103 research outputs found
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The discrimination ratio derived from Novel Object Recognition tasks as a measure of recognition memory sensitivity, not bias
Translational recognition memory research makes frequent use of the Novel Object Recognition (NOR) paradigm in which animals are simultaneously presented with one new and one old object. The preferential exploration of the new as compared to the old object produces a metric, the Discrimination Ratio (DR), assumed to represent recognition memory sensitivity. Human recognition memory studies typically assess performance using signal detection theory derived measures; sensitivity (d′) and bias (c). How DR relates to d′ and c and whether they measure the same underlying cognitive mechanism is, however, unknown. We investigated the correspondence between DR (eye-tracking-determined), d′ and c in a sample of 37 humans. We used dwell times during a visual paired comparison task (analogous to the NOR) to determine DR, and a separate single item recognition task to derive estimates of response sensitivity and bias. DR was found to be significantly positively correlated to sensitivity but not bias. Our findings confirm that DR corresponds to d′, the primary measure of recognition memory sensitivity in humans, and appears not to reflect bias. These findings are the first of their kind to suggest that animal researchers should be confident in interpreting the DR as an analogue of recognition memory sensitivity
M-Branes and Metastable States
We study a supersymmetry breaking deformation of the M-theory background
found in arXiv:hep-th/0012011. The supersymmetric solution is a warped product
of R^{2,1} and the 8-dimensional Stenzel space, which is a higher dimensional
generalization of the deformed conifold. At the bottom of the warped throat
there is a 4-sphere threaded by \tilde{M} units of 4-form flux. The dual
(2+1)-dimensional theory has a discrete spectrum of bound states. We add p
anti-M2 branes at a point on the 4-sphere, and show that they blow up into an
M5-brane wrapping a 3-sphere at a fixed azimuthal angle on the 4-sphere. This
supersymmetry breaking state turns out to be metastable for p / \tilde{M} <
0.054. We find a smooth O(3)-symmetric Euclidean bounce solution in the
M5-brane world volume theory that describes the decay of the false vacuum.
Calculation of the Euclidean action shows that the metastable state is
extremely long-lived. We also describe the corresponding metastable states and
their decay in the type IIA background obtained by reduction along one of the
spatial directions of R^{2,1}.Comment: 33 pages, 5 figures; v2 note adde
Exact results for static and radiative fields of a quark in N=4 super Yang-Mills
In this work (which supersedes our previous preprint arXiv:1112.2345) we
determine the expectation value of the N=4$ SU(N) SYM Lagrangian density
operator in the presence of an infinitely heavy static particle in the
symmetric representation of SU(N), by means of a D3-brane probe computation.
The result that we obtain coincides with two previous computations of different
observables, up to kinematical factors. We argue that these agreements go
beyond the D-brane probe approximation, which leads us to propose an exact
formula for the expectation value of various operators. In particular, we
provide an expression for the total energy loss by radiation of a heavy
particle in the fundamental representation.Comment: 14 pages. This submission supersedes our previous preprint
arXiv:1112.2345. v2: numerical factors fixed, minor clarifications, added
reference
D-Branes on the Conifold and N=1 Gauge/Gravity Dualities
We review extensions of the AdS/CFT correspondence to gauge/ gravity
dualities with N=1 supersymmetry. In particular, we describe the gauge/gravity
dualities that emerge from placing D3-branes at the apex of the conifold. We
consider first the conformal case, with discussions of chiral primary operators
and wrapped D-branes. Next, we break the conformal symmetry by adding a stack
of partially wrapped D5-branes to the system, changing the gauge group and
introducing a logarithmic renormalization group flow. In the gravity dual, the
effect of these wrapped D5-branes is to turn on the flux of 3-form field
strengths. The associated RR 2-form potential breaks the U(1) R-symmetry to
and we study this phenomenon in detail. This extra flux also leads to
deformation of the cone near the apex, which describes the chiral symmetry
breaking and confinement in the dual gauge theory.Comment: Based on I.R.K.'s lectures at the Les Houches Summer School Session
76, ``Gravity, Gauge Theories, and Strings'', August 2001, 42 pages, v2:
clarifications and references adde
The influence of D-branes' backreaction upon gravitational interactions between open strings
We argue that gravitational interactions between open strings ending on
D3-branes are largely shaped by the D3-branes' backreaction. To this end we
consider classical open strings coupled to general relativity in Poincare AdS5
backgrounds. We compute the linear gravitational backreaction of a static
string extending up to the Poincare horizon, and deduce the potential energy
between two such strings. If spacetime is non-compact, we find that the
gravitational potential energy between parallel open strings is independent of
the strings' inertial masses and goes like 1/r at large distance r. If the
space transverse to the D3-branes is suitably compactified, a collective mode
of the graviton propagates usual four-dimensional gravity. In that case the
backreaction of the D3-branes induces a correction to the Newtonian potential
energy that violates the equivalence principle. The observed enhancement of the
gravitational attraction is specific to string theory; there is no similar
effect for point-particles.Comment: 28 pages, 7 figures. Typos corrected, minor addition
Writing CFT correlation functions as AdS scattering amplitudes
We explore the Mellin representation of conformal correlation functions
recently proposed by Mack. Examples in the AdS/CFT context reinforce the
analogy between Mellin amplitudes and scattering amplitudes. We conjecture a
simple formula relating the bulk scattering amplitudes to the asymptotic
behavior of Mellin amplitudes and show that previous results on the flat space
limit of AdS follow from our new formula. We find that the Mellin amplitudes
are particularly useful in the case of conformal gauge theories in the planar
limit. In this case, the four point Mellin amplitudes are meromorphic functions
whose poles and their residues are entirely determined by two and three point
functions of single-trace operators. This makes the Mellin amplitudes the ideal
objects to attempt the conformal bootstrap program in higher dimensions.Comment: 23 pages + appendice
Absorption of scalars by nonextremal charged black holes in string theory
We analyze the low frequency absorption cross section of minimally coupled massless scalar fields by different kinds of charged static black holes in string theory, namely the D1–D5 system in d=5 and a four dimensional dyonic four-charged black hole. In each case we show that this cross section always has the form of some parameter of the solution divided by the black hole Hawking temperature. We also verify in each case that, despite its explicit temperature dependence, such quotient is finite in the extremal limit, giving a well defined cross section. We show that this precise explicit temperature dependence also arises in the same cross section for black holes with string \alpha' corrections: it is actually induced by them.This work has been supported by FEDER funds through Programa Operacional Fatores de Competitividade
– COMPETE and by Fundação para a Ciência e a Tecnologia (FCT) through projects EstC/MAT/UI0013/2011
and CERN/FP/123609/2011
Conformal field theories in anti-de Sitter space
In this paper we discuss the dynamics of conformal field theories on anti-de
Sitter space, focussing on the special case of the N=4 supersymmetric
Yang-Mills theory on AdS_4. We argue that the choice of boundary conditions, in
particular for the gauge field, has a large effect on the dynamics. For
example, for weak coupling, one of two natural choices of boundary conditions
for the gauge field leads to a large N deconfinement phase transition as a
function of the temperature, while the other does not. For boundary conditions
that preserve supersymmetry, the strong coupling dynamics can be analyzed using
S-duality (relevant for g_{YM} >> 1), utilizing results of Gaiotto and Witten,
as well as by using the AdS/CFT correspondence (relevant for large N and large
't Hooft coupling). We argue that some very specific choices of boundary
conditions lead to a simple dual gravitational description for this theory,
while for most choices the gravitational dual is not known. In the cases where
the gravitational dual is known, we discuss the phase structure at large 't
Hooft coupling.Comment: 57 pages, 1 figure. v2: fixed typo
Exact Results and Holography of Wilson Loops in N=2 Superconformal (Quiver) Gauge Theories
Using localization, matrix model and saddle-point techniques, we determine
exact behavior of circular Wilson loop in N=2 superconformal (quiver) gauge
theories. Focusing at planar and large `t Hooft couling limits, we compare its
asymptotic behavior with well-known exponential growth of Wilson loop in N=4
super Yang-Mills theory. For theory with gauge group SU(N) coupled to 2N
fundamental hypermultiplets, we find that Wilson loop exhibits non-exponential
growth -- at most, it can grow a power of `t Hooft coupling. For theory with
gauge group SU(N) x SU(N) and bifundamental hypermultiplets, there are two
Wilson loops associated with two gauge groups. We find Wilson loop in untwisted
sector grows exponentially large as in N=4 super Yang-Mills theory. We then
find Wilson loop in twisted sector exhibits non-analytic behavior with respect
to difference of two `t Hooft coupling constants. By letting one gauge coupling
constant hierarchically larger/smaller than the other, we show that Wilson
loops in the second type theory interpolate to Wilson loop in the first type
theory. We infer implications of these findings from holographic dual
description in terms of minimal surface of dual string worldsheet. We suggest
intuitive interpretation that in both type theories holographic dual background
must involve string scale geometry even at planar and large `t Hooft coupling
limit and that new results found in the gauge theory side are attributable to
worldsheet instantons and infinite resummation therein. Our interpretation also
indicate that holographic dual of these gauge theories is provided by certain
non-critical string theories.Comment: 52 pages, 7 figures v2. more figures embedded v3. minor stylistic
changes, v4. published versio
Wilson loops stability in the gauge/string correspondence
We study the stability of some classical string worldsheet solutions employed
for computing the potential energy between two static fundamental quarks in
confining and non-confining gravity duals. We discuss the fixing of the
diffeomorphism invariance of the string action, its relation with the
fluctuation orientation and the interpretation of the quark mass substraction
worldsheet needed for computing the potential energy in smooth (confining)
gravity background. We consider various dual gravity backgrounds and show by a
numerical analysis the existence of instabilities under linear fluctuations for
classical string embedding solutions having positive length function derivative
. Finally we make a brief discussion of 't Hooft loops in
non-conformal backgrounds.Comment: 34 pages, 36 figures. Reference added. Final version JHEP accepte
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