Why Fourier Transform doesn't show obvious periodicity? I have a clearly 4Hz periodic signal (see image below). However when I apply a fourier transform, there is no peak at 4Hz. Also, when I do a power spectrum density (using Matlab's pwelch algorithm), there is no peak at 4Hz. 

My original signal is sampled at 5ms, and is 141 bins (705ms) long. 

Why is this &amp; which alogithm will detect the 4Hz peak?

Thanks
An FFT doesn't necessarily show a fundamental periodicity, but instead spectral periodicities, which might be higher harmonics (or overtones) of the periodicity that you might perceive visually.

Also, some spectral peaks can be hidden under (the windowing artifacts of) a relatively  large enough DC offset.  If you subtract the mean of the data before doing the FFT, these frequency peaks might show up more clearly.