[Freesurfer] within-subject contrast between runs

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Dear Freesurfer Experts,

I have a very basic questions about creating subject-wise contrasts between runs

Background

Study with two runs per subject where each run corresponds two a different condition (i.e. Stimulation A (on - off) and Stimulation B (on - off).

I performed the preprocessing and 1st levels stats in the native subjects space

Questions

* How can I obtain a within-subjects contrast between two runs (the Stimulation A vs. Stimulation B ) obtained in two runs within one session? * Can I obtain this within-subject contrast by staying in the native subject space? * Do I have to concatenate both runs and create a new parradigm file?

Thanks for your support

Sincerely

Chris