[Alpha oscillations of human brain elicited by illusory contour perception]

We examined the changes in the phase-locked and total alpha oscillations of EEG during perception of illusory (Kanizsa square) and non-illusory images in 16 healthy adults. We applied a wavelet-based time-frequency analysis to compute alpha band power, statistical evaluation was performed using repeated-measures ANOVA and the non-parametric multi-way analysis. Results showed that both stimuli provoked an initial increase of total alpha power emerging not later than in 100 ms after the stimulus onset. The increase was caused by the phase-locked alpha response, and the illusory contour was systematically followed by a higher level of alpha power than the non-illusory one. Further suppression of total alpha power occurred within the time window of 200-500 ms after the stimulus onset and did not depend on the stimulus type. Both stages of the total alpha response were more prominent over the parieto-occipito-temporal scalp areas. We hypothesize that the first stage of alpha response is related to the modulation of the activity of neural circuits participating in processing of a coherent pattern whereas a later suppression of total alpha power links to the orienting processes.