Hi-

So, it is the *underlay* dataset that sets the grid for displaying. You have a high res and a low res image, but you want the low res one (the EPI) as underlay, then you can make a resampled version that is at a finer grid size: it will look quite similar, but perhaps slightly smoothed (because regridding does that).

So, how about trying this (where you will insert input or output filenames for each DSET* file, but the "IDENTITY" term is actually a keyword to leave as is; each keyword after "-final .." is also meant to be left written as is), where there are some different interpolants you can try, and see what looks best:

# A) use "nearest neighbor" interpolant
3dAllineate                        \
        -input   DSET_LOWRES              \
        -master  DSET_HIGHRES           \
        -prefix DSET_LOWRES_UP_A      \
        -1Dparam_apply IDENTITY        \
        -final        NN

# B) use "cubic" interpolant
3dAllineate                        \
        -input   DSET_LOWRES              \
        -master  DSET_HIGHRES           \
        -prefix DSET_LOWRES_UP_B      \
        -1Dparam_apply IDENTITY        \
        -final        cubic

# C) use "wsinc" interpolant
3dAllineate                        \
        -input   DSET_LOWRES              \
        -master  DSET_HIGHRES           \
        -prefix DSET_LOWRES_UP_C      \
        -1Dparam_apply IDENTITY        \
        -final        wsinc5

# D) use "linear" interpolant
3dAllineate                        \
        -input   DSET_LOWRES              \
        -master  DSET_HIGHRES           \
        -prefix DSET_LOWRES_UP_C      \
        -1Dparam_apply IDENTITY        \
        -final        linear

You can underlay each of DSET_LOWRES_UP_* in the viewer and see what looks most appropriate (each should be fairly quick to calculate.

--pt

ps: if you want a more compact/scripty way to do the above, then this will make 4 dsets, each with the interpolant name in the file name:

#!/bin/tcsh

foreach interp ( NN cubic wsinc5 linear )
  3dAllineate                        \
        -input   DSET_LOWRES              \
        -master  DSET_HIGHRES           \
        -prefix DSET_LOWRES_UP_${interp}      \
        -1Dparam_apply IDENTITY        \
        -final    ${interp}
end