Greetings everyone,
I have what I imagine to be a simple modeling question, but can't seem to find the answer by myself. Apologies if it's very basic indeed!
I have 2 stimulation conditions, each composed of 2 stimuli - AC and BD. I'm not interested in analyzing them separately, but rather in the activity associated with the fact of perceiving 2 stimuli (with respect to only one - other conditions I haven't mentionned for the sake of simplicity), whether it be AC or BD. Now I wonder what is the best approach for this:
1. model AC and BD separately with 2 regressors at the first level, and use a [1 1] contrast to look at the average response?
2. model both conditions using one single EV?
I've used both approaches, and they give pretty different outcomes: while with the first approach, both regressors are associated with activity within the same regions, which in turn show up for the average response contrast, I only get a small subset of the same areas with the second approach.
I'm assuming the question asked by modeling both conditions together differs from that asked by modeling them separately, but I'd like to understand the difference to chose the best approach. Could someone be kind enough to explain to me what happens in the second case?
Many thanks in advance!
Stephane
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