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Searching for pitch invariant representations in auditory cortex [oral presentation]
Pitch constancy relates to perceiving the same pitch from tones with differing spectral shapes and is one key criteria for identifying a pitch selective neural representation in auditory cortex. Here we used an event-related potential (ERP) adaptation study and a behavioural task (target same/different) to investigate whether pitch coding is invariant to changes in timbre. Adaptation is observed as a decrease in N100-P200 when the same stimulus is repeated because overlapping neuronal populations encode the stimulus. Reduced adaptation indicates that new neuronal populations are recruited to encode a change in an acoustic feature of interest (i.e. pitch, timbre or both). If neurons are selective to pitch (invariant to timbre), reduced adaptation should occur for pitch changes only. If selective to both (non-invariant to timbre), reduced adaptation should occur for pitch and timbre changes. Similarly, stimulus discrimination during the behavioural task should not require any additional processing resources if neurons are selective to pitch only, and hence reaction times and accuracy should be equivalent across conditions. If neurons are selective to both pitch and timbre, longer reaction times and poorer accuracy should be observed for timbre changes. We found reduced adaptation in the N100-P200 and increased reaction times and poorer accuracy for timbre changes. This suggests that neurons in auditory cortex are selective to both pitch and timbre, i.e. pitch coding is non-invariant to timbre. This supports recent evidence suggesting interdependence between pitch
Toward Interpretable Deep Reinforcement Learning with Linear Model U-Trees
Deep Reinforcement Learning (DRL) has achieved impressive success in many
applications. A key component of many DRL models is a neural network
representing a Q function, to estimate the expected cumulative reward following
a state-action pair. The Q function neural network contains a lot of implicit
knowledge about the RL problems, but often remains unexamined and
uninterpreted. To our knowledge, this work develops the first mimic learning
framework for Q functions in DRL. We introduce Linear Model U-trees (LMUTs) to
approximate neural network predictions. An LMUT is learned using a novel
on-line algorithm that is well-suited for an active play setting, where the
mimic learner observes an ongoing interaction between the neural net and the
environment. Empirical evaluation shows that an LMUT mimics a Q function
substantially better than five baseline methods. The transparent tree structure
of an LMUT facilitates understanding the network's learned knowledge by
analyzing feature influence, extracting rules, and highlighting the
super-pixels in image inputs.Comment: This paper is accepted by ECML-PKDD 201
Variability of Low-ionization Broad Absorption Line Quasars Based on Multi-epoch Spectra from The Sloan Digital Sky Survey
We present absorption variability results for 134 bona fide \mgii\ broad
absorption line (BAL) quasars at 0.46~~2.3 covering days
to 10 yr in the rest frame. We use multiple-epoch spectra from the Sloan
Digital Sky Survey, which has delivered the largest such BAL-variability sample
ever studied. \mgii-BAL identifications and related measurements are compiled
and presented in a catalog. We find a remarkable time-dependent asymmetry in EW
variation from the sample, such that weakening troughs outnumber strengthening
troughs, the first report of such a phenomenon in BAL variability. Our
investigations of the sample further reveal that (i) the frequency of BAL
variability is significantly lower (typically by a factor of 2) than that from
high-ionization BALQSO samples; (ii) \mgii\ BAL absorbers tend to have
relatively high optical depths and small covering factors along our line of
sight; (iii) there is no significant EW-variability correlation between \mgii\
troughs at different velocities in the same quasar; and (iv) the EW-variability
correlation between \mgii\ and \aliii\ BALs is significantly stronger than that
between \mgii\ and \civ\ BALs at the same velocities. These observational
results can be explained by a combined transverse-motion/ionization-change
scenario, where transverse motions likely dominate the strengthening BALs while
ionization changes and/or other mechanisms dominate the weakening BALs.Comment: 24 pages, accepted for publication in ApJ
X-ray Insights into the Nature of Quasars with Redshifted Broad Absorption Lines
We present observations of seven broad absorption line (BAL)
quasars at -2.516 with redshifted BAL troughs (RSBALs). Five of our
seven targets were detected by in 4-13 ks exposures with ACIS-S. The
values, values, and spectral energy
distributions of our targets demonstrate they are all X-ray weak relative to
expectations for non-BAL quasars, and the degree of X-ray weakness is
consistent with that of appropriately-matched BAL quasars generally.
Furthermore, our five detected targets show evidence for hard X-ray spectral
shapes with a stacked effective power-law photon index of . These findings support the presence of heavy X-ray
absorption ( cm) in RSBAL quasars,
likely by the shielding gas found to be common in BAL quasars more generally.
We use these X-ray measurements to assess models for the nature of RSBAL
quasars, finding that a rotationally-dominated outflow model is favored while
an infall model also remains plausible with some stipulations. The X-ray data
disfavor a binary quasar model for RSBAL quasars in general.Comment: 11 pages, 5 figures, and 3 table
X-ray and multi-epoch optical/UV investigations of BAL to non-BAL quasar transformations
We report on an X-ray and optical/UV study of eight Broad Absorption Line
(BAL) to non-BAL transforming quasars at 1.7-2.2 over 0.29-4.95
rest-frame years with at least three spectroscopic epochs for each quasar from
the SDSS, BOSS, , and ARC 3.5-m telescopes. New observations
obtained for these objects show their values of and
, as well as their spectral energy distributions, are
consistent with those of non-BAL quasars. Moreover, our targets have X-ray
spectral shapes that are, on average, consistent with weakened absorption with
an effective power-law photon index of . The newer and ARC 3.5-m spectra reveal
that the BAL troughs have remained absent since the BOSS observations where the
BAL disappearance was discovered. The X-ray and optical/UV results in tandem
are consistent with at least the X-ray absorbing material moving out of the
line-of-sight, leaving an X-ray unabsorbed non-BAL quasar. The UV absorber
might have become more highly ionized (in a shielding-gas scenario) or also
moved out of the line-of-sight (in a wind-clumping scenario).Comment: 14 pages, 5 figures, Accepted for publication in MNRA
Elevated trace elements in sediments and seagrasses at CO2 seeps
Seagrasses often occur around shallow marine CO2 seeps, allowing assessment of trace metal accumulation. Here, we measured Cd, Cu, Hg, Ni, Pb and Zn levels at six CO2 seeps and six reference sites in the Mediterranean. Some seep sediments had elevated metal concentrations; an extreme example was Cd which was 43x more concentrated at a seep site than its corresponding reference site. Three seeps had metal levels that were predicted to adversely affect marine biota, namely Vulcano (for Hg), Ischia (for Cu) and Paleochori (for Cd and Ni). There were higher-than-sediment levels of Zn and Ni in Posidonia oceanica and of Zn in Cymodocea nodosa, particularly in roots. High levels of Cu were found in Ischia seep sediments, yet seagrass was abundant there, and the plants contained low levels of Cu. Differences in bioavailability and toxicity of trace elements helps explain why seagrasses can be abundant at some CO2 seeps but not at others.Funding Agency
MARES 'Future Oceans' project
MARES _12_14
MARES Grant
Ghent University
FPA 2011-0016
Portuguese Foundation for Science and Technology
UID/Muli/04326/2019info:eu-repo/semantics/publishedVersio
Emergence and Variability of Broad Absorption Line Quasar Outflows
We isolate a set of quasars that exhibit emergent C iv broad absorption lines
(BALs) in their spectra by comparing spectra in the SDSS Data Release 7 and the
SDSS/BOSS Data Releases 9 and 10. After visually defining a set of emergent
BALs, follow-up observations were obtained with the Gemini Observatory for 105
quasars. We find an emergence rate consistent with the previously reported
disappearance rate of BAL quasars given the relative numbers of non-BAL and BAL
quasars in the SDSS. We find candidate newly emerged BALs are preferentially
drawn from among BALs with smaller balnicity indices, shallower depths, larger
velocities, and smaller widths. Within two rest-frame years (average) after a
BAL has emerged, we find it equally likely to continue increasing in equivalent
width in an observation six months later (average) as it is to start
decreasing. From the time separations between our observations, we conclude the
coherence time-scale of BALs is less than 100 rest-frame days. We observe
coordinated variability among pairs of troughs in the same quasar, likely due
to clouds at different velocities responding to the same changes in ionizing
flux; and the coordination is stronger if the velocity separation between the
two troughs is smaller. We speculate the latter effect may be due to clouds
having on average lower densities at higher velocities due to mass conservation
in an accelerating flow, causing the absorbing gas in those clouds to respond
on different timescales to the same ionizing flux variations.Comment: 37 pages, 25 figure
Sensing electric fields using single diamond spins
The ability to sensitively detect charges under ambient conditions would be a
fascinating new tool benefitting a wide range of researchers across
disciplines. However, most current techniques are limited to low-temperature
methods like single-electron transistors (SET), single-electron electrostatic
force microscopy and scanning tunnelling microscopy. Here we open up a new
quantum metrology technique demonstrating precision electric field measurement
using a single nitrogen-vacancy defect centre(NV) spin in diamond. An AC
electric field sensitivity reaching ~ 140V/cm/\surd Hz has been achieved. This
corresponds to the electric field produced by a single elementary charge
located at a distance of ~ 150 nm from our spin sensor with averaging for one
second. By careful analysis of the electronic structure of the defect centre,
we show how an applied magnetic field influences the electric field sensing
properties. By this we demonstrate that diamond defect centre spins can be
switched between electric and magnetic field sensing modes and identify
suitable parameter ranges for both detector schemes. By combining magnetic and
electric field sensitivity, nanoscale detection and ambient operation our study
opens up new frontiers in imaging and sensing applications ranging from
material science to bioimaging
Gene mobility promotes the spread of resistance in bacterial populations
Theory predicts that horizontal gene transfer (HGT) expands the selective conditions under which genes spread in bacterial populations. Whereas vertically inherited genes can only spread by positively selected clonal expansion, mobile genetic elements can drive fixation of genes by infectious HGT. We tested this using populations of Pseudomonas fluorescens and the conjugative mercury resistance (Hg R) plasmid pQBR57. HGT expanded the selective conditions allowing the spread of Hg R: Chromosomal Hg R only increased in frequency under positive selection, whereas plasmid-encoded Hg R reached fixation with or without positive selection. Tracking plasmid dynamics over time revealed that the mode of Hg R inheritance varied across mercury environments. Under mercury selection, the spread of Hg R was driven primarily by clonal expansion while in the absence of mercury Hg R dynamics were dominated by infectious transfer. Thus, HGT is most likely to drive the spread of resistance genes in environments where resistance is useless
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