Master's Thesis

Signatures of Chaos in Neural Timeseries

Final Thesis 2.72 MB Appendix 124.66 kB

Author of thesis: Bc. Ondřej Havelka

Acad. year: 2025/2026

Supervisor: doc. Ing. Luděk Nechvátal, Ph.D.

Reviewer: doc. Ing. Jiří Šremr, Ph.D.

Abstract:

This thesis investigates the use of nonlinear dynamical methods for the analysis of neural time series. The main motivation is the study of intracranial EEG recordings from patients with pharmacoresistant epilepsy, where the localisation of the seizure onset zone is clinically important.

The theoretical part introduces basic concepts from dynamical systems, chaos theory, phase-space reconstruction, Lyapunov exponents, fractal dimensions, and sample entropy. The practical part focuses on the implementation and comparison of selected features, mainly the largest Lyapunov exponent estimated using the Wolf algorithm. Additional quantities, including the Rosenstein estimate of the largest Lyapunov exponent, Higuchi fractal dimension, and sample entropy, are computed using the EPYCOM library.

The methods are applied to intracranial EEG recordings obtained from patients treated at Saint Anne's University Hospital in Brno. The computed descriptors are compared between contacts marked as seizure onset zone and non-SOZ tissue. The results are evaluated statistically and discussed with respect to their possible use in feature-based analysis of epileptic EEG signals.

The thesis shows that chaos and complexity features can capture differences between SOZ and NON-SOZ for some epileptic patients. However, the interpretation of results has limitations.

Keywords:

Chaos, EEG, epilepsy, EPYCOM, Lyapunov exponents, fractal dimension, sample entropy.

Date of defence

17.06.2026

Result of the defence

Defended (thesis was successfully defended)

znamkaEznamka

Grading

E

Process of defence

Student presented Master's thesis and sucessfully answered all of three questions of the reviewer. Then prof. Protasov asked about the definition of Lyapunov exponent. Student answered. Prof. Protasov is interested in the numerical results. Prof. Řehák asked about the choice of used numerical method (what aspects were considered).

Language of thesis

English

Faculty

Department

Study programme

Applied and Interdisciplinary Mathematics (N-AIM-A)

Composition of Committee

prof. RNDr. Josef Šlapal, CSc. (předseda)
doc. Ing. Luděk Nechvátal, Ph.D. (místopředseda)
doc. Ing. Petr Tomášek, Ph.D. (člen)
prof. Mgr. Pavel Řehák, Ph.D. (člen)
doc. Ing. Tomáš Kisela, Ph.D. (člen)
Prof. Vladimir Protasov (člen)

The presented diploma thesis aims to analyze EEG signals from a nonlinear dynamics perspective and identify characteristics of deterministic chaos within them. The topic is rather challenging, requiring a solid mathematical background and some experience in the subsequent statistical processing. Unfortunately, the student's approach to this task significantly reduced the overall quality of the work.

The thesis's greatest weakness is its considerable inconsistency and poor logical coherence; mainly in Section 2. In particular, many concepts appear to be taken out of context; on the other hand, some more important ones are missing. There is a lack of a smooth transition from theoretical definitions to practical implementation, and the chosen statistical methodology is not sufficiently introduced.

The formal side of the text is also rather poor. In particular, the thesis suffers from inconsistencies in mathematical notation. The typographical layout and overall typesetting could also be better. Clearly, the text did not undergo thorough final proofreading, which should be obvious for a submission of this type.

All the aforementioned shortcomings stem mainly from bad time management during thesis preparation. The student left the crucial part of the practical calculations and the writing of the text to the last minute. The resulting time pressure precluded any deeper iteration of the text, adequate consultation on partial problems, and meaningful revision of formal and logical errors. It also seems the given dataset offered greater potential for mathematical/statistical processing.

Despite the above-mentioned rather harsh criticism, the student eventually achieved results and performed a set of calculations. The thesis thus meets the minimum requirements and can be recommended for defense.
Evaluation criteria Grade
Splnění požadavků a cílů zadání C
Postup a rozsah řešení, adekvátnost použitých metod E
Vlastní přínos a originalita E
Schopnost interpretovat dosažené výsledky a vyvozovat z nich závěry D
Využitelnost výsledků v praxi nebo teorii D
Logické uspořádání práce a formální náležitosti D
Grafická, stylistická úprava a pravopis C
Práce s literaturou včetně citací D
Samostatnost studenta při zpracování tématu E

Grade proposed by supervisor: E

Reviewer’s report
doc. Ing. Jiří Šremr, Ph.D.

The present thesis is focused to the detection of chaos in neural time-series.

I would like to appreciate:

1. The author conducted numerous numerical simulations and composed a text that is quite consistent and meaningful, concerning an interesting and non-trivial topic.
2. The conclusion is quite well-formulated.

Unfortunately, I have many objections, in particular:

1. The mathematical background lacks precise definitions of some terms used in the text (e.g., trajectory, dense orbit, quasiperiodic orbit).
2. The crucial notion of the text - Lyapunov's exponent - is defined in a very vague way.
3. The description of Wolf's algorithm contains inaccuracies and typographical errors.
4. On p. 28, the author claims that "Sample entropy does not directly reflect chaos but can be a valuable indicator." - What can a sample entropy be an indicator of?
5. On p. 37, it is written that "we choose m=9" and on p. 39 that "we use m=10".
6. In Section 5.3, it is written that "the phase space reconstruction is still limited by the quality of the signal, which even after filtering out most of the noise might not fulfill the original assumption on smoothness. This problem can be visible in Figure 5.6, which was recorded on electrode closer to skull and therefore being more noisy.". Unfortunately, I do not see that at all. I would appreciate a better explanation.
7. In Section 5.3, it is not explained why the number of evolution steps used to estimate LLE is set to 30.
8. In Section 6.1, it is written that "In most patients, only a small number of contacts, typically between 2 and 10, are marked as SOZ, while the remaining contacts, sometimes up to approximately 200, are considered NON-SOZ." However, in the tables presenting the conducted statistics, there are three groups: NON-SOZ, Resection, and SOZ.
9. No assumptions about the data are stated. Consequently, I am not convinced that ANOVA is an appropriate method for analysing the data. This concern is particularly relevant given that only two groups are compared and that one of them has a very small sample size (as few as two observations).

CONCLUSION:

Although the submitted text is quite complete and meaningful, its form is closer to the first iteration. It seems to me that the author did not have time to make the necessary corrections in the text. However, in my opinion, the main goals of the thesis have been achieved except for item 3 in the assignment. In view of the above, I can recommend the thesis for defence and propose an evaluation of E.
Evaluation criteria Grade
Splnění požadavků a cílů zadání C
Postup a rozsah řešení, adekvátnost použitých metod D
Vlastní přínos a originalita C
Schopnost interpretovat dosaž. výsledky a vyvozovat z nich závěry D
Využitelnost výsledků v praxi nebo teorii C
Logické uspořádání práce a formální náležitosti B
Grafická, stylistická úprava a pravopis D
Práce s literaturou včetně citací A
Topics for thesis defence:
  1. Please explain what we should see in Figures 5.5 and 5.6.
  2. Please explain why you chose 30 evolution steps to estimate LLE.
  3. Please explain your formulation "Since for experimental time series we do not know the tangent directions of the underlying system, the separation of nearby trajectories cannot be renormalised in the exact direction." on p. 22.

Grade proposed by reviewer: E

Responsibility: Mgr. et Mgr. Hana Odstrčilová