Postdoctoral Fellows Seminars – Fall 2016

Introductory Talks from SAMSI Postdoctoral Fellows

September 14, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Benjamin Risk, Sarah Vallelian, Zhengwu Zhang, Duy Thai, David Jones, Hyungsuk Tak, David Stenning, Ahmed Attia, Sercan Yildiz, Peter Diao

Abstract

Each of our Postdoctoral fellows will perform a short introductory talk discussing their areas of academic interest and possibly an overview of their upcoming lectures throughout the fall.


Lecture: Spurious Activation from Multiband Acquisition fMRI

September 21, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Benjamin Risk

Abstract

In fMRI, conventional pulse sequences collect two-dimensional data one slice at a time and consequently require up to six seconds to acquire the dozens of slices composing a single volume with whole-brain coverage. Multiband acquisition techniques collect multiple slices in a single shot and can be used to decrease the time between acquisition of fMRI volumes, which can increase statistical power and better characterize the temporal dynamics of the blood-oxygen level dependent
(BOLD) signal. The technique requires an additional processing step in which the slices are separated, or unaliased, to recover the whole brain volume. However, this may result in signal leakage between aliased locations and lead to spurious activation (false positives). We examine the Slice-GRAPPA algorithm for image reconstruction at different acceleration factors. We found a high incidence of spurious activation in simulations. Perversely, more time points actually increase the detection of spurious activation, such that longer acquisition times can lead to poorer estimates of activation. We also show evidence of artifacts in fMRI data from the Human Connectome Project, which uses a higher acceleration factor than most studies and thus may be more susceptible to spurious activation.

Presentation

To see this presentation, click here


Lecture: Computationally efficient Markov chain Monte Carlo methods for hierarchical Bayesian inverse problems

September 28, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Sarah Vallelian

Abstract

In Bayesian inverse problems, the posterior distribution can be used to quantify uncertainty about the reconstructed solution. In practice, approximating the posterior requires Markov chain Monte Carlo (MCMC) algorithms, but these can be computationally expensive. We present a computationally efficient MCMC sampling scheme for ill-posed Bayesian inverse problems.

References

1) Computationally Efficient Markov Chain Monte Carlo Methods for Hierarchical Bayesian Inverse Problems: D. Andrew Brown, Arvind K. Saibaba, Sarah Vallélian

Presentation

To see this presentation, click here


Lecture: Nonparametric Bayes Models of Fiber Curves Connecting Brain Regions

October 5, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Zhengwu Zhang

Abstract

In studying structural inter-connections in the human brain, it is common to first estimate fiber bundles connecting different regions of the brain relying on diffusion tensor imaging. These fiber bundles act as three-dimensional highways for neural activity and communication, snaking through the brain and connecting different regions. Current statistical methods for analyzing these fibers reduce the rich information into an adjacency matrix, with the elements containing a count of the number of connections between pairs of regions. The goal of this article is to avoid discarding the rich functional data on the shape, size and orientation of fibers, developing flexible models for characterizing the population distribution of fibers between brain regions of interest within and across different individuals. We start by efficiently decomposing each fiber in each individual’s brain into a corresponding rotation matrix, shape and translation from a global reference curve. These components can then be viewed as data lying on a product space composed of different Euclidean spaces and manifolds. To non-parametrically model the distribution within and across individuals, we rely on a hierarchical mixture of product kernels specific to the component spaces. Taking a Bayesian approach to inference, we develop an efficient method for posterior sampling. The approach automatically produces clusters of fibers within and across individuals, and yields interesting new insight into variation in fiber tracks, while providing a useful starting point for more elaborate models relating fibers to covariates and neuropsychiatric traits.

Presentation

To see this presentation, click here


Lecture: TV-l2-like Model based Riesz Wavelet Frame

October 12, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Duy Thai

Abstract

In order to address a question: “which domain makes signal sparse?” Recently, there are two techniques: harmonic analysis and regularization. Sampling theory in harmonic analysis (bi)-orthogonally projects an image into scaling and wavelet spaces whose coefficients correspond to energy (lowpass signal) and singularity (edges), e.g. wavelet. Decomposition with frames (instead of bases) is known to enhance sparsity of an image, e.g. curvelet transform. Although these projection methods can define scales of oscillating components, e.g. texture or noise, it is based on linearly filtering technique, i.e. convolution with a kernel in the spatial domain, producing Gibbs effect. The total variation (TV)-l2 model, a regularization method in a Banach space, is more attractive to tackle these blurring artifact due to a shrinkage operator by the l1-norm, i.e. this model smooths a noisy image while preserving edges. We observe that a solution of this model can be rewritten as in a form of sampling theory with scaling/wavelet functions (which is similar to wavelet-like operator) and a shrinkage operator. However, their bandwidths are quite large which captures all frequencies components of fine scale structure, e.g. texture. Thus, this model cannot enhance enough sparsity under the l1-norm, i.e. although edges on a reconstructed image are kept, this model cannot remove texture while preserving contrast and it is known that stair case effect is produced in image denoising.

Benefit from these two techniques, we build a hybrid model, called TV-l2-like model based N-th order Riesz dyadic wavelet, to remove fine scale structure while preserving edges and its contrast. Implementation is simple and natural images with texture are performed for a comparison.

References

[1] D.H. Thai and L. Mentch. Multiphase Segmentation For Simultaneously Homogeneous and Textural Images: https://arxiv.org/pdf/1606.09281.pdf
[2] D.H. Thai and D. Banks. Directional Mean Curvature for Textured Image Demixing (in preparation and provided upon request)


Lecture: Detecting planets: jointly modeling radial velocity and stellar activity time series

October 19, 2016, 1:15pm – 2:15pm
Room 150
Speaker: David Jones

Abstract

The radial velocity technique is one of the two main approaches for detecting planets outside our solar system, or exoplanets as they are known in astronomy. The method works by detecting the Doppler shift resulting from the motion of a host star caused by an orbiting planet. Unfortunately, this Doppler signal is typically contaminated by various “stellar activity” phenomena, such as dark spots on the star surface. A principled approach to recovering the Doppler signal was proposed by Rajpaul et al. (2015), and involves the use of dependent Gaussian processes to jointly model the corrupted Doppler signal and multiple proxies for the stellar activity.

During the SAMSI ASTRO program, we aim to extend this work by (i) proposing more informative stellar activity proxies, (ii) adapting the model to incorporate a wider variety of proxies, and (iii) utilizing the new model to optimally schedule telescope observations. In my talk, I will introduce the problem and present a more general model that allows, for example, a lag between the current stellar activity and subsequent effects on the Doppler time series. I will then discuss potential scheduling methods and conclude.

References

Rajpaul, V., Aigrain, S., Osborne, M. A., Reece, S., & Roberts, S. (2015). A
Gaussian process framework for modelling stellar activity signals in radial velocity data. Monthly Notices of the Royal Astronomical Society, 452(3), 2269-2291. https://arxiv.org/abs/1506.07304

Boyle, P., & Frean, M. (2004). Dependent Gaussian processes. In Advances in
neural information processing systems, 217-224. http://papers.nips.cc/paper/2561-dependent-gaussian-processes.pdf

Presentation

To see this presentation, click here


Lecture: Signal Separation for Exoplanet Detection: A Diffusion Map Approach

October 26, 2016, 1:15pm – 2:15pm
Room 150
Speaker: David Stenning

Abstract

An active area of research in modern astronomy is the detection of exoplanets—planets that orbit stars other than the Sun. A promising approach for discovering Earth-like exoplanets is the radial velocity method, which involves detecting the Doppler shift in a star’s spectral lines resulting from the gravitational effects of an orbiting planet. However, the signal caused by a Doppler shift is often obscured by those resulting from stellar activity, such as dark spots rotating across the star’s surface.

We aim to disentangle the Doppler shift signal present in stellar spectra from those of stellar activity, thereby improving the efficacy of the radial velocity method. We rely on synthetic spectra generated by the SOAP 2.0 code (Dumusque et al. 2014) to test various methodologies. In this talk, I will present some preliminary results on the use of diffusion maps (e.g. Coifman and Lafon, 2006; Lafron and Lee, 2006; Richards et al., 2009)—a nonlinear dimension reduction technique—as part of our overall procedure for distinguishing between Doppler shifts and stellar activity.

References

Dumusque, X., Boisse, I., and Santos, N.C. (2014). SOAP 2.0: A Tool to Estimate the Photometric and Radial Velocity Variations Induced by Stellar Spots and Plages. The Astrophysical Journal, 796, 2.

Coifman, R.R. and Lafon, S. (2006). Diffusion maps. Applied and Computational Harmonic Analysis. 21: 5–30.

Lafon, S. and Lee, A.B. (2006). Diffusion Maps and Coarse-Graining: A Unified Framework for Dimensionality Reduction, Graph Partitioning, and Data Set Parameterization. IEEE Trans. Pattern Anal. Mach. Intell., vol. 28, pp. 1393-1403.

Richards, J.W., Freeman, P.E., Lee, A.B., and Schafer, C.M. (2009). Exploiting Low-Dimensional Structure in Astronomical Spectra. The Astrophysical Journal, 691, 32-42


Lecture: Strong Lens Time Delay Challenge 2

November 9, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Hyungsuk Tak

Abstract

The gravitational field of a galaxy can act as a lens and deflect the light emitted by a more distant object such as a quasar. If the galaxy is a strong gravitational lens, it can produce multiple images of the same quasar in the sky. Since the light in each gravitationally-lensed image traverses a different path length from the quasar to the Earth, fluctuations in the source brightness are observed in the several images at different times. The time delay between these fluctuations can be used to constrain cosmological parameters and can be inferred from the time series of brightness data or light curves of each image. Strong lens time delay challenge (TDC) is a blind competition with the aim of improving time delay estimation methods for application to realistic observational data sets in preparation for the era of the Large Synoptic Survey Telescope (LSST), the top-ranked ground-based telescope project in the 2010 Astrophysics Decadal Survey. Though the LSST will produce multi-band light curves (observed via multiple optical bands), the organizers of the first TDC simulated thousands of single-band data sets as a simplified starting point. The second TDC is about to begin with more realistic multi-band light curves. We introduce a couple of models developed from the first TDC model and demonstrate their potential to handle the multi-band light curves using several examples.

References

1. H. Tak, K. Mandel, D. A. van Dyk, V. L. Kashyap, X.-L. Meng, and A. Siemiginowska (in progress), “Bayesian Estimates of Astronomical Time Delays between Gravitationally Lensed Stochastic Light Curves,” tentatively accepted in Annals of Applied Statistics, ArXiv 1602.01462.

2. Liao et al. (2015), “Strong Lens Time Delay Challenge: II. Results of TDC1,” The Astrophysical Journal, 800, 11.

3. T. Treu and P. Marshall (in progress), “Time Delay Cosmography,” ArXiv 1605.05333


Lecture: Title Ongoing

November 16, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Ahmed Attia

Abstract

Abstract for this talk has not been provided. Please stay tuned for more details as our event gets closer.

References

To Be Determined


Lecture: Title Ongoing

November 30, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Sercan Yildiz

Abstract

Abstract for this talk has not been provided. Please stay tuned for more details as our event gets closer.

References

To Be Determined


Lecture: Title Ongoing

December 7, 2016, 1:15pm – 2:15pm
Room 150
Speaker: Peter Diao

Abstract

Abstract for this talk has not been provided. Please stay tuned for more details as our event gets closer.

References

To Be Determined