Presentation of the dissertation project and the research results to date (X.Li)

Presentation of the dissertation project and the research results to date:

In cognitive research, we can obtain valuable knowledge about neural processing through detecting the time-locked change in ERPs and ERD/ERS. In this project, we introduce the fractal analysis method into neurocognitive research. Fractal geometry has been proved useful in quantifying complexities of dynamical signals. The temporal fractal dimension of EEG signals provides a new neurophysiological measure.

In order to better understand, in some sense, what the complexity measure reveals about the underlying brain process, a further exploration of the neuronal generators of fractal geometry characteristics of EEG has been conducted in this project. A fractal-dimension based event-related complexity analysis approach for cognitive research was proposed. Event-related complexities are sensitive to changes in the aspect of diversity of the neuronal population activities. They offer more insights into the underlying neural processes, providing a new neurophysiological index of the cognitive processes.

We have investigated the fundamentals of the fractal property of the EEG signal. We found experimental evidence from the visual cortex and the sensorimotor cortex to demonstrate the correlation between the fractal property of the EEG signal and the underlying neural activities. The relations between ERCs and ERD/ERS were also studied. Our investigation suggests that the temporal fractal measure of EEG signals can be related to the activity diversity of neuronal population activities. The complexity measure also gives an indication on the change in synchronization under certain mental conditions. This work helps give an interpretation of the obtained results of the temporal complexity analysis on EEG signals and may be useful in further investigating the covert steps of brain information processing from the perspective of temporal morphology.

Currently, we are studying the neural correlates, through ERCs analysis, of semantic/syntactic violations in language perception in the context of written German sentence comprehension. A N400 component in the centro-parietal area is commonly believed to follow a semantic error. By contrast, a left anterior negativity (LAN) in the left-anterior area and a following P600 component which is distributed later are related to the syntactic process. Corresponding components were also found in our ERC results. These coinciding results testified the effectiveness of morphological measures in reflecting the change in the patterns of neuron activities.

In the next step, we are planning to get a deeper comprehension of the neurophysiological fundamentals of the ERC method by further comparing the fractal aspect and the ERP amplitude of EEG signals. More behaviour experiments in other cognitive research fields than language perception will also be employed in our project to validate our current findings.

This project is carried out in collaboration with the Biological Psychology and Neuropsychology Group of the Psychology department at Hamburg University. In this joint work, the language perception experiments were conducted by the partner group, including the language material preparation, participant recruitment,d experimental data collection etc.

We have also finished another brain–computer interface (BCI) project based on the platform of the IRTG. In this project, we employed the EEG analysis methods of event-related (de-)synchronization (ERS/ERD) analysis and Steady-state visually evoked potentials (SSVEP). This part of work used common equipment and common paradigm with a project of the Neural Engineering Department at the Institutes of Biomedicine at Tsinghua University. Frequent discussions between these two groups significantly advanced the progress of the project. A SSVEP-based BCI humoid robot control system was created in this project.

The cooperation within the framework of the International Research Training Group considerably helped my research work. I took the cooperation as a good chance for studying. It gave me face-to-face communication opportunities with many distinguished professors. During the period of my visit to Hamburg University, I have acquired considerable knowledge in biological psychology and neuropsychology needed for the project. High-quality experiment data was provided by the Biological Psychology and Neuropsychology Group. The discussions with the professors and students in the partner group gave me valuable directions. New ideas were inspired by the discussions or during lab visits. I also got useful experiences for BCI research from the discussions with students of the Neural Engineering Department in the Institute of Biomedicine at Tsinghua University. All these cooperations played important roles in the progress of our projects.