4,207 research outputs found
THE ROLE OF GAMMA OSCILLATIONS AND CORTICAL INHIBITION IN THE DEVELOPMENT OF WORKING MEMORY IN ADOLESCENCE
Adolescence is a dynamic period of social, cognitive, and biological changes. In particular, working memory, the ability to actively encode and maintain information over a short period of time, develops early in childhood and gradually increases in capacity and stability during adolescence. The precise neurophysiological mechanism by which working memory capacity increases during adolescence is unclear. The objective of this investigation was to evaluate the role of cortical gamma-band (\u3e 30 Hz) oscillationsâwhich are associated with working memory in adultsâfor the development of working memory capacity in adolescents, and to identify the extent to which the temporal profile of gamma-aminobutyric acid (GABA)-mediated cortical inhibition underlies these changes. I hypothesized that cortical gamma-band rhythms would become faster during adolescence in a manner that supports improved working memory capacity, and that the kinetics of cortical inhibition would also become faster to support these faster rhythms.
To this end, I recruited two cohorts of typically developing children (10 â 12 years) and adolescents (15 â 17 years) for a combined electrophysiology (EEG) and transcranial magnetic stimulation (TMS) study. First, I investigated the endogenous rhythmic activity generated by children and adolescence when performing a serially presented working memory task of varying set size. I found evidence of maturation in the generation of gamma-band rhythms which differed in power between groups, but identified no effects of a change in the central frequency of gamma-band activity. Next, I used TMS to exogenously evoke oscillatory activity in the left prefrontal cortex to identify the cortical natural (i.e., resonant) frequency. Using this measure, I found that adolescents exhibit higher median natural frequencies (MdCHILD = 16 Hz; MdADO = 24 Hz, Z = 2.35, p = 0.009), but that sex may play a mediating role when this change emerges. While this measure positively correlated with working memory capacity (rs = 0.47, p = 0.007), this effect disappeared when controlling for age and sex (rs = 0.29, p = 0.128).
Finally, I investigated the role of inhibitory timing as a potential mechanism for improved cognition and increased natural frequency using classic paired pulse TMS techniques. Six inter-pulse intervals (IPI) in the range of short- and long-intracortical inhibition (SICI, LICI) were tested to assess the temporal characteristics of GABA type-A and type-B receptor-mediated inhibition (GABAAR, GABABR, respectively). For SICI, I found alpha-band (9-14 Hz) facilitation in children and suppression in adolescents. For LICI, adolescents demonstrated greater suppression of gamma-band power compared to children, and equal suppression to children in the beta-band (15-30 Hz). I found no evidence for a change in timing of SICI- or LICI-induced modulations though LICI suppression of gamma- and beta-band power correlated with working memory capacity.
The overall hypothesis that the prefrontal cortex can produce faster rhythms during adolescent development was supported, but the hypothesized relationships between those rhythms, working memory capacity, and the timing of GABA-mediated inhibition were not. Rather, I observed several developmental differences in oscillatory power that suggest excitation-inhibition balance underlies the developmental increases in working memory capacity and gamma-band synchrony
Molecular Star Formation Rate Indicators in Galaxies
We derive a physical model for the observed relations between star formation
rate (SFR) and molecular line (CO and HCN) emission in galaxies, and show how
these observed relations are reflective of the underlying star formation law.
We do this by combining 3D non-LTE radiative transfer calculations with
hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We
demonstrate that the observed SFR-molecular line relations are driven by the
relationship between molecular line emission and gas density, and anchored by
the index of the underlying Schmidt law controlling the SFR in the galaxy.
Lines with low critical densities (e.g. CO J=1-0) are typically thermalized and
trace the gas density faithfully. In these cases, the SFR will be related to
line luminosity with an index similar to the Schmidt law index. Lines with high
critical densities greater than the mean density of most of the emitting clouds
in a galaxy (e.g. CO J=3-2, HCN J=1-0) will have only a small amount of
thermalized gas, and consequently a superlinear relationship between molecular
line luminosity and mean gas density. This results in a SFR-line luminosity
index less than the Schmidt index for high critical density tracers. One
observational consequence of this is a significant redistribution of light from
the small pockets of dense, thermalized gas to diffuse gas along the line of
sight, and prodigious emission from subthermally excited gas. At the highest
star formation rates, the SFR-Lmol slope tends to the Schmidt index, regardless
of the molecular transition. The fundamental relation is the Kennicutt-Schmidt
law, rather than the relation between SFR and molecular line luminosity. We use
these results to make imminently testable predictions for the SFR-molecular
line relations of unobserved transitions.Comment: ApJ Accepted - Results remain same as previous version. Content
clarified with Referee's comment
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
Low-Frequency Carbon Recombination Lines in the Orion Molecular Cloud Complex
We detail tentative detections of low-frequency carbon radio recombination lines from within the Orion molecular cloud complex observed at 99â129 MHz. These tentative detections include one alpha transition and one beta transition over three locations and are located within the diffuse regions of dust observed in the infrared at 100 Âľm, the Ha emission detected in the optical, and the synchrotron radiation observed in the radio. With these observations, we are able to study the radiation mechanism transition from collisionally pumped to radiatively pumped within the H ii regions within the Orion molecular cloud complex
Plans for a 10-m Submillimeter-wave Telescope at the South Pole
A 10 meter diameter submillimeter-wave telescope has been proposed for the
NSF Amundsen-Scott South Pole Station. Current evidence indicates that the
South Pole is the best submillimeter-wave telescope site among all existing or
proposed ground-based observatories. Proposed scientific programs place
stringent requirements on the optical quality of the telescope design. In
particular, reduction of the thermal background and offsets requires an
off-axis, unblocked aperture, and the large field of view needed for survey
observations requires shaped optics. This mix of design elements is well-suited
for large scale (square degree) mapping of line and continuum radiation from
submillimeter-wave sources at moderate spatial resolutions (4 to 60 arcsecond
beam size) and high sensitivity (milliJansky flux density levels). the
telescope will make arcminute angular scale, high frequency Cosmic Microwave
Background measurements from the best possible ground-based site, using an
aperture which is larger than is currently possible on orbital or airborne
platforms. Effective use of this telescope will require development of large
(1000 element) arrays of submillimeter detectors which are background-limited
when illuminated by antenna temperatures near 50 K.Comment: 12 pages, 3 figure
Novel dimeric β-helical model of an ice nucleation protein with bridged active sites
<p>Abstract</p> <p>Background</p> <p>Ice nucleation proteins (INPs) allow water to freeze at high subzero temperatures. Due to their large size (>120 kDa), membrane association, and tendency to aggregate, an experimentally-determined tertiary structure of an INP has yet to be reported. How they function at the molecular level therefore remains unknown.</p> <p>Results</p> <p>Here we have predicted a novel β-helical fold for the INP produced by the bacterium <it>Pseudomonas borealis</it>. The protein uses internal serine and glutamine ladders for stabilization and is predicted to dimerize via the burying of a solvent-exposed tyrosine ladder to make an intimate hydrophobic contact along the dimerization interface. The manner in which <it>Pb</it>INP dimerizes also allows for its multimerization, which could explain the aggregation-dependence of INP activity. Both sides of the <it>Pb</it>INP structure have tandem arrays of amino acids that can organize waters into the ice-like clathrate structures seen on antifreeze proteins.</p> <p>Conclusions</p> <p>Dimerization dramatically increases the 'ice-active' surface area of the protein by doubling its width, increasing its length, and presenting identical ice-forming surfaces on both sides of the protein. We suggest that this allows sufficient anchored clathrate waters to align on the INP surface to nucleate freezing. As <it>Pb</it>INP is highly similar to all known bacterial INPs, we predict its fold and mechanism of action will apply to these other INPs.</p
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