Discovering How The Brain Works Through Computation

A team led by professor Christos Papadimitriou proposes a new computational system to expand the understanding of the brain at an intermediate level, between neurons and cognitive phenomena such as language.

CS Undergrads Recognized by the Computing Research Association

For this year’s Outstanding Undergraduate Researcher Award, three computer science students received honorable mentions – Lalita Devadas, Dave Epstein, and Jessy Xinyi Han. The Computing Research Association (CRA) recognized the undergraduates for their work in an area of computing research.

Secure Montgomery Multiplication and Repeated Squares for Modular Exponentiation
Lalita Devadas Columbia University and Justin Bloom Oregon State University

The researchers worked on using some recent advances in garbling of arithmetic circuits for secure exponentiation mod N, a vital operation in many cryptosystems, including in the RSA public-key cryptosystem.

A garbled circuit is a cryptographic protocol which allows for secure two-party computation, in which two parties, Alice and Bob, each with a private input, want to compute some shared function of their inputs without either party learning the other’s input.

Their novel approach implemented the Montgomery multiplication method, which uses clever arithmetic to avoid costly division by the modulus being multiplied in. The best method they found had each wire in a circuit representing one digit of a number in base p. They developed a system of base p arithmetic which is asymptotically more efficient in the given garbled circuit architecture than any existing protocols. 

They measured performance for both approaches by counting the ciphertexts communicated for a single multiplication (a typical measure of efficiency for garbled circuit operations). They found that the base p Montgomery multiplication implementation vastly outperformed all other implementations for values of N with bit length greater than 500 (i.e., all N used for applications like RSA encryption).

“Unfortunately, our best implementations showed only incremental improvement over existing non-Montgomery-based implementations for values of N used in practice,” said Lalita Devadas. “We are still looking into further optimizations using Montgomery multiplication.”

Secure multiparty computation has many applications outside of computer science. For example, suppose five friends want to know their cumulative net worth without anyone learning anyone else’s individual net worth. This is actually a secure computation problem, since the friends want to perform some computation of their inputs while keeping said inputs private from other parties.

Oops! Predicting Unintentional Action in Video
Dave Epstein Columbia University, Boyuan Chen Columbia University, and Carl Vondrick Columbia University

The paper trains models to detect when human action is unintentional using self-supervised computer vision, an important step towards machines that can intelligently reason about the intentions behind complex human actions.

Despite enormous scientific progress over the last five to ten years, machines still struggle with tasks learned quickly and autonomously by young children, such as understanding human behavior or learning to speak a language. Epstein’s research tackles these types of problems by using self-supervised computer vision, a paradigm that predicts information naturally present in large amounts of input data such as images or videos. This stands in contrast with supervised learning, which relies on humans manually labelling data (e.g. “this is a picture of a dog”).

“I was surprised to learn that failure is an expected part of research and that it can take a long time to realize you’re failing,” said Dave Epstein. “Taking a failed idea, identifying the promising parts, and trying again leads to successful research.”

Seeding Network Influence in Biased Networks and the Benefits of Diversity
Ana-Andreea Stoica Columbia University, Jessy Xinyi Han Columbia University, and Augustin Chaintreau Columbia University

The paper explores the problem of social influence maximization and how information is diffused in a social network.

For example, it might be about what kind of news people read on social media, how many people know about job opportunities or who hears about the latest loan options from a bank. So given a social network, classical algorithms are focused on picking the best k early-adopters based on how central they are in a network, say, based on their number of connections, to maximize outreach.

However, since social inequalities are reflected in the uneven networks, classical algorithms which ignore demographics often amplify such inequalities in information access.

“We were wondering if we can do better than an algorithm that ignores demographics,” said Jessy Xinyi Han. “‘Better’ here means more people in total and more people from the disadvantaged group can receive the information.”

Through a network model with unequal communities, they developed new heuristics to take demographics into account, showing that including sensitive features in the input of most natural seed selection algorithms substantially improves diversity but also often leaves efficiency untouched or even provides a small gain.

Such analytical condition turned out to be a closed-form condition on the number of early adopters. They also validated this result on the real CS co-authorship network from DBLP.

21 papers from CS researchers accepted to NeurIPS 2019

The 33rd Conference on Neural Information Processing Systems (NeurIPS 2019) fosters the exchange of research on neural information processing systems in their biological, technological, mathematical, and theoretical aspects. 

The annual meeting is one of the premier gatherings in artificial intelligence and machine learning that featured talks, demos from industry partners as well as tutorials. Professor Vishal Misra, with colleagues from the Massachusetts Institute of Technology (MIT), held a tutorial on synthetic control.

At this year’s NeurIPS, 21 papers from the department were accepted to the conference. Computer science professors and students worked with researchers from the statistics department and the Data Science Institute.

Noise-tolerant Fair Classification
Alex Lamy Columbia University, Ziyuan Zhong Columbia University, Aditya Menon Google, Nakul Verma Columbia University

Fairness-aware learning involves designing algorithms that do not discriminate with respect to some sensitive feature (e.g., race or gender) and is usually done under the assumption that the sensitive feature available in a training sample is perfectly reliable.

This assumption may be violated in many real-world cases: for example, respondents to a survey may choose to conceal or obfuscate their group identity out of fear of potential discrimination. In the paper, the researchers show that fair classifiers can still be used given noisy sensitive features by simply changing the desired fairness-tolerance. Their procedure is empirically effective on two relevant real-world case-studies involving sensitive feature censoring.

Poisson-randomized Gamma Dynamical Systems
Aaron Schein UMass Amherst, Scott Linderman Columbia University, Mingyuan Zhou University of Texas at Austin, David Blei Columbia University, Hanna Wallach MSR NYC

This paper presents a new class of state space models for count data. It derives new properties of the Poisson-randomized gamma distribution for efficient posterior inference.

Using Embeddings to Correct for Unobserved Confounding in Networks
Victor Veitch Columbia University, Yixin Wang Columbia University, David Blei Columbia University

This paper address causal inference in the presence of unobserved confounder when proxy is available for the confounders in the form of a network connecting the units. For example, the link structure of friendships in a social network reveals information about the latent preferences of people in that network. The researchers show how modern network embedding methods can be exploited to harness the network estimation for efficient causal adjustment.

Variational Bayes Under Model Misspecification
Yixin Wang Columbia University, David Blei Columbia University

The paper characterizes the theoretical properties of a popular machine learning algorithm, variational Bayes (VB). The researchers studied the VB under model misspecification, which is the setting that is most aligned with the practice, and show that the VB posterior is asymptotically normal and centers at the value that minimizes the Kullback-Leibler (KL) divergence to the true data-generating distribution. 

As a consequence, they found that the model misspecification error dominates the variational approximation error in VB posterior predictive distributions. In other words, VB pays a negligible price in producing posterior predictive distributions. It explains the widely observed phenomenon that VB achieves comparable predictive accuracy with MCMC even though VB uses an approximating family.

Poincaré Recurrence, Cycles and Spurious Equilibria in Gradient-Descent-Ascent for Non-Convex Non-Concave Zero-Sum Games
Emmanouil-Vasileios Vlatakis-Gkaragkounis Columbia University, Lampros Flokas Columbia University, Georgios Piliouras Singapore University of Technology and Design

The paper introduces a model that captures a min-max competition over complex error landscapes and shows that even a simplified model can provably replicate some of the most commonly reported failure modes of GANs (non-convergence, deadlock in suboptimal states, etc).

Moreover, the researchers were able to understand the hidden structure in these systems — the min-max competition can lead to system behavior that is similar to that of energy preserving systems in physics (e.g. connected pendulums, many-body problems, etc). This makes it easier to understand why these systems can fail and gives new tools in the design of algorithms for training GANs. 

Near-Optimal Reinforcement Learning in Dynamic Treatment Regimes
Junzhe Zhang Columbia University, Elias Bareinboim Columbia University

Dynamic Treatment Regimes (DTRs) are particularly effective for managing chronic disorders and is arguably one of the key aspects towards more personalized decision-making. The researchers developed the first adaptive algorithm that achieves near-optimal regret in DTRs in online settings, while leveraging the abundant, yet imperfect confounded observations. Applications are given to personalized medicine and treatment recommendation in clinical decision support.

Paraphrase Generation with Latent Bag of Words
Yao Fu Columbia University, Yansong Feng Peking University, John Cunningham University of Columbia

The paper proposes a latent bag of words model for differentiable content planning and surface realization in text generation. This model generates paraphrases with clear steps, adding interpretability and controllability of existing neural text generation models. 

Adapting Neural Networks for the Estimation of Treatment Effects
Claudia Shi Columbia University, David Blei Columbia University, Victor Veitch Columbia University

This paper addresses how to design neural networks to get very accurate estimates of causal effects from observational data. The researchers propose two methods based on insights from the statistical literature on the estimation of treatment effects. 

The first is a new architecture, the Dragonnet, that exploits the sufficiency of the propensity score for estimation adjustment. The second is a regularization procedure, targeted regularization, that induces a bias towards models that have non-parametrically optimal asymptotic properties “out-of-the-box”. Studies on benchmark datasets for causal inference show these adaptations outperform existing methods.

Efficiently Avoiding Saddle Points with Zero Order Methods: No Gradients Required
Emmanouil-Vasileios Vlatakis-Gkaragkounis Columbia University, Lampros Flokas Columbia University, Georgios Piliouras Singapore University of Technology and Design

The researchers prove that properly tailored zero-order methods are as effective as their first-order counterparts. This analysis requires a combination of tools from optimization theory, probability theory and dynamical systems to show that even without perfect knowledge of the shape of the error landscape, effective optimization is possible.

Metric Learning for Adversarial Robustness
Chengzhi Mao Columbia University, Ziyuan Zhong Columbia University, Junfeng Yang Columbia University, Carl Vondrick Columbia University, Baishakhi Ray Columbia University

Deep networks are well-known to be fragile to adversarial attacks. The paper introduces a novel Triplet Loss Adversarial (TLA) regulation that is the first method that leverages metric learning to improve the robustness of deep networks. This method is inspired by the evidence that deep networks suffer from distorted feature space under adversarial attacks. The method increases the model robustness and efficiency for the detection of adversarial attacks significantly.

Efficient Symmetric Norm Regression via Linear Sketching
Zhao Song University of Washington, Ruosong Wang Carnegie Mellon University, Lin Yang Johns Hopkins University, Hongyang Zhang TTIC, Peilin Zhong Columbia University

The paper studies linear regression problems with general symmetric norm loss and gives efficient algorithms for solving such linear regression problems via sketching techniques.

Rethinking Generative Coverage: A Pointwise Guaranteed Approach
Peilin Zhong Columbia University, Yuchen Mo Columbia University, Chang Xiao Columbia University, Pengyu Chen Columbia University, Changxi Zheng Columbia University

The paper presents a novel and  formal definition of mode coverage for generative models. It also gives a boosting algorithm to achieve this mode coverage guarantee.

How Many Variables Should Be Entered in a Principal Component Regression Equation?
Ji Xu Columbia University, Daniel Hsu Columbia University

The researchers studied the least-squares linear regression over $N$ uncorrelated Gaussian features that are selected in order of decreasing variance with the number of selected features $p$ can be either smaller or greater than the sample size $n$. And give an average-case analysis of the out-of-sample prediction error as $p,n,N \to \infty$ with $p/N \to \alpha$ and $n/N \to \beta$, for some constants $\alpha \in [0,1]$ and $\beta \in (0,1)$. In this average-case setting, the prediction error exhibits a “double descent” shape as a function of $p$. This also establishes conditions under which the minimum risk is achieved in the interpolating ($p>n$) regime.

Adaptive Influence Maximization with Myopic Feedback
Binghui Peng Columbia University, Wei Chen Microsoft Research

The paper investigates the adaptive influence maximization problem and provides upper and lower bounds for the adaptivity gaps under myopic feedback model. The results confirm a long standing open conjecture by Golovin and Krause (2011).

Towards a Zero-One Law for Column Subset Selection
Zhao Song University of Washington, David Woodruff Carnegie Mellon University, Peilin Zhong Columbia University

The researchers studied low-rank matrix approximation with general loss function and showed that if the loss function has several good properties, then there is an efficient way to compute a good low-rank approximation. Otherwise, it could be hard to compute a good low-rank approximation efficiently.

Average Case Column Subset Selection for Entrywise l1-Norm Loss
Zhao Song University of Washington, David Woodruff Carnegie Mellon University, Peilin Zhong Columbia University

The researchers studied how to compute an l1-norm loss low-rank matrix approximation to a given matrix. And showed that if the given matrix can be decomposed into a low-rank matrix and a noise matrix with a mild distributional assumption, we can obtain a (1+eps) approximation to the optimal solution.

A New Distribution on the Simplex with Auto-Encoding Applications
Andrew Stirn Columbia University, Tony Jebara Spotify, David Knowles Columbia University

The researchers developed a surrogate distribution for the Dirichlet that offers explicit, tractable reparameterization, the ability to capture sparsity, and has barycentric symmetry properties (i.e. exchangeability) equivalent to the Dirichlet. Previous works have used the Kumaraswamy distribution in a stick-breaking process to create a non-exchangeable distribution on the simplex. The method was improved by restoring exchangeability and demonstrating that approximate exchangeability is efficiently achievable. Lastly, the method was showcased in a variety of VAE semi-supervised learning tasks.

Discrete Flows: Invertible Generative Models of Discrete Data
Dustin Tran Google Brain, Keyon Vafa Columbia University, Kumar Agrawal Google AI Resident, Laurent Dinh Google Brain, Ben Poole Google Brain

While normalizing flows have led to significant advances in modeling high-dimensional continuous distributions, their applicability to discrete distributions remains unknown. The researchers extend normalizing flows to discrete events, using a simple change-of-variables formula not requiring log-determinant-Jacobian computations. Empirically, they find that discrete flows obtain competitive performance with or outperform autoregressive baselines on various tasks, including addition, Potts models, and language models.

Characterization and Learning of Causal Graphs with Latent Variables from Soft Interventions
Murat Kocaoglu MIT-IBM Watson AI Lab IBM Research, Amin Jaber Purdue University, Karthikeyan Shanmugam MIT-IBM Watson AI Lab IBM Research NY, Elias Bareinboim Columbia University

This work is all about learning causal relationships – the classic aim of which is to characterize all possible sets that could produce the observed data. In the paper, the researchers provide a complete characterization of all possible causal graphs with observational and interventional data involving so-called ‘soft interventions’ on variables when the targets of soft interventions are known.

This work potentially could lead to discovery of other novel learning algorithms that are both sound and complete.

Identification of Conditional Causal Effects Under Markov Equivalence
Amin Jaber Purdue University, Jiji Zhang Lingnan University, Elias Bareinboim Columbia University

Causal identification is the problem of deciding whether a causal distribution is computable from a combination of qualitative knowledge about the underlying data-generating process, which is usually encoded in the form of a causal graph, and an observational distribution. Despite the obvious need for identifying causal effects throughout the data-driven sciences, in practice, finding the causal graph is a notoriously challenging task.

In this work, the researchers provide a relaxation of the requirement of having to specify the causal graph (based on substantive knowledge) and allow the input of the inference to be an equivalence class of causal graphs, which can be inferred from data. Specifically, they propose the first general algorithm to learn conditional causal effects entirely from data. This result is particularly useful for evaluating the impact of conditional plans and stochastic policies, which appear both in AI (in the context of reinforcement learning) and in the data-driven sciences.

Efficient Identification in Linear Structural Causal Models with Instrumental Cutsets
Daniel Kumor Purdue University, Bryant Chen Brex Inc., Elias Bareinboim Columbia University

Regression analysis is one of the most common tools used in modern data science. While there is a great understanding and powerful technology to perform regression analysis in high dimensional spaces, the output of such a method is purely associational and devoid of any causal interpretation.

The researchers studied the problem of identification of structural (causal) coefficients in linear systems (deciding whether regression coefficients are amenable to causal interpretation, etc). Building on a technique called instrumental variables, they developed a new method called Instrumental Cutset, which partitions the systems into tractable components such that identification can be decided more efficiently. The resulting algorithm was efficient and strictly more powerful than the current state-of-the-art methods.

When Does Failure Become a Good Thing?

Assistant Professor Allison Bishop takes a look at failure and how people can learn from “unsuccessful” research.

When it comes to research and getting papers into cryptography conferences, there usually has to be a “positive” result — either a new theorem must be proven, a new algorithm must be presented, or a successful attack on an existing algorithm must be obtained. If researchers try to accomplish a lofty goal and fall short, but manage to achieve a smaller goal, they typically present only the smaller goal as if it was the point on its own.

Allison Bishop

“I’ve found that not every research paper magically comes together and has a “great” result,” said Allison Bishop, who has been teaching since 2013. “Our community doesn’t really talk about the research process and I wanted to highlight research where even if it “failed” there is still something to learn from it.”

Through the years Bishop noticed the lack of a venue to talk about all kinds of research. When she and other researchers studied obfuscation it resulted in a paper “In Pursuit of Clarity In Obfuscation”. In the paper they talked about how they “failed” but managed to still learn from their mistakes. Their topic on failure was not considered a “standard” that could be published and they were not able to submit it to a conference. But Bishop, along with PhD students Luke Kowalczyk and Kevin Shi, really wanted to get their findings out and share it with other researchers.

And so, a conference dedicated to disseminating insightful failures of the cryptology research community was born. The Conference for Failed Approaches and Insightful Losses in Cryptology or CFAIL featured seven previously unpublished papers for a day of talks by computer scientists on insightful failures spanning the full range from cryptanalysis (trying to break systems) to cryptographic theory and design (constructing new systems and proving things about specific systems or about abstract systems, etc.).

“CFAIL is great for our field in that it promotes openness and accessibility for these kinds of ideas which are typically sort of intimate,” said Luke Kowalczyk, who completed his PhD in November of last year. “When approaching new problems, it’s always helpful to see the approaches of other researchers, even if they were not successful. However, it’s rare to see failed approaches explained in a public and formal setting.”

They were not alone in thinking about the lack of dialogue on research failures. At the time of the conference, a thread on Hacker News (a tech news aggregator) discussed the incentive structures of academia. Shared Kowalczyk, “I was proud to see CFAIL cited as an example of a scientific field with a formal venue to help promote this kind of openness.”

“There is a deeply ingrained human tendency to fear that being open about failure will make other people think you are dumb,” said Bishop. On the contrary, the researchers at CFAIL were some of the “most creative, bold, and deeply intelligent people.” And the atmosphere it created was energizing for the participants — the audience got pretty involved and felt comfortable asking questions, and even started thinking about some of the open research problems in real time. Continued Bishop, ”I think talking about failure is probably the best scientific communication strategy left that is severely underused.”

Bishop will continue to promote openness in scientific research with another CFAIL at Crypto 2020. This time around it will be a workshop at the conference and a call for papers will be out soon.

Baishakhi Ray Receives 2019 IBM Faculty Award

IBM has selected assistant professor Baishakhi Ray for an IBM Faculty Award. The highly selective award is given to professors in leading universities worldwide to foster collaboration with IBM researchers. Ray will use the funds to continue research on artificial intelligence-driven program analysis to understand software robustness. 

Although much research has been done, there are still countless vulnerabilities that make system robustness brittle. Hidden vulnerabilities are discovered all the time – either through a system hack or monitoring system’s functionalities. Ray is working to automatically detect system weaknesses using artificial intelligence (AI) with her project, “Improving code representation for enhanced deep learning to detect and remediate security vulnerabilities”.

One of the major challenges in AI-based security vulnerability detection is finding the best source code representation that can distinguish between vulnerable versus benign code. Such representation can further be used as an input in supervised learning settings for automatic vulnerability detection and fixes. Ray is tackling this problem by building new machine-learning models for source code and applying machine learning techniques such as code embeddings. This approach could open new ways of encoding source code into feature vectors. 

“It will provide new ways to make systems secure,” said Ray, who joined the department in 2018. “The goal is to reduce the hours of manual effort spent in automatically detecting vulnerabilities and fixing them.”

A successful outcome of this project will produce a new technique to encode source code with associated trained models that will be able to detect and remediate a software vulnerability with increased accuracy.

IBM researchers Jim Laredo and Alessandro Morari will collaborate closely with Ray and her team on opportunities around design, implementation, and evaluation of this research.

CS Researchers Create Technology That Helps Developers with App Performance Testing

Younghoon Jeon and Junfeng Yang

The application performance testing company NimbleDroid was recently acquired by mobile performance platform HeadSpin. Co-founded by professor Junfeng Yang and PhD student Younghoon Jeon, the tool is used to detect bugs and performance bottlenecks during the development and testing phase of mobile apps and websites.

“I’ve always liked programming but hated the manual debugging process,” said Junfeng Yang, who joined the department in 2008. “So I thought it would be good to use artificial intelligence and program analysis to automate the task of debugging.”

NimbleDroid scans apps for bugs and bottlenecks and sends a report back with a list of issues. The New York Times wrote about how they used it to identify bottleneck issues with the start up time of their android app and speed it up by four times. Pinterest also used the tool during a testing phase and was able to resolve issues within 21 hours. Previously they would hear about problems from users once the app was already released, and spent “multiple days” to identify and fix the problems.

NimbleDroid has a premier list of customers, some of which also use HeadSpin.  These common customers connected Yang and HeadSpin’s CEO Manish Lachwani. They thought that NimbleDroid would be a great addition to HeadSpin’s suite of mobile testing and performance solutions. The acquisition was recently announced with Yang named as Chief Scientist of HeadSpin.

Because the initial technology for NimbleDroid started at Columbia, Yang and Jeon worked with Columbia Technology Ventures (CTV) to license the technology. They started a company, got the exclusive license for the technology, and further developed the tool. CTV is the technology transfer office that facilitates the transition of inventions from academic research labs to the market.

“We are excited that our research is widely used by unicorn and Fortune 1000 companies and helps over a billion users,” said Yang. “But our work is not done, we are developing more technologies to make it easy to launch better software faster.”