Phase stability in the SSVEP responses

Steady State Visual Evoked Potentials (SSVEP) are natural brain responses to flicker stimuli. Stimulation within 5 – 50 Hz induces oscillations at corresponding frequency and its harmonics in the EEG over visual areas of the scalp. This phenomenon is used in Brain – Computer Interfaces (BCI). It is commonly assumed that SSVEP correspond to a true steady state signal with fixed
properties over the whole stimulation signals. This assumption may be tested in amplitude and phase domain which in principle are two independent indicators of stationary of the response signal.

Our initial results suggest that the amplitude of the subsequent responses is not constant and habituates with time. In the present study we investigated stability of phase of the SSVEP. We used EEG signal recorded during flickering stimulation. The Fourier Transform was computed and then phase of the stimulation frequency component was computed in a sliding overlaping window. Using these metods we have shown that the phase of the response signal is stabile during whole stimulation sequences and it's value is aproximately constant among trials.