597 research outputs found
Explaining two circumnuclear star forming rings in NGC5248
The distribution of gas in the central kiloparsec of a galaxy has a
dynamically rapid evolution. Nonaxisymmetries in the gravitational potential of
the galactic disk, such as a large scale stellar bar or spiral, can lead to
significant radial motion of gaseous material from larger radii to the central
region. The large influx of gas and the subsequent star formation keep the
central region constantly changing. However, the ability of gas to reach the
nucleus proper to fuel an AGN phase is not guaranteed. Gas inflow can be halted
at a circumnuclear star forming ring several hundred parsec away. The nearby
galaxy NGC5248 is especially interesting in this sense since it is said to host
2 circumnuclear star forming rings at 100pc and 370pc from its quiescent
nucleus. Here we present new subarcsecond PdBI+30m CO(2-1) emission line
observations of the central region. For the first time the molecular gas
distribution at the smallest stellar ring is resolved into a gas ring,
consistent with the presence of a quiescent nucleus. However, the molecular gas
shows no ring structure at the larger ring. We combine analyses of the gaseous
and stellar content in the central kiloparsec of this galaxy to understand the
gas distribution and dynamics of this star forming central region. We discuss
the probability of two scenarios leading to the current observations, given our
full understanding of this system, and discuss whether there are really two
circumnuclear star forming rings in this galaxy.Comment: Accepted for publication in A&A, 14pages + long tabl
The ontology of biological taxa
Motivation: The classification of biological entities in terms of species and taxa is an important endeavor in biology. Although a large amount of statements encoded in current biomedical ontologies is taxon-dependent there is no obvious or standard way for introducing taxon information into an integrative ontology architecture, supposedly because of ongoing controversies about the ontological nature of species and taxa
HST/WFPC2 imaging of the circumnuclear structure of LLAGNs. I Data and nuclear morphology
To advance our knowledge of the nature of the central source in LLAGNs and
its relation with stellar clusters, we are carrying out several imaging
projects with HST at near-UV, optical and near-IR wavelengths. In this paper,
we present the first results obtained with observations of the central regions
of 57 LLAGNs imaged with the WFPC2 through any of the V (F555W, F547M, F614W)
and I (F791W, F814W) filters that are available in the HST archive. The sample
contains 34% of the LINERs and 36% of the TOs in the Palomar sample. The mean
spatial resolution of these images is 10 pc. With these data we have built an
atlas that includes structural maps for all the galaxies, useful to identify
compact nuclear sources and, additionally, to characterize the circumnuclear
environment of LLAGNs, determining the frequency of dust and its morphology.
The main results obtained are: 1) We have not found any correlation between the
presence of nuclear compact sources and emission-line type. Thus, nucleated
LINERs are as frequent as nucleated TOs. 2) The nuclei of "Young-TOs" are
brighter than the nuclei of "Old-TOs" and LINERs. These results confirm our
previous results that Young-TOs are separated from other LLAGNs classes in
terms of their central stellar population properties and brightness. 3)
Circumnuclear dust is detected in 88% of the LLAGNs, being almost ubiquitous in
TOs. 4) The dust morphology is complex and varied, from nuclear spiral lanes to
chaotic filaments and nuclear disk-like structures. Chaotic filaments are as
frequent as dust spirals; but nuclear disks are mainly seen in LINERs. These
results suggest an evolutionary sequence of the dust in LLAGNs, LINERs being
the more evolved systems and Young-TOs the youngest. The full collection of
figures are at http://www.iaa.es/~rosa/research/LLAGNs2007/LLAGNs-HSTIma1.htmlComment: Paper accepted in AJ, pdf file and the full collection of figures are
at the ULR: http://www.iaa.es/~rosa/research/LLAGNs2007/LLAGNs-HSTIma1.htm
Ketamine decreases resting state functional connectivity between networks via the dorsal nexus: implications for major depression
Question: Increasing preclinical and clinical evidence underscores the strong and rapid antidepressant properties of the glutamate modulating NMDA receptor antagonist ketamine [1, 2]. Targeting the glutamatergic system might thus provide a novel therapeutic strategy for antidepressant drug treatment [3]. Since glutamate is the most abundand and major excitatory neurotransmitter in the human brain, pathophysiological changes in glutamatergic signalling are likely to affect neurobehavioural plasticity, information processing and large-scale changes in functional brain connectivity underlying certain symptoms of major depressive disorder (MDD) [4]. Using resting state functional MRI (rsfMRI), the âdorsal nexusâ (DN) was recently identified as a bilateral dorsal medial prefrontal cortex (DMPFC) region showing dramatically increased depression-associated fMRI connectivity with large portions of the cognitive control network (CCN), the default mode network (DMN), and the affective network (AN) [5]. Hence, Sheline and colleagues [5] proposed that reducing increased connectivity of the DN might play a critical role in reducing depressive symptomatology and thus represent a potential therapeutic target for affective disorders. Since little is known about how ketamine affects large-scale neural network dynamics in the human brain, we aimed to test the hypothesis that ketamine as an antidepressant glutamatergic agent decreases resting state connectivties via the DN. Methods: Study design: 17 healthy subjects (mean age, 40.5 +/- 7.5 [SD]; 9 males) completed four resting state fMRI sessions in a double-blind, randomized, crossover study design (s. Fig 1). The baseline scan was followed by an intravenous infusion (45 mins) of either S-ketamine (0.25 mg/kg) or placebo (saline) outside the scanner. Since the antidepressant effect of ketamine is most prominent after one day [1], the followup scans were scheduled 24 hours after the ketamine or placebo infusion in order to assess the mid-term effects on neuronal network dynamics that might contribute to its antidepressant efficacy. To avoid a possible carry-over effect, the time lag between the two baseline measurements was set to at least 10 days. rsfMRI data acquisition and analysis: Measurements were performed on a Philips Achieva TX 3-T whole-body MR unit equipped with an 8-channel SENSE head coil. During each session a total of 200 functional images were collected in 10 minute runs (eyes closed) using the following acquisition parameters: TE = 35 ms, TR = 3000 ms (Ξ = 82°), FOV = 22 cm, acquisition matrix = 80 x 80 interpolated to 128 x 128, voxel size = 2.75 x 2.75 x 4 mm, 32 contiguous axial slices (placed along the anterior-posterior commissure plane), and sensitivity-encoded acceleration factor R = 2.0. A 3-dimensional T1-weighted anatomical scan was obtained for structural reference. Data were analyzed using the SPM8 (Wellcome Trust Center for Neuroimaging, London, England) based data processing assistant for resting state fMRI (DPARSF, by Yan Chao-Gan et al.) which includes a resting state fMRI data analysis toolkit (REST, by Song Xiao-Wei et al.). The postprocessing steps followed the standard protocol described by Yan and Zang (2010) [6]. Results: To test our hypothesis, we created a seed region of interest in the left and right DMPFC (10 mm sphere at ± 6 51 24) representing the DN. 24 h following ketamine administration, functional connectivity was exclusively reduced to the posterior cingulate cortex (PCC), to the subgenual anterior cingulate cortex (sgACC), and to anterior and mediodorsal parts of the thalamus (compared to placebo). The backprojection from a seed in the PCC confirmed these results and revealed an additional significant reduction of functional connectivity to the pregenual ACC (PACC) and medioprefrontal cortex (MPFC). For details, see Fig. 2 A, B and bar diagrams (functional connectivity change, paired t tests). Conclusion: While pharmacological effects of ketamine on task induced fMRI BOLD signals have been studied extensively, this is the first randomized, placebo-controlled, double-blind, crossover study demonstrating changes in resting state functional connectivity in response to ketamine administration in healthy subjects. Here, we report a significant decrease in functional connectivity of the sgACC (AN) and the PCC (DMN) via the DN 24 hours following ketamine administration, thus reflecting a neuronal pattern of normalization with regard to MDD where increased connectivities of the AN and DMN via the DN have been observed [5]. As critical hub of the AN, the sgACC plays an important role in mood regulation. Subgenual cortical activity was shown to be elevated in MDD and effective antidepressant treatment was associated with a reduction in sgACC activity [for review see ref. 7]. In addition, the observed reduction in functional connectivity between anterior (PACC/MPFC) and posterior parts of the DMN (PCC) may partially reverse the disrupted neurobehavioral homeostasis in MDD where a failure to normally down-regulate activity within the DMN during emotional stimulation was found [8], with increasing levels of DMN dominance being associated with higher levels of maladaptive, depressive rumination and lower levels of adaptive, reflective rumination [9]. Finally, reductions in cortico-thalamic connectivity may reflect functional alterations in thalamocortical loops via the prefrontal cortex. Based on the fact that the antidepressant effect of ketamine peaks one day after a single intravenous administration [1], we conclude that pharmacologically reducing the hyperconnectivity via the DN may play a critical role in reducing depressive symptomatology and in representing a systems level mechanism of treatment response for major depression
Gender and rapid alterations of hemispheric dominance during planning
BACKGROUND: Mental planning and carrying out a plan provoke specific cerebral hemodynamic responses. Gender aspects of hemispheric laterality using rapid cerebral hemodynamics have not been reported.
METHOD: Here, we applied functional transcranial Doppler sonography to examine lateralization of cerebral hemodynamics of the middle cerebral arteries of 28 subjects (14 women and 14 men) performing a standard planning task. There were easy and difficult problems, and mental planning without motor activity was separated from movement execution.
RESULTS: Difficult mental planning elicited lateralization to the right hemisphere after 2 or more seconds, a feature that was not observed during movement execution. In females, there was a dominance to the left hemisphere during movement execution. Optimized problem solving yielded an increased laterality change to the right during mental planning.
CONCLUSIONS: Gender-related hemispheric dominance appears to be condition-dependent, and change of laterality to the right may play a role in optimized performance. Results are of relevance when considering laterality from a perspective of performance enhancement of higher cognitive functions, and also of psychiatric disorders with cognitive dysfunctions and abnormal lateralization patterns such as schizophrenia
- âŠ