I have collected some fluorescence time series data from some cortical cells. Let's consider a single cell, where the data is described by a row vector. In our case it is described by the signal variable. A low pass filter has been applied. The cutoff frequency in such a case is close to 1Hz. I used the code that follows to filter and create the power spectrum of the data. From my understanding, this would allow me to see the rate with which the cell fires which is supposed to be constant and be shown as a single spike after applying the FFT algorithm, but I get a messy spectrum with quite a lot of spikes, out of which none seems very prominent.
Wn=1;
n=2;
ftype='low';
[b,a]=butter(n,Wn/(Fs/2),ftype);
dataF=filtfilt(b,a,signal);
figure();
subplot(221); plot(signal);
subplot(222); plot(dataF);
F=fft(signal); L=length(signal);
P2=abs(F/L);
P1=P2(1:L/2+1);
P1(2:end-1)=2*P1(2:end-1);
f=Fs*(0:(L/2))/L;
subplot(223); plot(f,P1);
