1. You don't *have* to make the duration of the baseline the same as the experimental condition. (Although see Lindquist et al 2009 NeuroImage)
2. I don't think comparison 2 is possible (for reasons similar to my response to you a few weeks ago). For comparison 3, if you think the subjects will habituate after a few responses (I assume this is why you only want to compare the first few responses), then is there a way you can turn this into an event-related design? Your design seems like it's getting too long, in terms of the separation between tasks.
I don't understand why you would insert "some random cognitive task" into your design. That would just unnecessarily complicate the model.
Rik Henson posted to the list semi-recently about the variable duration method (search the list). According to him, your betas will represent "activation per unit time" (instead of per event/trial/block). I take that to mean that you don't have to mess around with changing the length of your baseline epochs.
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From: SPM (Statistical Parametric Mapping) [[log in to unmask]] On Behalf Of Talya Sadeh [[log in to unmask]]
Sent: Sunday, July 25, 2010 10:11 AM
To: [log in to unmask]
Subject: [SPM] Baseline condition duration and other design issues
Dear SPMers,
I've posted a similar message before, but a response has yet to come… So, I've decided to post this message again and tried to make it a little shorter and friendlier this time. Any help would be very much appreciated.
I am running a behavioural pilot for an fMRI experiment with a quite an unconventional design, so there are a few issues I'm uncertain about.
Our paradigm involves two different semantic verbal fluency tasks (task A and task B) with self-initiated responses. We give quite a few blocks of each of the tasks. According to our pilot data, the rate of responses is quick at the beginning of the each block (around 3-4 responses in the first 2-4 seconds) and then the response rate decreases (to a response every 2-7 seconds). I am interested in the following comparisons:
1. All of task A compared to resting baseline (and all of task B compared to baseline)
2. The first 2-4 responses of the blocks compared to a few responses from the middle/end of the blocks
3. The first 2-4 responses of blocks of task A compared to the first few responses of blocks of task B
I plan to analyze the data using the variable duration epoch method (Grinband et al., Neuroimage), where the duration of each epoch is the time between two consecutive self-initiated responses.
The baseline trials cannot be added within the experimental blocks because we assume subjects are actively performing the task throughout the block.
My questions concern the baseline condition:
1. I understood that I have to make sure that the duration of the baseline condition is as long as the duration of the experimental condition(s). So, for comparison #1, if each block of the task lasts 30 seconds, should I give a 30-sec baseline following each experimental block?
2. For comparisons #2 and #3, I compare two conditions whose durations are shorter than the entire block (for each block, the total duration of each condition—which includes a few trials— is 2-7 seconds). Does this mean that for these comparisons I should use shorter baseline epochs (or is it OK to use the entire 30-sec baseline epochs)?
If I need shorter baseline epochs does it make sense to introduce another part to the experiment where I will present short blocks of some random cognitive task interleaved with short blocks of the baseline task?
I would appreciate, of-course, any other input regarding this non-trivial design (in case I'm missing anything important…).
Thanks a lot!
Talya
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