2,474 research outputs found
Differential Impact of Interference on Internally- and Externally-Directed Attention.
Attention can be oriented externally to the environment or internally to the mind, and can be derailed by interference from irrelevant information originating from either external or internal sources. However, few studies have explored the nature and underlying mechanisms of the interaction between different attentional orientations and different sources of interference. We investigated how externally- and internally-directed attention was impacted by external distraction, how this modulated internal distraction, and whether these interactions were affected by healthy aging. Healthy younger and older adults performed both an externally-oriented visual detection task and an internally-oriented mental rotation task, performed with and without auditory sound delivered through headphones. We found that the addition of auditory sound induced a significant decrease in task performance in both younger and older adults on the visual discrimination task, and this was accompanied by a shift in the type of distractions reported (from internal to external). On the internally-oriented task, auditory sound only affected performance in older adults. These results suggest that the impact of external distractions differentially impacts performance on tasks with internal, as opposed to external, attentional orientations. Further, internal distractibility is affected by the presence of external sound and increased suppression of internal distraction
The first detection time of a quantum state under random probing
We solve for the statistics of the first detection of a quantum system in a
particular desired state, when the system is subject to a projective
measurement at independent identically distributed random time intervals. We
present formulas for the probability of detection in the th attempt. We
calculate as well the mean and mean square both of the number of the first
successful detection attempt and the time till first detection. We present
explicit results for a particle initially localized at a site on a ring of size
, probed at some arbitrary given site, in the case when the detection
intervals are distributed exponentially. We prove that, for all interval
distributions and finite-dimensional Hamiltonians, the mean detection time is
equal to the mean attempt number times the mean time interval between attempts.
We further prove that for the return problem when the initial and target state
are identical, the total detection probability is unity and the mean attempts
till detection is an integer, which is the size of the Hilbert space
(symmetrized about the target state). We study an interpolation between the
fixed time interval case to an exponential distribution of time intervals via
the Gamma distribution with constant mean and varying width. The mean arrival
time as a function of the mean interval changes qualitatively as we tune the
inter-arrival time distribution from very narrow (delta peaked) to exponential,
as resonances are wiped out by the randomness of the sampling
Driving quantum systems with repeated conditional measurements
We investigate the effect of conditional null measurements on a quantum
system and find a rich variety of behaviors. Specifically, quantum dynamics
with a time independent in a finite dimensional Hilbert space are
considered with repeated strong null measurements of a specified state. We
discuss four generic behaviors that emerge in these monitored systems. The
first arises in systems without symmetry, along with their associated
degeneracies in the energy spectrum, and hence in the absence of dark states as
well. In this case, a unique final state can be found which is determined by
the largest eigenvalue of the survival operator, the non-unitary operator
encoding both the unitary evolution between measurements and the measurement
itself. For a three-level system, this is similar to the well known shelving
effect. Secondly, for systems with built-in symmetry and correspondingly a
degenerate energy spectrum, the null measurements dynamically select the
degenerate energy levels, while the non-degenerate levels are effectively wiped
out. Thirdly, in the absence of dark states, and for specific choices of
parameters, two or more eigenvalues of the survival operator match in
magnitude, and this leads to an oscillatory behavior controlled by the
measurement rate and not solely by the energy levels. Finally, when the control
parameters are tuned, such that the eigenvalues of the survival operator all
coalesce to zero, one has exceptional points that corresponds to situations
that violate the null measurement condition, making the conditional measurement
process impossible
Structural and functional integrity of neural circuits
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, September 2011.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections."September, 2011." Cataloged from student submitted PDF version of thesis.Includes bibliographical references.This dissertation documents how healthy aging and Parkinson's disease (PD) affect brain anatomy and physiology and how these neural changes relate to measures of cognition and perception. While healthy aging and PD are both accompanied by a wide-range of cognitive impairments, the neural underpinnings of cognitive decline in each is likely mediated by deterioration of different systems. The four chapters of this dissertation address specific aspects of how healthy aging and PD affect the neural circuits that support sensory processes and high-level cognition. The experiments in Chapters 2 and 3 examine the effects of healthy aging on the integrity of neural circuits that modulate cognitive control processes. In Chapter 2, we test the hypothesis that the patterns of age-related change differ between white matter and gray matter regions, and that changes in the integrity of anterior regions correlate most strongly with performance on cognitive control tasks. In Chapter 3, we build upon the structural findings by examining the hypothesis that age-related changes in white matter integrity are associated with disrupted oscillatory dynamics observed during a visual search task. Chapter 4 investigates healthy age-related changes in somatosensory mu rhythms and evoked responses and uses a computational model of primary somatosensory cortex to predict the underlying cellular and neurophysiolgical bases of these alterations. In contrast to the widespread cortical changes seen in healthy OA, the cardinal motor symptoms of PD are largely explained by degeneration of the dopaminergic substantia nigra, pars compacta (SNc). Cognitive sequelae of PD, however, likely result from disruptions in multiple neurotransmitter systems, including nondopaminergic nuclei, but research on these aspects of the disease has been hindered by a lack of sensitive MRI biomarkers for the affected structures. Chapter 5 presents new multispectral MRI tools that visualize the SNc and the cholinergic basal forebrain (BF). We applied these methods to test the hypothesis that degenerative processes in PD affect the SNc before the BF. This experiment lays important groundwork for future studies that will examine the relative contribution of the SNc and BF to cognitive impairments in PD.by David A. Ziegler.Ph.D
Single-kernel ionomic profiles are highly heritable indicators of genetic and environmental influences on elemental accumulation in maize grain (Zea mays)
Peer reviewedPublisher PD
W/SiC x-ray multilayers optimized for use above 100 keV
We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E ~150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail, compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope
Variation of magnetostriction with temperature in Tb5Si2.2Ge1.8 single crystal
The Tb5(SixGe4âx) alloy system is similar to the better known Gd5(SixGe4âx), except it has a more complex magnetic and structural phase diagram. Gd5(SixGe1âx)4 has received much attention recently due to its giant magnetocaloric effect, colossal magnetostriction and giant magnetoresistance in the vicinity of a first order combined magnetic-structural phase transition. The magnetostriction changes that accompany the phase transitions of single crystal Tb5(Si2.2Ge1.8) have been investigated at temperatures between 20 and150âK by measurements of magnetostriction along the a axis. Over this temperature range the shape and slope of the magnetostriction curves change, indicative of changes in the magnetic state, crystal structure, and magnetic anisotropy. The results appear to indicate a phase transition that occurs near 106âK (onset-completion range of 116â100âK). The steepness of the strain transition, its unusual hysteresis, and its temperature dependence appear to indicate a first order phase transition which is activated by applied magnetic field in addition to temperature (see Fig. 1). Magnetostriction measurements at temperature below the transition region appear to indicate a magnetostriction of small overall magnitude (about 30Ă10â6) but high anisotropy, with anistropy showing considerable temperature dependence
Thermoregulation of \u3ci\u3eEscherichia coil pap\u3c/i\u3e Transcription: H-NS is a Temperature-Dependent DNA Methylation Blocking Factor
The expression of Pap pili that facilitate the attach- ment of Escherichia coli to uroepithelial cells is shut off outside the host at temperatures below 268C. Ribo- nuclease protection analysis showed that this thermo- regulatory response was rapid as evidenced by the absence of papBA transcripts, coding for Pap pilin, after only one generation of growth at 238C. The his- tone-like nucleoid structuring protein H-NS and DNA sequences within papB were required for thermoregu- lation, but the PapB and PapI regulatory proteins were not. In vivo analysis of pap DNA methylation patterns indicated that H-NS or a factor regulated by H-NS bound within the pap regulatory region at 238C but not at 378C, as evidenced by H-NS-dependent inhibi- tion of methylation of the pap GATC sites designated GATC-I and GATC-II. These GATC sites lie upstream of the papBAp promoter and have been shown pre- viously to play a role in controlling Pap pili expression by regulating the binding of Lrp, a global regulator that is essential for activating papBAp transcription. Competitive electrophoretic mobility shift analysis showed that H-NS bound specifically to a pap DNA fragment containing the GATC-I and GATC-II sites. Moreover, H-NS blocked methylation of these pap GATC sites in vitro : H-NS blocked pap GATC methyla- tion at 1.4 mM but was unable to do so at higher con- centrations at which non-specific binding occurred. Thus, non-specific binding of H-NS to pap DNA was not sufficient to inhibit methylation of the pap GATC sites. These results suggest that the ability of H-NS to act as a methylation blocking factor is dependent upon the formation of a specific complex of H-NS with pap regulatory DNA. We hypothesize that a func- tion of H-NS such as oligomerization was altered at 238C, which enabled H-NS to repress pap gene expres- sion through the formation of a specific nucleoprotein complex
Star formation activity of intermediate redshift cluster galaxies out to the infall regions
We present a spectroscopic analysis of two galaxy clusters out to ~4Mpc at
z~0.2. The two clusters VMF73 and VMF74 identified by Vikhlinin et al. (1998)
were observed with MOSCA at the Calar Alto 3.5m telescope. Both clusters lie in
the ROSAT PSPC field R285 and were selected from the X-ray Dark Cluster Survey
(Gilbank et al. 2004) that provides optical V- and I-band data. VMF73 and VMF74
are located at respective redshifts of z=0.25 and z=0.18 with velocity
dispersions of 671 km/s and 442 km/s, respectively. The spectroscopic
observations reach out to ~2.5 virial radii. Line strength measurements of the
emission lines H_alpha and [OII]3727 are used to assess the star formation
activity of cluster galaxies which show radial and density dependences. The
mean and median of both line strength distributions as well as the fraction of
star forming galaxies increase with increasing clustercentric distance and
decreasing local galaxy density. Except for two galaxies with strong H_alpha
and [OII] emission, all of the cluster galaxies are normal star forming or
passive galaxies. Our results are consistent with other studies that show the
truncation in star formation occurs far from the cluster centre.Comment: 15 pages, 12 figures. A&A in pres
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