afni_proc.py tshift

AFNI Message Board
1

Hello everyone,

Thank you for accepting me into the community. I came across an oddity that I wanted to clear up. By default the tshift option in afni_proc.py will put the reference volume at t=0 while 3dTshift itself has the default reference volume at TR/2. Most commonly and mathematically plausible would be the TR/2 option, so I was wondering what the reasoning is, in putting it to the first volume?

Thanks for the help.

Norman

2

Hi Norman,

Using t=0 generally aligns it with the stimulus timing. If the EPI timing were shifted to TR/2, then the event times should really be offset by that same amount of time (in the other direction), so that the timing matches in the linear regression.

  • rick
3

Ah ok, thank you Rick! thought so, but its good to have that cleared up!
Have a nice weekend!