It is usually the case that the anatomical dataset (SPGR or MPGRAGE) has thinner slices than the EPI dataset from which the functional maps are derived. It is also usually the case that the in-slice EPI voxels are larger than the in-slice anatomical MRI voxels. This means that as you scroll through the anatomical voxels one-by-one (in any direction), you will be in the same EPI/functional voxel for several scrolling steps.

To see this relationship precisely, turn on the "Datamode->Misc->Voxel Coords?" button, and turn on "See Function". Then the coordinates shown in the upper-left corner of the AFNI controller will be voxel indexes, rather than spatial coordinates in mm, and the voxel indexes from both the anat and functional datasets will be displayed. As you move around the anat 1 voxel/slice at a time, you will see one of the anat indexes change, but the func indexes will only change occasionally, as you cross the border from one large EPI voxel to the next.

All of this is true in +orig coordinates. In a resampled dataset (+tlrc), the voxel dimensions will all usually be the same (1 mm). However, the functional datasets may still look blocky, since the default interpolation method for functional datasets is NN.

bob cox