\"o, is approached here
constraining two copies of the same system at different external parameters to
have a fixed overlap. We find that imposing an overlap different from the
minimal one implies an extensive cost in free energy. This confirms by a
different method the Kondor's finding that equilibrium states corresponding to
different values of the external parameters are completely uncorrelated. We
also consider the Generalized Random Energy Model of Derrida as an example of
system with strong correlations among states at different temperatures.Comment: 19 pages, Late
Complex order parameter symmetry and thermal conductivity
Thermal behaviour of superconductors with complex order parameter symmetry is
studied within a weak coupling theory. It is shown numerically, that the
thermal nature of the different components of complex order parametrs are
qualitatively different. Within the complex order parameter scenario, the
recent experimental observations by Krishna {\it et al.}, [Science {\bf 277},
83 (1997)] on magnetothermal conductivity and by J. Ma {\it et al.},
[Science {\bf 267}, 862 (1995)] on temperature dependent gap anisotropy for
high temperature superconductors can have natural explanation.Comment: 6 pages, 3 figures and macros attached, Europhysics Letters (1998) in
pres
Entropy-based measure of structural order in water
We analyze the nature of the structural order established in liquid TIP4P
water in the framework provided by the multi-particle correlation expansion of
the statistical entropy. Different regimes are mapped onto the phase diagram of
the model upon resolving the pair entropy into its translational and
orientational components. These parameters are used to quantify the relative
amounts of positional and angular order in a given thermodynamic state, thus
allowing a structurally unbiased definition of low-density and high-density
water. As a result, the structurally anomalous region within which both types
of order are simultaneously disrupted by an increase of pressure at constant
temperature is clearly identified through extensive molecular-dynamics
simulations.Comment: 5 pages, 2 figures, to appear in Phys. Rev. E (Rapid Communication
How do we grasp (virtual) objects in three-dimensional space?
Jeannerod (1981,1984) studied extensively the relationships between object size and grasping parameters, which has been influential for the interpretation of grasping data. The maximum grip aperture (MGA) scales linearly with object size, but the slope is less than 1 (app. 0.82, cf. Smeets Brenner 99). Here, we investigated if the location of the object in three-dimensional space influences the MGA. As well we addressed the question if the grasping of virtual objects shows the same characteristics as natural prehension. Virtual environments could enable experimenters to easily vary objects after the movement onset and therefore to explore the mechanisms of online control in visually guided movements. A virtual disc (36, 40, or 44 mm in diameter) was rendered using stereo computer graphics in 27 positions in different heights and locations relative to the observer. Virtual, haptic feedback was given using two robot arms (PHANToM TM). One robot arm was connected to the index finger, one to the thumb. Ten participants grasped the discs and transported them to a goal area, where they dropped the discs. The stereoscopically rendered discs were viewed through a mirror, such that the visual and haptic feedback matched. The position of the finger tips was measured using the two robot arms and an Optotrak (TM), in order to test for the accuracy of the PHANToM devices. The MGA was dependent on the distance of the object with respect to the observers body but not on the height of the disc. Participants scaled their MGA according to the size of the virtual disc, but with a slightly smaller slope (0.64+/−0.06) compared to natural environments. This could indicate that tactile feedback (in addition to haptic feedback) is needed to perform natural grasping movements
Syntheses of small cluster oligosaccharide mimetics
We designed multiple Small Cluster Oligosaccharide Mimetics (SCOMs) - potential glycosidase inhibitors - to be metabolically stable and small enough to enter cells or bacteria. Therefore, minimal scaffolds (urea, amide, ammonia) or simply non-glycosidic linkages of carbohydrate structures were central to our synthetic strategy, including: (a) coupling of several natural carbohydrate precursors; (b) total syntheses of aminomethyl tetrahydropyrans and their chiral amides with quinic acid; (c) glycopyranosyl cyanide reduction to prepare crowded clusters on a urea scaffold; (d) total syntheses via cycloadditions leading to amide-linked C-glycosides; (e) reduction of nitromethyl C-glycosides; and (f) a synthesis of hydroxylated 1,2- cyclohexanedicarboxylic acids
Quasiparticle interference patterns as a test for the nature of the pseudogap phase in the cuprate superconductors
Electrons, when scattered by static random disorder, form standing waves that
can be imaged using scanning tunneling microscopy. Such interference patterns,
observable by the recently developed technique of Fourier transform scanning
tunneling spectroscopy (FT-STS), are shown to carry unique fingerprints
characteristic of the electronic order present in a material. We exploit this
feature of the FT-STS technique to propose a test for the nature of the
enigmatic pseudogap phase in the high- cuprate superconductors. Through
their sensitivity to the quasiparticle spectra and coherence factors, the
FT-STS patterns in principle carry enough information to unambiguously
determine the nature of the condensate responsible for the pseudogap
phenomenon. We argue that the next generation of FT-STS experiments, currently
underway, should be able to distinguish between the pseudogap dominated by the
remnants of superconducting order from the pseudogap dominated by some
competing order in the particle-hole channel. Using general arguments and
detailed numerical calculations, we point to certain fundamental differences
between the two scenarios and discuss the prospects for future experiments.Comment: 15 pages REVTeX + 9 ps figures. For related work and info visit
http://www.physics.ubc.ca/~franz; version 2 to appear in IJMP
On the integral cohomology of smooth toric varieties
Let be a smooth, not necessarily compact toric variety. We show
that a certain complex, defined in terms of the fan , computes the
integral cohomology of , including the module structure over the
homology of the torus. In some cases we can also give the product. As a
corollary we obtain that the cycle map from Chow groups to integral Borel-Moore
homology is split injective for smooth toric varieties. Another result is that
the differential algebra of singular cochains on the Borel construction of
is formal.Comment: 10 page
Excitonic Dynamical Franz-Keldysh Effect
The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap
absorption in the presence of intense THz electric fields. It bridges the gap
between the DC Franz- Keldysh effect and multi-photon absorption and competes
with the THz AC Stark Effect in shifting the energy of the excitonic resonance.
A theoretical model which includes the strong THz field non-perturbatively via
a non-equilibrium Green Functions technique is able to describe the Dynamical
Franz-Keldysh Effect in the presence of excitonic absorption.Comment: 4 pages in revtex with 5 figures included using epsf. Submitted to
Physical Review Letter
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