What could be causing these humps every 10 Hz on my PSD estimate data? I am working with EEG recordings about 1,000,000 data pts long, recorded at 4,000 Hz.  

I'm generating PSD estimates for these recordings using the matlab "periodogram" function like so:

[power,f]=periodogram(testdata,hamming(length(testdata)),[],4000);


and plotted like so:

scatter(f,log(power),1)


When I look at my plotted data (frequency vs. power), I see these strange, camel-like humps with maxima at every 10 Hz. The humps kind of resemble the plot of the absolute value of a sine wave. These humps accompany and are quite distinct from what I believe to the the non-artifactual baseline data.

When I run the periodogram function on smaller portions of the data as opposed to the full 1,000,000 point recording, the humps and the baseline appear to merge together.

Does anyone know what these humps might represent?
These are indeed artefacts from the stimulation

x = zeros(1,10*4000)
x(1) = 1;
x(401) = 1;
[power,f]=periodogram(x,hamming(length(x)),[],4000);
scatter(f,log(power),1);
xlim([0 200]);