528 research outputs found
Excitonic Funneling in Extended Dendrimers with Non-Linear and Random Potentials
The mean first passage time (MFPT) for photoexcitations diffusion in a
funneling potential of artificial tree-like light-harvesting antennae
(phenylacetylene dendrimers with generation-dependent segment lengths) is
computed. Effects of the non-linearity of the realistic funneling potential and
slow random solvent fluctuations considerably slow down the center-bound
diffusion beyond a temperature-dependent optimal size. Diffusion on a
disordered Cayley tree with a linear potential is investigated analytically. At
low temperatures we predict a phase in which the MFPT is dominated by a few
paths.Comment: 4 pages, 4 figures, To be published in Phys. Rev. Let
Disorder and Funneling Effects on Exciton Migration in Tree-Like Dendrimers
The center-bound excitonic diffusion on dendrimers subjected to several types
of non-homogeneous funneling potentials, is considered. We first study the
mean-first passage time (MFPT) for diffusion in a linear potential with
different types of correlated and uncorrelated random perturbations. Increasing
the funneling force, there is a transition from a phase in which the MFPT grows
exponentially with the number of generations , to one in which it does so
linearly. Overall the disorder slows down the diffusion, but the effect is much
more pronounced in the exponential compared to the linear phase. When the
disorder gives rise to uncorrelated random forces there is, in addition, a
transition as the temperature is lowered. This is a transition from a
high- regime in which all paths contribute to the MFPT to a low- regime
in which only a few of them do. We further explore the funneling within a
realistic non-linear potential for extended dendrimers in which the dependence
of the lowest excitonic energy level on the segment length was derived using
the Time-Dependent Hatree-Fock approximation. Under this potential the MFPT
grows initially linearly with but crosses-over, beyond a molecular-specific
and -dependent optimal size, to an exponential increase. Finally we consider
geometrical disorder in the form of a small concentration of long connections
as in the {\it small world} model. Beyond a critical concentration of
connections the MFPT decreases significantly and it changes to a power-law or
to a logarithmic scaling with , depending on the strength of the funneling
force.Comment: 13 pages, 9 figure
Nitrogen-vacancy singlet manifold ionization energy
The singlet states of the negatively-charged nitrogen-vacancy centers in
diamond play a key role in its optical spin control and readout. In this work,
the hitherto unknown ionization energy of the singlet is measured
experimentally and found to be between 1.91-2.25 eV. This is obtained by
analyzing photoluminescence measurements incorporating spin control and NV
charge state differentiation, along with simulations based on the
nitrogen-vacancy's master equation. This work establishes a protocol for a more
accurate estimate of this ionization energy, which can possibly lead to
improved read-out methods
The shadows of waiting and care: on discourses of waiting in the history of the British National Health Service
Waiting is at the centre of experiences and practices of healthcare. However, we know very little about the relationship between the subjective experiences of patients who wait in and for care, health practitioners who ‘prescribe’ and manage waiting, and how this relates to broader cultural meanings of waiting. Waiting features heavily in the sociological, managerial, historical and health economics literatures that investigate UK healthcare, but the focus has been on service provision and quality, with waiting (including waiting lists and waiting times) drawn on as a key marker to test the efficiency and affordability of the NHS. In this article, we consider the historical contours of this framing of waiting, and ask what has been lost or occluded through its development. To do so, we review the available discourses in the existing literature on the NHS through a series of ‘snapshots’ or key moments in its history. Through its negative imprint, we argue that what shadows these discourses is the idea of waiting and care as phenomenological temporal experiences, and time as a practice of care. In response, we begin to trace the intellectual and historical resources available for alternative histories of waiting – materials that might enable scholars to reconstruct some of the complex temporalities of care marginalized in existing accounts of waiting, and which could help reframe both future historical accounts and contemporary debates about waiting in the NHS
The cognitive and emotional effects of cognitive bias modification in interpretations in behaviorally inhibited youth
Cognitive bias modification (CBM) procedures follow from the view that interpretive biases play an important role in the development and maintenance of anxiety. As such, understanding the link between interpretive biases and anxiety in youth at risk for anxiety (e.g., behaviorally inhibited children) could elucidate the mechanisms involved in the development of pediatric anxiety. However, to date, the majority of CBM-I work only studies adult populations. The present article presents the results of a CBM study examining effects of positive interpretive bias modification on mood, stress vulnerability, and threat-related attention bias in a group of behaviorally inhibited children (n = 45). Despite successful modification of interpretive bias in the at-risk youth, minimal effects on stress vulnerability or threat-related attention bias were found. The current findings highlight the need for continued research on cognitive biases in anxiety
An Open Pilot Study of Training Hostile Interpretation Bias to Treat Disruptive Mood Dysregulation Disorder
Objective: Irritability in disruptive mood dysregulation disorder (DMDD) may be associated with a biased tendency to judge ambiguous facial expressions as angry. We conducted three experiments to explore this bias as a treatment target. We tested: 1) whether youth with DMDD express this bias; 2) whether judgment of ambiguous faces can be altered in healthy youth by training; and 3) whether such training in youth with DMDD is associated with reduced irritability and associated changes in brain function. Methods: Participants in all experiments made happy versus angry judgments of faces that varied along a happy to angry continuum. These judgments were used to quantify a “balance point,” the facial expression at which a participant's judgment switches from predominantly happy to predominantly angry. We first compared balance points in youth with DMDD (n = 63) versus healthy youth (n = 26). We then conducted a double-blind, randomized controlled trial of active versus sham balance-point training in 19 healthy youth. Finally, we piloted open, active balance-point training in 14 youth with DMDD, with 10 completing an implicit functional MRI (fMRI) face-emotion processing task. Results: Relative to healthy youth, DMDD youth manifested a shifted balance point, expressed as a tendency to classify ambiguous faces as angry rather than happy. In both healthy and DMDD youth, active training is associated with a shift in balance point toward more happy judgments. In DMDD, evidence suggests that active training may be associated with decreased irritability and changes in activation in the lateral orbitofrontal cortex. Conclusions:These results set the stage for further research on computer-based treatment targeting interpretation bias of angry faces in DMDD. Such treatment may decrease irritability and alter neural responses to subtle expressions of happiness and anger
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Neural traces of stress: cortisol related sustained enhancement of amygdala-hippocampal functional connectivity
Stressful experiences modulate neuro-circuitry function, and the temporal trajectory of these alterations, elapsing from early disturbances to late recovery, heavily influences resilience and vulnerability to stress. Such effects of stress may depend on processes that are engaged during resting-state, through active recollection of past experiences and anticipation of future events, all known to involve the default mode network (DMN). By inducing social stress and acquiring resting-state functional magnetic resonance imaging (fMRI) before stress, immediately following it, and 2 h later, we expanded the time-window for examining the trajectory of the stress response. Throughout the study repeated cortisol samplings and self-reports of stress levels were obtained from 51 healthy young males. Post-stress alterations were investigated by whole brain resting-state functional connectivity (rsFC) of two central hubs of the DMN: the posterior cingulate cortex (PCC) and hippocampus. Results indicate a ’recovery’ pattern of DMN connectivity, in which all alterations, ascribed to the intervening stress, returned to pre-stress levels. The only exception to this pattern was a stress-induced rise in amygdala-hippocampal connectivity, which was sustained for as long as 2 h following stress induction. Furthermore, this sustained enhancement of limbic connectivity was inversely correlated to individual stress-induced cortisol responsiveness (AUCi) and characterized only the group lacking such increased cortisol (i.e., non-responders). Our observations provide evidence of a prolonged post-stress response profile, characterized by both the comprehensive balance of most DMN functional connections and the distinct time and cortisol dependent ascent of intra-limbic connectivity. These novel insights into neuro-endocrine relations are another milestone in the ongoing search for individual markers in stress-related psychopathologies
Additional services for Psychological Medicine: Email alerts: Click here Subscriptions: Click here Commercial reprints: Click here Terms of use : Click here Perturbed threat monitoring following a traumatic event predicts risk for post-traumatic stress di
Background. Post-traumatic stress disorder (PTSD) is a chronic and difficult to treat psychiatric disorder. Objective, performance-based diagnostic markers that uniquely index risk for PTSD above and beyond subjective self-report markers could inform attempts to improve prevention and early intervention. We evaluated the predictive value of threat-related attention bias measured immediately after a potentially traumatic event, as a risk marker for PTSD at a 3-month follow-up. We measured the predictive contribution of attentional threat bias above and beyond that of the more established marker of risk for PTSD, self-reported psychological dissociation. Method. Dissociation symptoms and threat-related attention bias were measured in 577 motor vehicle accident (MVA) survivors (mean age = 35.02 years, 356 males) within 24 h of admission to an emergency department (ED) of a large urban hospital. PTSD symptoms were assessed at a 3-month follow-up using the Clinician-Administered PTSD Scale (CAPS). Results. Self-reported dissociation symptoms significantly accounted for 16% of the variance in PTSD at follow-up, and attention bias toward threat significantly accounted for an additional 4% of the variance in PTSD. Conclusions. Threat-related attention bias can be reliably measured in the context of a hospital ED and significantly predicts risk for later PTSD. Possible mechanisms underlying the association between threat bias following a potentially traumatic event and risk for PTSD are discussed. The potential application of an attention bias modification treatment (ABMT) tailored to reduce risk for PTSD is suggested
Emotional orientation, brain function and genetics in adults and children : implications for development, and psychopathology
The ability to attend or avoid emotional stimuli is important to our
survival. Attending to potential threats can help us avoid danger; while attending to
positive stimuli is important for our social function. For example, when we see a man
with a knife it is important to run away, or avoid the threat so we are not harmed. Just
as the knife warns us of the threatening stiuation, a smiling face indicates a friendly
person. We are drawn to this cue to possibly receive a rewarding social interaction.
Attention orientation to both negative and positive stimuli may be impacted by
development, psychopathology and genetics. The dot probe task yields both behavioral
and neural indices of attention biases towards or away from an emotional cue (angry or
happy face). This thesis includes three studies to determine the effects of development,
psychopathology, and genetics on attention orientating.
In Study I, we examined age-related correlations in attention-orienting
biases to negative and positive faces in a healthy sample using functional magnetic
resonance imaging (fMRI) and a dot probe task. Behavioral response data indicated a
positive correlation between age and attention bias towards happy faces, such that
younger participants showed less bias towards happy, relative to neutral, faces, than
older subjects. Attention bias towards angry faces did not correlate with age. Relative to
older, younger participants demonstrated greater activation in the left cuneus and left
caudate on the contrast of trials used to assess happy-face attention bias.
In Study II, using the dot probe task in a home setting, we studied parents
that were highly exposed to the attack on the World Trade Center in 2001 and their
children. We found that psychiatrically healthy parents who experienced severe trauma
showed greater attention bias towards threat than parents experiencing no such trauma, but trauma experienced by parents did was not predictive of attention bias in their children.
In Study III, using an fMRI on 5-HTTLPR genotyped adults performing
dot probe task; we compared amygdala response to threat bias contrasts. The 5-
HTTLPR has been previously linked to amygdala reactivity and the amygdala has been
implicated in the orienting of attention towards threat. Behavioral data indicated no
difference between the two genotyped subject populations for the 5-HTTLPR
polymorphism (l/l and s-carrier). However, fMRI data did reveal between-group
differences in the amygdala activation. Specifically, relative to l/l, s-carriers showed
greater right amygdala activation to trials with angry faces. Because similar levels of
threat bias were found in the two genotype groups, these findings suggest that s-carriers exhibit a lower threshold for engaging the amygdala within the context of the task.
In total, these three studies explore the effect of both the environment and
genes on behavior and brain function. Studies I and II focus on environment,
specifically, how their environment affects their emotional orientation. On the genetic
side, Study III focuses on the effect of genetics on emotional orientation
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