10,674 research outputs found
Sampling Properties of the Spectrum and Coherency of Sequences of Action Potentials
The spectrum and coherency are useful quantities for characterizing the
temporal correlations and functional relations within and between point
processes. This paper begins with a review of these quantities, their
interpretation and how they may be estimated. A discussion of how to assess the
statistical significance of features in these measures is included. In
addition, new work is presented which builds on the framework established in
the review section. This work investigates how the estimates and their error
bars are modified by finite sample sizes. Finite sample corrections are derived
based on a doubly stochastic inhomogeneous Poisson process model in which the
rate functions are drawn from a low variance Gaussian process. It is found
that, in contrast to continuous processes, the variance of the estimators
cannot be reduced by smoothing beyond a scale which is set by the number of
point events in the interval. Alternatively, the degrees of freedom of the
estimators can be thought of as bounded from above by the expected number of
point events in the interval. Further new work describing and illustrating a
method for detecting the presence of a line in a point process spectrum is also
presented, corresponding to the detection of a periodic modulation of the
underlying rate. This work demonstrates that a known statistical test,
applicable to continuous processes, applies, with little modification, to point
process spectra, and is of utility in studying a point process driven by a
continuous stimulus. While the material discussed is of general applicability
to point processes attention will be confined to sequences of neuronal action
potentials (spike trains) which were the motivation for this work.Comment: 33 pages, 9 figure
Possible High-Redshift, Low-Luminosity AGN Activity in the Hubble Deep Field
In the Hubble Deep Field (HDF), twelve candidate sources of high-redshift (z
> 3.5) AGN activity have been identified. The color selection criteria were
established by passing spectra of selected quasars and Seyfert galaxies
(appropriately redshifted and modified for "Lyman forest" absorption), as well
as stars, observed normal and starburst galaxies, and galaxy models for various
redshifts through the filters used for the HDF observations. The actual
identification of AGN candidates also involved convolving a
Laplacian-of-Gaussian filter with the HDF images, thereby removing relatively
flat galactic backgrounds and leaving only the point-like components in the
centers. Along with positions and colors, estimated redshifts and absolute
magnitudes are reported, with the candidates falling toward the faint end of
the AGN luminosity function. One candidate has been previously observed
spectroscopically, with a measured redshift of 4.02. The number of sources
reported here is consistent with a simple extrapolation of the observed quasar
luminosity function to magnitude 30 in B_Johnson. Implications for ionization
of the intergalactic medium and for gravitational lensing are discussed.Comment: 10 pages LaTex plus 2 separate files (Table 1 which is a two-page
landscape LaTex file; and Figure 6 which is a large (0.7 MB) non-encapsulated
postscript file). Accepted for publication in the Astronomical Journa
Weak Lensing Determination of the Mass in Galaxy Halos
We detect the weak gravitational lensing distortion of 450,000 background
galaxies (20<R<23) by 790 foreground galaxies (R<18) selected from the Las
Campanas Redshift Survey (LCRS). This is the first detection of weak lensing by
field galaxies of known redshift, and as such permits us to reconstruct the
shear profile of the typical field galaxy halo in absolute physical units
(modulo H_0), and to investigate the dependence of halo mass upon galaxy
luminosity. This is also the first galaxy-galaxy lensing study for which the
calibration errors are negligible. Within a projected radius of 200 \hkpc, the
shear profile is consistent with an isothermal profile with circular velocity
164+-20 km/s for an L* galaxy, consistent with typical disk rotation at this
luminosity. This halo mass normalization, combined with the halo profile
derived by Fischer et al (2000) from lensing analysis SDSS data, places a lower
limit of (2.7+-0.6) x 10^{12}h^{-1} solar masses on the mass of an L* galaxy
halo, in good agreement with satellite galaxy studies. Given the known
luminosity function of LCRS galaxies, and the assumption that for galaxies, we determine that the mass within 260\hkpc of normal
galaxies contributes to the density of the Universe (for
) or for . These lensing data suggest
that (95% CL), only marginally in agreement with the usual
Faber-Jackson or Tully-Fisher scaling. This is the most
complete direct inventory of the matter content of the Universe to date.Comment: 18 pages, incl. 3 figures. Submitted to ApJ 6/7/00, still no response
from the referee after four months
A NEW APPROACH FOR ASSESSING THE COSTS OF LIVING WITH WILDLIFE IN DEVELOPING COUNTRIES
The costs of living with wildlife are assessed using Namibian subsistence farmers willingness to pay (WTP) for deterrents to attacks on crops and livestock as a measure of damage costs. A utility-theoretic approach jointly estimates household WTP for deterrent programs in two currencies, maize and cash. This has a double payoff. Use of a noncash staple increases respondent comprehension and provides more information about preferences, improving the accuracy of results. The household shadow value of maize is also identified. Significant costs from living with elephants and other types of wildlife are demonstrated. Compensation for farmers may be warranted on equity and efficiency grounds. Uncontrolled domestic cattle generate even higher costs to farmers than wildlife, highlighting the need to clarify property rights among these farmers.Resource /Energy Economics and Policy,
The environment and host haloes of the brightest z~6 Lyman-break galaxies
By studying the large-scale structure of the bright high-redshift Lyman-break
galaxy (LBG) population it is possible to gain an insight into the role of
environment in galaxy formation physics in the early Universe. We measure the
clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a
halo occupation distribution (HOD) model to measure their typical halo masses.
We find that the clustering amplitude and corresponding HOD fits suggests that
these sources are highly biased (b~8) objects in the densest regions of the
high-redshift Universe. Coupled with the observed rapid evolution of the number
density of these objects, our results suggest that the shape of high luminosity
end of the luminosity function is related to feedback processes or dust
obscuration in the early Universe - as opposed to a scenario where these
sources are predominantly rare instances of the much more numerous M_UV ~ -19
population of galaxies caught in a particularly vigorous period of star
formation. There is a slight tension between the number densities and
clustering measurements, which we interpret this as a signal that a refinement
of the model halo bias relation at high redshifts or the incorporation of
quasi-linear effects may be needed for future attempts at modelling the
clustering and number counts. Finally, the difference in number density between
the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is
shown to be consistent with the cosmic variance implied by the clustering
measurements.Comment: 19 pages, 8 figures, accepted MNRAS 23rd March 201
On Combining Lensing Shear Information from Multiple Filters
We consider the possible gain in the measurement of lensing shear from
imaging data in multiple filters. Galaxy shapes may differ significantly across
filters, so that the same galaxy offers multiple samples of the shear. On the
other extreme, if galaxy shapes are identical in different filters, one can
combine them to improve the signal-to-noise and thus increase the effective
number density of faint, high redshift galaxies. We use the GOODS dataset to
test these scenarios by calculating the covariance matrix of galaxy
ellipticities in four visual filters (B,V,i,z). We find that galaxy shapes are
highly correlated, and estimate the gain in galaxy number density by combining
their shapes.Comment: 8 pages, no figures, submitted to JCA
A long-term comparison of wind and tide measurements in the upper mesosphere recorded with an imaging Doppler interferometer and SuperDARN radar at Halley, Antarctica
International audienceData from a co-located imaging Doppler interferometer and SuperDARN radar recorded since 1996 have been analysed in a consistent manner to determine daily mean winds and tides in the upper mesosphere. By comparing only days when both techniques were recording good quality data it is shown that the SuperDARN radar winds and tides correlate best with the IDI height bin 90?95 km. On timescales of one hour the winds derived from each technique correlate poorly, whereas the daily mean winds are in much better agreement suggesting that the two radars are sensitive to different parts of the gravity wave spectrum. Regression analysis reveals that the observed SuperDARN daily mean meridional wind strength is approximately 65% that recorded by the IDI while the zonal winds are of similar magnitude, in good quantitative agreement with previous studies which have shown contamination to SuperDARN-derived winds due to the significant back lobe of the radar radiation pattern. Climatologically the two techniques observe similar monthly mean winds with the SuperDARN meridional winds suppressed compared to the IDI which tends to record winds more poleward and eastward than those derived by the SuperDARN radar during the summer months, and to be slightly more equatorward during the winter. The 12-h tidal amplitude and phase in both the zonal and meridional components derived from both techniques are in excellent agreement, whereas the 24-h tides are seen much more strongly in the SuperDARN radar, especially in wintertime, with poor phase agreement. Long term comparison of the two techniques reveals a tendency for the IDI meridional winds to be more poleward during solar maximum especially during summer time; an effect which is not reproduced in the meridional winds derived from the SuperDARN radar. These results are discussed in the context of previous studies to independently determine the veracity of each technique
Using Machine Learning to Predict Swine Movements within a Regional Program to Improve Control of Infectious Diseases in the US.
Between-farm animal movement is one of the most important factors influencing the spread of infectious diseases in food animals, including in the US swine industry. Understanding the structural network of contacts in a food animal industry is prerequisite to planning for efficient production strategies and for effective disease control measures. Unfortunately, data regarding between-farm animal movements in the US are not systematically collected and thus, such information is often unavailable. In this paper, we develop a procedure to replicate the structure of a network, making use of partial data available, and subsequently use the model developed to predict animal movements among sites in 34 Minnesota counties. First, we summarized two networks of swine producing facilities in Minnesota, then we used a machine learning technique referred to as random forest, an ensemble of independent classification trees, to estimate the probability of pig movements between farms and/or markets sites located in two counties in Minnesota. The model was calibrated and tested by comparing predicted data and observed data in those two counties for which data were available. Finally, the model was used to predict animal movements in sites located across 34 Minnesota counties. Variables that were important in predicting pig movements included between-site distance, ownership, and production type of the sending and receiving farms and/or markets. Using a weighted-kernel approach to describe spatial variation in the centrality measures of the predicted network, we showed that the south-central region of the study area exhibited high aggregation of predicted pig movements. Our results show an overlap with the distribution of outbreaks of porcine reproductive and respiratory syndrome, which is believed to be transmitted, at least in part, though animal movements. While the correspondence of movements and disease is not a causal test, it suggests that the predicted network may approximate actual movements. Accordingly, the predictions provided here might help to design and implement control strategies in the region. Additionally, the methodology here may be used to estimate contact networks for other livestock systems when only incomplete information regarding animal movements is available
Epidemic of youth nicotine addiction? What does the National Youth Tobacco Survey reveal about high school e-cigarette use in the USA? (Preprint)
Background: In November 2018 the US Food and Drug Administration announced restrictions on e-cigarette manufacturers in response to a perceived epidemic of e-cigarette use among high school students. The stimulus was headline figures from the 2018 National Youth Tobacco Survey (NYTS). We analyse e-cigarette use and dependence in the NYTS in relation to lifetime history of use of tobacco products.
Design and setting: Nationally representative survey of high school students in 2017 and 2018.
Participants: 10,186 students in 2017 and 10,991 in 2018.
Measurements: Any use of e-cigarettes in past 30 days, frequent e-cigarette use (â„20 of past 30 days) and indicators of dependence (craving in past 30 days; use within 30 minutes of waking and days used in lifetime) were analysed in relation to lifetime tobacco product use history, ranging from never use through to lifetime smoking of >100 cigarettes.
Findings: Past-30-day e-cigarette use increased by 78% from 11.7% in 2017 to 20.8% in 2018. In both years, use was strongly associated with lifetime tobacco use history: it was seen in 8.4% of never tobacco users in 2018, in 29.0% of those who had tried a non-combustible, but never a combustible, product (OR 4.4 (CI 2.8-7.2) by comparison with never tobacco users), and in 71.0 % of those who had smoked more than 100 cigarettes in their lifetime (OR 26.8, CI 17.7-40.5) Frequent use occurred in 0.1% of never tobacco users in 2017 and 1.0% in 2018. Findings from 2014 and 2015 showed that first product tried was overwhelmingly cigarettes among those with a substantial lifetime cigarette history. Among past-30-day e-cigarette users who had never tried tobacco products in 2018, 3.8% reported craving, 3.1% reported wanting to use within 30 minutes of waking, and 61.8% said they had used e-cigarettes on â€10 days in their life.
Conclusions: Data from the NYTS do not support claims of a new epidemic of nicotine addiction stemming from use of e-cigarettes, nor concerns that declines in youth tobacco addiction stand to be reversed after years of progress. Among current e-cigarette users who had never tried tobacco products, responses consistently pointed to minimal dependence
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