How to bandpass filter an 4D image in Matlab?

Hello

I have implemented in Matlab a bandpass filter. The script is in the attachment.

I have now applied the filter to a 4D fMRI image. The first three dimensions are spatial, the last is temporal. The image is of size 80 x 35 x 12 x 350.

I have now made plots of the average time series before and after applying the bandpass filter.

Here is the plot after applying bandpass filtering:

As can be seen there are distortions at the beginning and end. Thus, from the result I had to throw away the first two and last two time points. The resulting images is thus of size 80 x 35 x 12 x 346.

Here is it after bandpass filtering and throwing away first and last two time points:

This one is before bandpass filtering:

Why is the shape of the curve exactly the same but just shifted on the y-axis? Did I made a mistake in my script?

By the way, I have used for the filter order n = 4. I don't know if it is right.