afni.nimh.nih.gov

I used a mask to measure activity in the hippocampus and received the following error message:
+ WARNING: Dataset lag4_percent+tlrc has 7221032 voxels/nodes while the mask has 4643401 - skipping.
afni then closed and that was it

How do I measure brain activity using a mask? I don't understand how I can make both mask and *+tlrc the same voxel/node resolution.

Thank you!