afni.proc.py regress_basis option

AFNI Message Board
1

Hello!

I am a relatively new user of afni.proc.py and have a question about what regress_basis option that I should use for my analysis.

The fMRI data that I am analyzing is an event-related design memory task, which includes three types of events (i.e., negative rating, neutral rating, binding). For each type of event (e.g., negative rating), the duration is the same across the whole task (e.g., 2.5s) and across all the participants (the stimulus onset times are different across participants). Given this design, is it correct to use the “-regress_basis 'BLOCK(duration,1)” option? Any guidance would be greatly appreciated!!

The part of my afni.proc.py script for the first-level GLM analysis (including the -regress_basis part) is detailed below:
-regress_stim_times negative_rating.txt
neutral_rating.txt
binding.txt
-regress_stim_labels negative_rating
neutral_rating
binding
-regress_basis ‘BLOCK(2.5,1)’
‘BLOCK(2.5,1)’
‘BLOCK(3,1)’ \

2

Hello,

These are reasonable basis functions, but there is a subtlety when combining them. Since they are all scaled to 1, all of the betas will be in terms of percent signal change. However if they were the exact same task, one event is 2.5 s and another is 3.0. If you perform a contrast between them, the longer event has an unfair advantage. If you are NOT going to compare negative or neutral ratings against binding, then it is fine as is. But if you do want to compare them, it might be more appropriate to exclude the ‘,1’ scaling in the functions.

  • rick
3

Hi Rick,

Thank you so much for such a detailed and helpful response! It not only helped me understand the basis function that I am using for my analysis but also taught me about the nuisances of options that can be applied to different situations/contrasts! :slight_smile: