Strange spectopo shape

[Violet]

Hi
I am new user of Biosemi and eeglab. My data shows strange shapes when are imported and plotted with eeglab. As it is shown in the attached image, the spectopo have a high power on low frequencies (1 to 6 Hz) and a very low power on frequencies from 6 Hz and above. These higher frequencies shows a very strange peaks at each 4 Hz intervals (10, 14, 18, 22 Hz, etc).
What could be the origin of this effect?

[Coen]

Not a proper window applied in your FFT routine, see https://en.wikipedia.org/wiki/Window_function.

Best regards, Coen (BioSemi)

[Violet]

Thank you so much