Papers in Preparation
Federated Block-Term Tensor Regression for Heart Diseases in Realistic Healthcare Settings
Axel Faes, Liesbet Peeters
2024 Spotlight
This paper presents an extension of the Block-Term Tensor Regression (BTTR) algorithm, termed Federated Block-Term Tensor Regression (FBTTR), designed to address the unique challenges of heart disease prediction in realistic healthcare settings. Traditional machine learning models often rely on centralized data, which poses significant privacy risks and limits the scalability of predictive models across multiple institutions. FBTTR adapts BTTR to federated learning scenarios, allowing decentralized data from various healthcare institutions to be utilized without compromising patient privacy. The proposed FBTTR method leverages the strengths of tensor regression in handling multi-dimensional data, enhancing predictive accuracy through collaborative learning while ensuring data confidentiality. By incorporating federated learning principles, FBTTR facilitates secure model training across different sites, effectively mitigating data silos and promoting a more comprehensive understanding of heart disease predictors. Empirical evaluations demonstrate the robustness of FBTTR, showing that it achieves high predictive performance comparable to traditional centralized models. Moreover, the algorithm exhibits efficient computational performance, making it suitable for deployment in real-world healthcare environments where timely and accurate predictions are crucial. The results highlight FBTTR's potential in advancing predictive analytics in healthcare by providing a scalable, privacy-preserving solution for heart disease prediction, ultimately contributing to better patient outcomes and more effective healthcare delivery.
How stimulus type and task structure can affect ERP signatures
Valentina Pergher*, Axel Faes*, Yide Li, Marc M. Van Hulle (* equal contribution)
Submitted to Frontiers in Psychology, section Cognition (2.6 IF) (Under review) 2024
The N-Back working memory (WM) updating task is commonly used in cognitive neuroscience research. Many studies investigated behavioral responses, while only few studies have sought to identify its electrophysiological signatures. Nevertheless, it is still unclear what the role is of stimulus type and task structure in the outcomes. Here, we address this issue using nine different variations of an N-Back task. Results show differences in electrode clusters between words vs. pictures for both N2 and P3-ERP components, specifically in the fronto-central brain regions. In conclusion, our findings suggest that stimulus type contributes to differences in electrophysiological responses during N-Back task performance, which needs to be accounted for when interpreting findings across studies.
Finger abduction trajectory prediction from high-density ECoG
Axel Faes, Eva Calvo Merino, Anais Van Hoylandt, Elina Keirse, Tom Theys, Marc M. Van Hulle
Submitted to Journal of Neural Engineering (5.4 IF) (Under review) 2024
Objective: A case study is conducted to show that not only finger flexion- but also finger abduction trajectories can be decoded from high-density electrocorticography (ECoG) recordings. Approach: Two patients, temporarily implanted with high-density ECoG grids as part of their clinical workup, performed a cued single finger flexion task, a cued sign language alphabet task and a cued finger abduction task. The extended Block-Term Tensor Regression (eBTTR) model was adopted to predict finger trajectories after being developed on simultaneous data glove/ECoG recordings. The model performance on the single finger flexion task provides a reference for comparing that on the finger abduction task. Finally, the sign language alphabet task involves realistic, complex finger movements, calling upon both finger flexions and abductions. Main results: We show that finger abduction trajectories can be decoded from ECoG recordings, with comparable precision as single finger flexion trajectories, and even when expressing sign language gestures that involve joint finger flexions and abductions. Significance: Our findings show for the first time that finger abduction trajectories can be decoded from ECoG even when part of Sign Language Alphabet gestures.
Decoding Sign Language Finger Movements from high-density ECoG using Graph-Optimized Block Term Tensor Regression
Axel Faes, Mariana P. Branco, Anais Van Hoylandt, Elina Keirse, Tom Theys, Nick F. Ramsey, Marc M. Van Hulle
Submitted to Journal of Neural Engineering (5.4 IF) (Under review) 2024
Objective A novel method is introduced to regress over the Sign Language finger movements from human electrocorticography (ECoG) recordings. Approach The proposed Graph-Optimized Block-Term Tensor Regression (Go-BTTR) method consists of two components: a deflation-based regression model that sequentially Tucker-decomposes multiway ECoG data into a series of blocks, and a Causal Graph Process (CGP) that accounts for the complex relationship between finger movements when expressing sign language gestures. Prior to each regression block, CGP is applied to decide which fingers should be kept separate or grouped and should therefore be referred to BTTR or its extended version eBTTR, respectively. Main results Two ECoG datasets were used, one recorded in 5 patients expressing 4 hand gestures of the American Sign Language Alphabet, and another in 2 patients expressing all gestures of the Flemish Sign Language Alphabet. As Go-BTTR combines fingers in a flexible way, it can better account for the nonlinear relationship ECoG exhibits when expressing hand gestures, including unintentional finger co-activations. This is reflected by the superior joint finger trajectory predictions compared to eBTTR, and predictions that are on par with BTTR in single finger scenarios. Subject 5, Dataset 1, during the movement of hand gestures reached 0.82 Pearson correlation for the pinky for Go-BTTR compared to 0.8 and 0.72 for eBTTR and BTTR respectively. Significance Our findings show that Go-BTTR is capable of decoding complex hand gestures taken from the Sign Language Alphabet. Go-BTTR also demonstrates computational efficiency, providing a notable benefit when intracranial electrodes are inserted during a patient’s pre-surgical evaluation. This efficiency helps reduce the time required for developing and testing a Brain-Computer Interface (BCI) solution.
International Journal Papers
The role of distinct ECoG frequency features in decoding finger movement
Eva Calvo Merino, Axel Faes, Marc M. Van Hulle
Journal of Neural Engineering (5.4 IF) 2023
Objective. To identify the electrocorticography (ECoG) frequency features that encode distinct finger movement states during repeated finger flexions. Approach. We used the publicly available Stanford ECoG dataset of cue-based, repeated single finger flexions. Using linear regression, we identified the spectral features that contributed most to the encoding of movement dynamics and discriminating movement events from rest, and combined them to predict finger movement trajectories. Furthermore, we also looked into the effect of the used frequency range and the spatial distribution of the identified features. Main results. Two frequency features generate superior performance, each one for a different movement aspect: high gamma band activity distinguishes movement events from rest, whereas the local motor potential (LMP) codes for movement dynamics. Combining these two features in a finger movement decoder outperformed comparable prior work where the entire spectrum was used as the average correlation coefficient with the true trajectories increased from 0.45 to 0.5, both applied to the Stanford dataset, and erroneous predictions during rest were demoted. In addition, for the first time, our results show the influence of the upper cut-off frequency used to extract LMP, yielding a higher performance when this range is adjusted to the finger movement rate. Significance. This study shows the benefit of a detailed feature analysis prior to designing the finger movement decoder.
Finger movement and coactivation predicted from intracranial brain activity using extended Block-Term Tensor Regression
Axel Faes, Marc M. Van Hulle
Journal of Neural Engineering (5.4 IF) 2023 Spotlight
Multiway- or tensor-based decoding techniques for Brain-Computer Interfaces (BCI) are believed to better account for the multilinear structure of brain signals than conventional vector- or matrix-based ones. However, despite their outlook on significant performance gains, the used parameter optimization approach is often too computationally demanding, so conventional techniques are still preferred. We propose two novel tensor factorizations which we integrate into our Block-Term Tensor Regression (BTTR) algorithm and further introduce a marginalization procedure that guarantees robust predictions while reducing the risk of overfitting (generalized regression). BTTR accounts for the underlying (hidden) data structure in a fully automatic and computationally-efficient manner, leading to a significant performance gain over conventional vector- or matrix-based techniques in a challenging real-world application. As a challenging real-world application, we apply BTTR to accurately predict single finger movement trajectories from intracranial recordings in human subjects. We compare the obtained performance with that of the state-of-the-art.
Single finger trajectory prediction from intracranial brain activity using Block-Term Tensor Regression with fast and automatic component extraction
Axel Faes, Flavio Camarrone, Marc M. Van Hulle
IEEE Transactions on Neural Networks and Learning Systems (14.25 IF) 2022
Objective We introduce extended Block-Term Tensor Regression (eBTTR), a novel regression method designed to account for the multilinear nature of human intracranial finger movement recordings. Approach The proposed method relies on recursive Tucker decomposition combined with automatic component extraction. Main results eBTTR outperforms state-of-the-art regression approaches, including multilinear and deep learning ones, in accurately predicting finger trajectories as well as unintentional finger coactivations. Significance eBTTR rivals state-of-the-art approaches while being less computationally expensive which is an advantage when intracranial electrodes are implanted acutely, as part of the patient's presurgical workup, limiting time for decoder development and testing.
Source space reduction for eLORETA
Axel Faes, Aurelie de Borman, Marc M. Van Hulle
Journal of Neural Engineering (5.4 IF) 2021
Objective We introduce Sparse eLORETA, a novel method for estimating a nonparametric solution to the source localization problem. Its goal is to generate a sparser solution compared to other source localization methods including eLORETA while benefitting from the latter's superior source localization accuracy. Approach Sparse eLORETA starts by reducing the source space of the Lead Field Matrix using Structured Sparse Bayesian Learning (SSBL) from which a Reduced Lead Field Matrix is constructed, which is used as input to eLORETA. Main results With Sparse eLORETA, source sparsity can be traded against signal fidelity; the proposed optimum is shown to yield a much sparser solution than eLORETA's with only a slight loss in signal fidelity. Significance When pursuing a data-driven approach, for cases where it is difficult to choose specific regions of interest (ROIs), or when subsequently a connectivity analysis is performed, source space reduction could prove beneficial.
Neural Networks for Directed Connectivity Estimation in Source-Reconstructed EEG Data
Axel Faes, Iris Vantieghem, Marc M. Van Hulle
Applied Sciences (2.9 IF) 2022
Directed connectivity between brain sources identified from scalp electroencephalography (EEG) can shed light on the brain's information flows and provide a biomarker of neurological disorders. However, as volume conductance results in scalp activity being a mix of activities originating from multiple sources, the correct interpretation of their connectivity is a formidable challenge despite source localization being applied with some success. Traditional connectivity approaches rely on statistical assumptions that usually do not hold for EEG, calling for a model-free approach. We investigated several types of Artificial Neural Networks in estimating Directed Connectivity between Reconstructed EEG Sources and assessed their accuracy with respect to several ground truths. We show that a Long Short-Term Memory neural network with Non-Uniform Embedding yields the most promising results due to its relative robustness to differing dipole locations. We conclude that certain network architectures can compete with the already established methods for brain connectivity analysis.
Conference Papers
Web Performance Characteristics of HTTP/2 and comparison to HTTP/1.1
Robin Marx, Maarten Wijnants, Peter Quax, Axel Faes, Wim Lamotte
International Conference on Web Information Systems and Technologies, pg 87-114 2018
The HTTP/1.1 protocol has long been a staple on the web, for both pages and apps. However, it has started to show its age, especially with regard to page load performance and the overhead it entails due to its use of multiple underlying connections. Its successor, the newly standardized HTTP/2, aims to improve the protocol's performance and reduce its overhead by (1) multiplexing multiple resources over a single TCP connection, (2) using advanced prioritization strategies, and by introducing new features such as (3) Server Push and (4) HPACK header compression. This work provides an in-depth overview of these four HTTP/2 performance aspects, discussing both synthetic and realistic experiments, to determine the gains HTTP/2 can provide in comparison to HTTP/1.1 in various settings. We find that the single multiplexed connection can actually become a significant performance bottleneck in poor network conditions with high packet loss and that HTTP/2 rarely improves much on HTTP/1.1, except in terms of reduced overhead. Prioritization strategies, Server Push, and HPACK compression are found to have a relatively limited impact on web performance, but together with other observed HTTP/2 performance problems, this could also be due to faulty current implementations, of which we have discovered various examples.
Concatenation, embedding and sharding: Do HTTP/1 performance best practices make sense in HTTP/2?
Robin Marx, Peter Quax, Axel Faes, Wim Lamotte
WEBIST 2017 - Proceedings of the 13th International Conference on Web Information Systems and Technologies 2017
Web page performance is becoming increasingly important for end users but also more difficult to provide by web developers, in part because of the limitations of the legacy HTTP/1 protocol. The new HTTP/2 protocol was designed with performance in mind, but existing work comparing its improvements to HTTP/1 often shows contradictory results. It is unclear for developers how to profit from HTTP/2 and whether current HTTP/1 best practices such as resource concatenation, resource embedding, and hostname sharding should still be used. In this work, we introduce the Speeder framework, which uses established tools and software to easily and reproducibly test various setup permutations. We compare and discuss results over many parameters (e.g., network conditions, browsers, metrics), both from synthetic and realistic test cases. We find that in most non-extreme cases, HTTP/2 is on a par with HTTP/1 and that most HTTP/1 best practices are applicable to HTTP/2. We show that situations in which HTTP/2 currently underperforms are mainly caused by inefficiencies in implementations, not due to shortcomings in the protocol itself.
Other publication
Efficient compilation of algebraic effects and handlers
Matija Pretnar, Amr Hany Shehata Saleh, Axel Faes, Tom Schrijvers
2017 - CW Reports, CW708, 32 pp. Leuven, Belgium: Department of Computer Science, KU Leuven.
The popularity of algebraic effect handlers as a programming language feature for user-defined computational effects is steadily growing. Yet, even though efficient runtime representations have already been studied, most handler-based programs are still much slower than hand-written code. In this paper, we show that the performance gap can be drastically narrowed (in some cases even closed) by means of type-and-effect directed optimizing compilation. Our approach consists of two stages. Firstly, we combine elementary source-to-source transformations with judicious function specialization in order to aggressively reduce handler applications. Secondly, we show how to elaborate the source language into a handler-less target language in a way that incurs no overhead for pure computations. This work comes with a practical implementation: an optimizing compiler from Eff, an ML-style language with algebraic effect handlers, to OCaml. Experimental evaluation with this implementation demonstrates that in a number of benchmarks, our approach eliminates much of the overhead of handlers and yields competitive performance with hand-written OCaml code.
Thesis
Finger Movement Decoding: From Source-Localisation to Tensor Regression Modelling
Axel Faes
PhD Thesis 2023
Brain-Computer Interfaces (BCIs) are hailed for bypassing defective neural pathways by translating brain activity directly into actions that convey the user's intent. How the kinematics of muscular activity relate to the motor- and somatosensory activity in the brain has been the focus of recent advancements. With such motor BCIs, amputees are able to gain control over a prosthesis and stroke patients to regain control over a paralyzed limb via electrical stimulation of their dysfunctional muscles or via an exoskeleton that supports the intended movements. The superior spatio-temporal resolution, bandwidth, and recording stability of electrocorticography (ECoG), a partially invasive brain recording technique, yields a new outlook on motor BCI applications. Despite some stunning successes in arm- and hand movement control from ECoG, the precise decoding of finger movements, which is essential for daily activities, is still lacking. A possible reason is that current decoders rely on conventional one- or two-way regression models, which might not adequately capture the intricate relation between neural activity and intended and unintended (such as coactivations) finger movements. The main objective of this PhD is to develop a robust, accurate, and quick-to-train decoder that predicts single- and coordinated finger trajectories from ECoG recordings. We used multiway decoders as they preserve the multilinear structure of the data while taking advantage of potentially hidden multilinear components. We demonstrated cutting-edge performance with the proposed decoders. As multiway models tend to be slow to train, which may become a significant obstacle for their clinical adoption, we also investigated whether the proposed multiway decoders could be used in a real-time setting. The findings support the relevance of the proposed multiway decoders for real-time ECoG-based finger activity, providing an outlook on achieving hand dexterity.
An Information Theoretical Approach to EEG Source-Reconstructed Connectivity
Axel Faes
Advanced Master's Thesis 2018
Determining how distinct brain regions are connected and communicate with each other will shed light on how behavior emerges. In EEG studies, interpreting connectivity measures can be problematic due to the high correlation between signals recorded from the scalp surface, a result of the volume conductance of the scalp and skin. Therefore, meaningful connectivity patterns can be measured only from the spatiotemporal distribution of localized cortical sources, generally referred to as source reconstruction. Still, spurious connectivity issues may persist in source-reconstructed EEG data, making it vital to choose an appropriate measure of connectivity. In this work, an information theoretical approach, which concerns model-free, probability-based methods such as mutual information, conditional mutual information, and interaction information, is taken. An information theoretical framework for Python is developed to operate on source-reconstructed activity. This framework is used to perform a connectivity analysis of a high-density source-reconstructed EEG dataset, which was constructed in an experiment regarding the semantic processing of abstract and concrete words.
Algebraic Subtyping for Algebraic Effects and Handlers
Axel Faes
Master's Thesis 2018
Algebraic effects and handlers benefit from a custom type-&-effect system, a type system that also tracks which effects can happen in a program. Several such type-&-effect systems have been proposed in the literature, but all are unsatisfactory. Recently, Stephen Dolan (University of Cambridge, UK) presented a novel type system that combines subtyping and parametric polymorphism in a particularly attractive and elegant fashion. A cornerstone of his design is the algebraic properties that the subtyping relation should respect. In this work, a type-&-effect system is derived that extends Dolan's elegant type system with effect information. This type-&-effect system inherits Dolan's harmonious combination of subtyping (in our case induced by a lattice structure on the effect information) with parametric polymorphism and preserves all of its desirable properties (both low-level algebraic properties and high-level meta-theoretical properties like type soundness and the existence of principal types). This type-&-effect system has been implemented in the Eff programming language in order to provide a proof-of-concept.
Machine learning techniques for flow-based network intrusion detection systems
Axel Faes
Bachelor's thesis 2016
Large data centers are storing and sending more and more data. In order to check whether the network traffic does not contain intrusions, an intrusion detection system is used. Such a system analyzes data from the network and gives an alert if it finds an intrusion. Since data centers have so much data traffic, it is difficult to process everything. That's where IP flows come into the picture. They are aggregated from packet data but do not contain any information about the payload data. This thesis explains which attacks can be detected and how IP flows can be used for intrusion detection. It would also be cost-efficient if an intrusion detection system could operate automatically and detect attacks with a high probability. For this, machine learning can be used. Machine learning is a type of Artificial Intelligence which allows programs to learn and find patterns within data. However, there are many different types of machine learning. This thesis gives an introduction to machine learning concepts and provides an overview of different machine learning algorithms such as Support Vector Machines and K-Nearest Neighbors. An explanation is given on how these algorithms can be used in an intrusion detection system. The algorithms are evaluated on different datasets which consist of both labeled training data and unlabeled real-world data. The evaluation is done using learning curves and F-scores. In the evaluation, it is found that supervised learning gives better and more detailed predictions compared to unsupervised learning. K-Nearest Neighbors gives the best results among the tested supervised learning algorithms. The results show that machine learning is a viable option to detect intrusions using IP flows. It can also be noted that using extra information such as the TCP flags is a useful addition and increases the performance significantly.
Extended Abstracts
Individual and Coordinated Finger Movements Decoding from High-Density EEG and Its Implication in Hand Exoskeleton Control
Qiang Sun, Axel Faes, Marc M. Van Hulle
European Congress of NeuroRehabilitation 2023 2023
Modulation of LMPs using the gamma band increases the stability of finger trajectories decoded from ECoG
Eva Calvo Merino, Axel Faes, Marc M. Van Hulle
BCI (Brain-computer interfaces) - Society 2023 2023
Reconstructing single finger trajectories from intracranial brain activity
Axel Faes, Benjamin Wittevrongel, Marc M. Van Hulle
III International Conference "Volga Neuroscience Meeting 2021" 2021
Causal Graphical Modelling of Functional Connectivity from Reconstructed EEG Sources
Axel Faes, Mansoureh Fahimi Hnazaee, Marc M. Van Hulle
8th International BCI Meeting (2021) 2021
Towards a Core Language with Row-Based Effects for Optimised Compilation
Axel Faes, Tom Schrijvers
International Conference on Functional Programming 2017 Student Research Competition 2017
Purpose-Centric Appropriation of Everyday Objects as Game Controllers
Kashyap Todi, Brent Berghmans, Axel Faes, Matthijs Kaminski
CHI EA '16: Extended Abstracts of the SIGCHI Conference on Human Factors in Computing Systems. Late Breaking Work 2016
Household Survival: Immersive Room-Sized Gaming Using Everyday Objects as Weapons
Kashyap Todi, Donald Degraen, Brent Berghmans, Axel Faes, Matthijs Kaminski, Kris Luyten
CHI EA '16: Extended Abstracts of the SIGCHI Conference on Human Factors in Computing Systems. Student Game Competition 2016
Posters
Towards a Core Language with Row-Based Effects for Optimised Compilation
Axel Faes, Tom Schrijvers
International Conference on Functional Programming 2017 Student Research Competition 2017
Other
Co-lecturer (co-titularis) 2024-2025
Bio-informatica (3740)
UHasselt 2024
Guest Associate Editor 2024-2025
Use of Big Data and Artificial Intelligence in Multiple Sclerosis
Frontiers in Immunology 2024
Student Research Competition Judge
Student Research Competition Judge
ICFP (International Conference on Functional Programming) 2024 2024
Flanders BCI (2022-2024)
Data Science Consultant
Laboratory of Neuro- and Psychophysiology, KU Leuven
Brain-Computer Interfacing (B-KUL-H08M0A)
Guest lecturer 2022-2023
KU Leuven 2022
Fundamental Research Grant 2018-2022
FWO
Belgium
FWO fundamental research grant for the project "Finger movement decoding: From source-localisation to graph-based regression modelling"
Students
(Federated) Machine Learning for Health Data
Dongho Chun
Master of Science in Computer Science (2024-2025)
Federated Learning for Regression modelling with Cardiovascular Health Data
Dries Cornelissen
Master of Science in Computer Science (2024-2025)
Brain-Computer Interfacing - population-based finger movement decoding
Ward Ceyssens
Master of Science in Computer Science (2024-2025)
Hand exoskeleton dexterity achieved by shared control with a semi-invasive brain-computer interface
Qiang Sun
Doctoral Program in Biomedical Sciences (daily supervision 2022-2023)
Restoring finger dexterity with an exoskeleton controlled by human intracranial recordings
Eva Calvo Merino
Doctoral Program in Biomedical Sciences (daily supervision 2022-2023)
Investigating the effect of Source Mixing on Directed Connectivity estimated between Simulated Reconstructed EEG Sources
Aurélie de Borman
Internship Student 2021
Estimating the effectiveness of source localized EEG for BCIs
Diogo Sousa Morais
Internship Student 2021
Cluster Permutation Analysis of N-Back related EEG-ERP Data
Guilherme de Borras Silva
Internship Student 2021
Using Neural Networks to derive Directed Connectivity between Reconstructed EEG Sources
Iris Vantieghem
Master of Science in Artificial Intelligence (2020-2021)
Neural Network Approach to the Inverse Problem
Didier Quintius
Master of Science in Artificial Intelligence (2020-2021)
Talks, presentations and other media
Decoding finger movements from invasive recordings in human motor cortex
2023
Mindseed event Leuven, NeuroTech Leuven
Guest Lecture "Finger movement decoding - brain-computer interfacing"
2023
KULeuven
Coordinated Finger Movements Predicted from Intracranial Brain Activity
2022
International Congress Humanities vs Sciences & the Knowledge Accelerating in Modern World: Parallels an Interaction,
Guest Lecture "Decoding single and coordinated finger actions from intracranial brain activity"
2022
KULeuven
Finger abduction trajectory prediction from high-density ECoG
2022
Leuven AI Scientific Workshop
Decoding single and coordinated finger actions from intracranial brain activity.
2021
XIV World Scientific Congress - SCIENCE FOR PEACE Modern Science, Global and Regional Theory and Practice
Guest Lecture "Finger Movement Decoding - From Source-Localisation to Tensor Regression Modelling"
2021
KULeuven
Reconstructing single finger trajectories from intracranial brain activity
2021
III International Conference "Volga Neuroscience Meeting 2021"
BCI demo - Dag van de Wetenschap
2021
Technopolis (geannuleerd wegens de covid-19 situatie)
Presentation "'MINDSPELLER' Medical Research Project on Brain Computer Interfaces" and concert (with Tigran Maytesian and his Mind Speller Chamber Orchestra)
2019
Kathedraal van Sint-Michiel en Sint-Goedele, Brussel
Honours student Axel Faes wins bronze medal in ACM SIGPLAN
2017
KU Leuven, Department of Computer Science
Student Axel Faes wins bronze medal in the ACM SIGPLAN Student Research Competition in ICFP conference
2017
KU Leuven, Department of Computer Science, DTAI