What could be causing these humps every 10 Hz on my PSD estimate data?
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]);