Temporal characteristic of SSVEP response
Anna Duszyk ( University of Social Sciences and Humanities ), Rafał Kuś (University of Warsaw, Faculty of Physics), Piotr Durka (University of Warsaw, Faculty of Physics)
Steady State Evoked Potentials (SSEP) are brain responses to repetitive stimulation with a certain frequency. Increase of power in frequency of the stimulation can be observed in brain activity, measured with electroencephalograph (EEG) from electrodes located above the primary area of the cortex corresponding to stimulation modalities. SSEP are commonly divided into three groups: Steady-State Visual Evoked Potentials (SSVEP), Steady-State Somatosensory Evoked Potentials (SSSEP) and Auditory Steady-State Responses (ASSR). Characterization of particular ERP components of the brain activity during such stimulation has been tested only in the case of ASSR. SSVEP frequency-specific responses are widely used in the technology of Brain-Computer Interfaces (BCI), so their spectral characteristics has been widely analyzed. However, temporal properties of SSVEP has not been adequately evaluated so far. The purpose of this experiment has been to examine SSVEP shape, its changes over time and its fading after the end of the stimulation.
The study has involved five subjects. Four white squares displayed on LCD were backlighted by LEDs flashing with different frequencies. Subject was supposed to concentrate on the indicated square, while EEG data was recorded. We tested four frequencies: 14, 17, 25 and 30 Hz. The stimulation of each frequency was repeated over 30 trials, with each trial including five seconds of stimulation and five seconds of rest period.
EEG registration was synchronized to the offset of stimulation epoch and then averaged over trials. Matching Pursuit Algorithm has been applied to the averaged EEG signal.
Results indicate that brain activity during repetitive stimulation consist mostly of 3 stages. At the beginning (0-200 ms) ERP with typical components P1, N1, P2 is observed in EEG. Next part of the response reveal regular periodic shape with frequency of the stimulation and its harmonics. After the end the stimulation, SSVEP response and its harmonics last for two more periods. These results are consistent with the temporal characteristics of ASSR.