24 research outputs found
Neural bases for individual differences in the subjective experience of short durations (less than 2 seconds).
The current research was designed to establish whether individual differences in timing performance predict neural activation in the areas that subserve the perception of short durations ranging between 400 and 1600 milliseconds. Seventeen participants completed both a temporal bisection task and a control task, in a mixed fMRI design. In keeping with previous research, there was increased activation in a network of regions typically active during time perception including the right supplementary motor area (SMA) and right pre-SMA and basal ganglia (including the putamen and right pallidum). Furthermore, correlations between neural activity in the right inferior frontal gyrus and SMA and timing performance corroborate the results of a recent meta-analysis and are further evidence that the SMA forms part of a neural clock that is responsible for the accumulation of temporal information. Specifically, subjective lengthening of the perceived duration were associated with increased activation in both the right SMA (and right pre-SMA) and right inferior frontal gyrus
Time singularities of correlators from Dirichlet conditions in AdS/CFT
Within AdS/CFT, we establish a general procedure for obtaining the leading
singularity of two-point correlators involving operator insertions at different
times. The procedure obtained is applied to operators dual to a scalar field
which satisfies Dirichlet boundary conditions on an arbitrary time-like surface
in the bulk. We determine how the Dirichlet boundary conditions influence the
singularity structure of the field theory correlation functions. New
singularities appear at boundary points connected by null geodesics bouncing
between the Dirichlet surface and the boundary. We propose that their
appearance can be interpreted as due to a non-local double trace deformation of
the dual field theory, in which the two insertions of the operator are
separated in time. The procedure developed in this paper provides a technical
tool which may prove useful in view of describing holographic thermalization
using gravitational collapse in AdS space.Comment: 30 pages, 3 figures. Version as in JHE
CFT dual of the AdS Dirichlet problem: Fluid/Gravity on cut-off surfaces
We study the gravitational Dirichlet problem in AdS spacetimes with a view to
understanding the boundary CFT interpretation. We define the problem as bulk
Einstein's equations with Dirichlet boundary conditions on fixed timelike
cut-off hypersurface. Using the fluid/gravity correspondence, we argue that one
can determine non-linear solutions to this problem in the long wavelength
regime. On the boundary we find a conformal fluid with Dirichlet constitutive
relations, viz., the fluid propagates on a `dynamical' background metric which
depends on the local fluid velocities and temperature. This boundary fluid can
be re-expressed as an emergent hypersurface fluid which is non-conformal but
has the same value of the shear viscosity as the boundary fluid. The
hypersurface dynamics arises as a collective effect, wherein effects of the
background are transmuted into the fluid degrees of freedom. Furthermore, we
demonstrate that this collective fluid is forced to be non-relativistic below a
critical cut-off radius in AdS to avoid acausal sound propagation with respect
to the hypersurface metric. We further go on to show how one can use this
set-up to embed the recent constructions of flat spacetime duals to
non-relativistic fluid dynamics into the AdS/CFT correspondence, arguing that a
version of the membrane paradigm arises naturally when the boundary fluid lives
on a background Galilean manifold.Comment: 71 pages, 2 figures. v2: Errors in bulk metrics dual to
non-relativistic fluids (both on cut-off surface and on the boundary) have
been corrected. New appendix with general results added. Fixed typos. 82
pages, 2 figure
Holographic Charged Fluid with Anomalous Current at Finite Cutoff Surface in Einstein-Maxwell Gravity
The holographic charged fluid with anomalous current in Einstein-Maxwell
gravity has been generalized from the infinite boundary to the finite cutoff
surface by using the gravity/fluid correspondence. After perturbing the boosted
Reissner-Nordstrom (RN)-AdS black brane solution of the Einstein-Maxwell
gravity with the Chern-Simons term, we obtain the first order perturbative
gravitational and Maxwell solutions, and calculate the stress tensor and
charged current of the dual fluid at finite cutoff surfaces which contains
undetermined parameters after demanding regularity condition at the future
horizon. We adopt the Dirichlet boundary condition and impose the Landau frame
to fix these parameters, finally obtain the dependence of transport
coefficients in the dual stress tensor and charged current on the arbitrary
radical cutoff . We find that the dual fluid is not conformal, but it has
vanishing bulk viscosity, and the shear viscosity to entropy density ratio is
universally . Other transport coefficients of the dual current turns
out to be cutoff-dependent. In particular, the chiral vortical conductivity
expressed in terms of thermodynamic quantities takes the same form as that of
the dual fluid at the asymptotic AdS boundary, and the chiral magnetic
conductivity receives a cutoff-dependent correction which vanishes at the
infinite boundary.Comment: 19 pages, v2: references added, v3: typos corrected, v5: typos
corrected, version accepted for publication in JHE
Charged, conformal non-relativistic hydrodynamics
We embed a holographic model of an U(1) charged fluid with Galilean
invariance in string theory and calculate its specific heat capacity and
Prandtl number. Such theories are generated by a R-symmetry twist along a null
direction of a N=1 superconformal theory. We study the hydrodynamic properties
of such systems employing ideas from the fluid-gravity correspondence.Comment: 31 pages, 1 figure, JHEP3 style, refs added, typos corrected, missing
terms in spatial charge current and field corrections added, to be published
in JHE
Holographic Brownian Motion in Magnetic Environments
Using the gauge/gravity correspondence, we study the dynamics of a heavy
quark in two strongly-coupled systems at finite temperature: Super-Yang-Mills
in the presence of a magnetic field and non-commutative Super-Yang-Mills. In
the former, our results agree qualitatively with the expected behavior from
weakly-coupled theories. In the latter, we propose a Langevin equation that
accounts for the effects of non-commutativity and we find new interesting
features. The equation resembles the structure of Brownian motion in the
presence of a magnetic field and implies that the fluctuations along
non-commutative directions are correlated. Moreover, our results show that the
viscosity is smaller than the commutative case and that the diffusion
properties of the quark are unaffected by non-commutativity. Finally, we
compute the random force autocorrelator and verify that the
fluctuation-dissipation theorem holds in the presence of non-commutativity.Comment: 34 pages. v2: typos corrected. v3: title and abstract slightly
modified in order to better reflect the contents of the paper; footnote 3 and
one reference were also added; version accepted for publication in JHE
Temporal Accumulation and Decision Processes in the Duration Bisection Task Revealed by Contingent Negative Variation
The duration bisection paradigm is a classic task used to examine how humans and other animals perceive time. Typically, participants first learn short and long anchor durations and are subsequently asked to classify probe durations as closer to the short or long anchor duration. However, the specific representations of time and the decision rules applied in this task remain the subject of debate. For example, researchers have questioned whether participants actually use representations of the short and long anchor durations in the decision process rather than merely a response threshold that is derived from those anchor durations. Electroencephalographic (EEG) measures, like the contingent negative variation (CNV), can provide information about the perceptual and cognitive processes that occur between the onset of the timing stimulus and the motor response. The CNV has been implicated as an electrophysiological marker of interval timing processes such as temporal accumulation, representation of the target duration, and the decision that the target duration has been attained. We used the CNV to investigate which durations are involved in the bisection categorization decision. The CNV increased in amplitude up to the value of the short anchor, remained at a constant level until about the geometric mean (GM) of the short and long anchors, and then began to resolve. These results suggest that the short anchor and the GM of the short and long anchors are critical target durations used in the bisection categorization decision process. In addition, larger mean N1P2 amplitude differences were associated with larger amplitude CNVs, which may reflect the participant’s precision in initiating timing on each trial across a test session. Overall, the results demonstrate the value of using scalp-recorded EEG to address basic questions about interval timing
Spectral function of the supersymmetry current
We continue our study of the retarded Green's function of the universal
fermionic supersymmetry current ("supercurrent") for the most general class of
d=3 N=2 SCFTs with D=10 or D=11 supergravity duals by studying the propagation
of the Dirac gravitino in the electrically charged AdS-Reissner-Nordstr\"om
black-brane background of N=2 minimal gauged supergravity in D=4. We expand
upon results presented in a companion paper, including the absence of a Fermi
surface and the appearance of a soft power-law gap at zero temperature. We also
present the analytic solution of the gravitino equation in the AdS_2 X R^2
background which arises as the near-horizon limit at zero temperature. In
addition we determine the quasinormal mode spectrum.Comment: 65 pages, 6 Figs; version published in journa