Dear Shengwei,
did you accidentally apply modulation ("preserve amount") when normalising the EPIs? With modulation enabled this alising can be expected, see the manual:
Specify the full-width at half maximum (FWHM) of the Gaussian blurring kernel in mm. Three
values should be entered, denoting the FWHM in the x, y and z directions. Note that you can
also specify [0 0 0], but any \modulated' data will show aliasing (see eg Wikipedia), which occurs
because of the way the warped images are generated.
and also https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1304&L=spm&D=0&P=450526 . For EPI normalisation one typically does NOT use modulation. If you want to use modulation nonetheless (or if this alising should also occur with modulation turned off, not sure right now) you can avoid the alising by adjusting the Gaussian FWHM.
Best,
Helmut
