3,248 research outputs found
Neuroimaging in Human Category Learning: A Comparison Between Functional Near-Infrared Spectroscopy (fNIR) and Functional Magnetic Resonance Imaging (fMRI)
The objective of this thesis is to examine the validity of functional near-infrared spectroscopy (fNIR) to examine brain regions involved in rule based (RB) and information integration (II) category learning. We predicted similar patterns of activation found by past studies that used fMRI scans. Our goal was to test if fNIR would be able to detect changes in blood oxygenation levels of participants who learned to categorize (learners) vs. those that did not (non learners). The stimulus set comprised of lines that differed in length and orientation. Participants had to learn to categorize by trial and error based on the feedback provided. Behavioral and neuroimaging data was recorded for both RB and II conditions. Results showed an upward trend in response accuracy over trials for participants identified as learners. Furthermore, blood oxygenation levels reported by fNIR indicated a systematic increase in oxygen consumption for learners as compared to non learners. These areas of increased prefrontal cortex activity recorded by fNIR correspond to the same areas found to be involved in categorization by fMRI. This paper reviews the background of category learning, explores various neuroimaging techniques in categorization research, and investigates the efficacy of fNIR as a relatively new neuroimaging modality by comparing it to fMRI
Applying Functional Near Infrared (fNIR) Spectroscopy to Enhance MIS Research
This review paper introduces the emerging technology of optical brain imaging, also known as functional near infrared (fNIR) spectroscopy, and discusses its potential role in enhancing theory and methodology used in MIS research. We discuss basic fNIR principles including the technique’s safe and portable nature, which allows ambulatory brain activity assessment in real world environments. We then touch on the neural correlates that fNIR measures, and the cortical oxygenation changes in the dorsal and anterior regions of the prefrontal cortex. We compare fNIR with traditional neuroimaging methods such as fMRI and PET. We also list case studies, future directions, and potential approaches relevant to MIS. fNIR may be used to inform theory and improve assessments in MIS-based studies, including informing theory, by identifying neural correlates, studying constructs that could not easily if at all be measured with traditional methods, applying objective constructs that subjects are unaware of, and designing better surveys
A search for hyperluminous X-ray sources in the XMM-Newton source catalog
We present a new method to identify luminous off-nuclear X-ray sources in the
outskirts of galaxies from large public redshift surveys, distinguishing them
from foreground and background interlopers. Using the 3XMM-DR5 catalog of X-ray
sources and the SDSS DR12 spectroscopic sample of galaxies, with the help of
this off-nuclear cross-matching technique, we selected 98 sources with inferred
X-ray luminosities in the range , compatible with hyperluminous X-ray objects (HLX). To validate
the method, we verify that it allowed us to recover known HLX candidates such
as ESO 24349 HLX1 and M82 X1. From a statistical study, we
conservatively estimate that up to of these sources may be fore- or
background sources, statistically leaving at least 16 that are likely to be
HLXs, thus providing support for the existence of the HLX population. We
identify two good HLX candidates and using other publicly available datasets,
in particular the VLA FIRST in radio, UKIDSS in the near-infrared, GALEX in the
ultra-violet and CFHT Megacam archive in the optical, we present evidence that
these objects are unlikely to be foreground or background X-ray objects of
conventional types, e.g. active galactic nuclei, BL Lac objects, Galactic X-ray
binaries or nearby stars. However, additional dedicated X-ray and optical
observations are needed to confirm their association with the assumed host
galaxies and thus secure their HLX classification.Comment: 20 pages, 6 figures, accepted to Ap
Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature
The direct measurement of Berry phases is still a great challenge in
condensed matter systems. The bottleneck has been the ability to adiabatically
drive an electron coherently across a large portion of the Brillouin zone in a
solid where the scattering is strong and complicated. We break through this
bottleneck and show that high-order sideband generation (HSG) in semiconductors
is intimately affected by Berry phases. Electron-hole recollisions and HSG
occur when a near-band gap laser beam excites a semiconductor that is driven by
sufficiently strong terahertz (THz)-frequency electric fields. We carried out
experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum
wells. The observed HSG spectra contain sidebands up to the 90th order, to our
knowledge the highest-order optical nonlinearity observed in solids. The
highest-order sidebands are associated with electron-hole pairs driven
coherently across roughly 10% of the Brillouin zone around the \Gamma point.
The principal experimental claim is a dynamical birefringence: the sidebands,
when the order is high enough (> 20), are usually stronger when the exciting
near-infrared (NIR) and the THz electric fields are polarized perpendicular
than parallel; the sideband intensities depend on the angles between the THz
field and the crystal axes in samples with sufficiently weak quenched disorder;
and the sidebands exhibit significant ellipticity that increases with
increasing sideband order, despite nearly linear excitation and driving fields.
We explain dynamical birefringence by generalizing the three-step model for
high order harmonic generation. The hole accumulates Berry phases due to
variation of its internal state as the quasi-momentum changes under the THz
field. Dynamical birefringence arises from quantum interference between
time-reversed pairs of electron-hole recollision pathways
Optical frequency combs from high-order sideband generation
We report on the generation of frequency combs from the recently-discovered
phenomenon of high-order sideband generation (HSG). A near-band gap
continuous-wave (cw) laser with frequency was transmitted
through an epitaxial layer containing GaAs/AlGaAs quantum wells that were
driven by quasi-cw in-plane electric fields between 4 and 50
kV/cm oscillating at frequencies between 240 and 640 GHz.
Frequency combs with teeth at
( even) were produced, with maximum reported , corresponding to a
maximum comb span THz. Comb spectra with the identical product
were found to have similar spans and shapes
in most cases, as expected from the picture of HSG as a scattering-limited
electron-hole recollision phenomenon. The HSG combs were used to measure the
frequency and linewidth of our THz source as a demonstration of potential
applications
Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission
Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk.
Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars.
Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data.
Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (N_(CO) 10¹⁸ cm⁻²) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (T_(gas) ~ T_(dust)).
Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future
- …