Hello everyone,
I am interested in learning how to model the within-subjects effect of time on a two-stimulus
presentation.
Each participant viewed two stimuli (CS+, CS-) for a total of ten trials (5 per stimuli).
In my first-level FEAT, I have created 2 original EVs and added the temporal derivates to
investigate the main effect of stimulus type. Thus, the contrasts:
1 -1 (CS+ > CS-)
-1 1 (CS+ < CS-)
1 0 (CS+ > rest)
-1 0 (CS+ < rest)
0 1 (rest > CS-)
0 -1 (rest < CS-)
We also belive the signal strength in our ROI may be greatly affected by repeated stimulus
exposure. For some participants, the signal may habituate, in others it may sensitize.
Do I need to create 5 original EVs to represent the 5 trials of stimulus exposure, and then create
contrasts that model each potential "shape" of the within-subjects' time effect?
I belive I would model a linear signal habituation as:
2 1 0 -1 -2
(and a sensitization model would be the opposite).
For a cubic effect, would I model:
2 1 -6 1 2
The interaction of the main effects of stimulus and time are also of interest, as we predict CS- to
remain constant over time and CS+ to change.
Are all interaction effects analyzed if I select the "interaction" basic shape of an addition EV?
Thank you so much for your help,
Deane
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