27,561 research outputs found
The neural correlates of phonological short-term memory: A repetitive transcranial magnetic stimulation study
Neuropsychological reports and activation studies by means of positron emission tomography anti functional magnetic resonance imaging have suggested that the neural correlates of phonological short-term memory are located in the left hemisphere, with Brodmann's area (BA) 40 being, responsible for short-term storage and BA 44 for articulatory rehearsal. However, a careful review of the literature on the role of left BA 40 shows that the data are equivocal. We tested We hypotheses by means of repetitive transcranial magnetic stimulation (rTMS). Participants performed four tasks: two phonological judgements, thought to require only articulatory rehearsal Without the contribution of short-term storage digit span, which involves both short-term storage and articudlatory rehearsal: and a pattern span, this last heing the control task. The sites of stimulation were left BA 40. left BA 44 anti the electrode location V-W plus a baseline without TMS. Reaction times increased and accuracy decreased in the case of the phonological judgement and digit span after stimulation of both left sites, suggesting that BA 40, in addition to BA 44. is involved in phonological judgements. Possible explanations are discussed, namely, the possibility that (i) the neural correlates of rehearsal are not limited to BA and (ii) phonological judgements invlove processes other than rehearsal. We also consider the effects of using different tasks and responses to resolve some of the descrepancies in the literature
Coherent States of Accelerated Relativistic Quantum Particles, Vacuum Radiation and the Spontaneous Breakdown of the Conformal SU(2,2) Symmetry
We give a quantum mechanical description of accelerated relativistic
particles in the framework of Coherent States (CS) of the (3+1)-dimensional
conformal group SU(2,2), with the role of accelerations played by special
conformal transformations and with the role of (proper) time translations
played by dilations. The accelerated ground state of first
quantization is a CS of the conformal group. We compute the distribution
function giving the occupation number of each energy level in
and, with it, the partition function Z, mean energy E and entropy S, which
resemble that of an "Einstein Solid". An effective temperature T can be
assigned to this "accelerated ensemble" through the thermodynamic expression
dE/dS, which leads to a (non linear) relation between acceleration and
temperature different from Unruh's (linear) formula. Then we construct the
corresponding conformal-SU(2,2)-invariant second quantized theory and its
spontaneous breakdown when selecting Poincar\'e-invariant degenerated
\theta-vacua (namely, coherent states of conformal zero modes). Special
conformal transformations (accelerations) destabilize the Poincar\'e vacuum and
make it to radiate.Comment: 25 pages, LaTeX, 3 figures. Additional information (resulting in four
extra pages) and a slight change of focus has been introduced in order to
make the line of arguments more clear. Title changed accordingl
Runaway massive stars as variable gamma-ray sources
Runaway stars are ejected from their formation sites well within molecular
cores in giant dark clouds. Eventually, these stars can travel through the
molecular clouds, which are highly inhomogeneous. The powerful winds of massive
runaway stars interact with the medium forming bowshocks. Recent observations
and theoretical modelling suggest that these bowshocks emit non-thermal
radiation. As the massive stars move through the inhomogeneous ambient gas the
physical properties of the bowshocks are modified, producing changes in the
non-thermal emission. We aim to compute the non-thermal radiation produced in
the bowshocks of runaway massive stars when travelling through a molecular
cloud. We calculate the non-thermal emission and absorption for two types of
massive runaway stars, an O9I and an O4I, as they move through a density
gradient. We present the spectral energy distributions for the runaway stars
modelled. Additionally, we obtain light curves at different energy ranges. We
find significant variations in the emission over timescales of 1 yr. We
conclude that bowshocks of massive runaway stars, under some assumptions, might
be variable gamma-ray sources, with variability timescales that depend on the
medium density profile. These objects might constitute a population of galactic
gamma-ray sources turning on and off within years.Comment: 10 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
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