I’m analyzing an event related fMRI learning task. The first level matrix comprises of one regressor coding the correct responses with a parametric modulation (repetition). I’ve written a script to generate matfiles containing the onsets & durations for the correct responses and the parametric modulation. Now I have 2 questions:
1. Duration of the regressor coding the correct responses
I’ve modeled the duration of the correct responses as the stimulus-response duration of 2500ms. In the context of an event related analysis would it be more reasonable to model the duration with duration = 0ms?
2. Parametric modulation & mean centering
The script I’ve written codes the parametric modulation (repetition) with values from 0-5 (0 = first presentation, 5 = 6 presentation). Should the parametric regressor be mean centered (values -2 -1 0 1 2) or does SPM8 automatically mean center the parametric modulator?
Thanks in advance & kind regards,