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Hi,

You would leave fixation unmodeled and then it acts as the baseline.  So if you had a single task and fixation you would have a single regressor that models your task and the parameter estimate corresponding to this task would be the activation of task compared to baseline.

In general you must have at least 1 thing left unmodeled to serve as the baseline in your model and fixation is typically this thing.

Jittering fixation time would help tease apart the signal of two different trials types (vs baseline) that occur before and after the fixation, which isn't what you have here. 

Hope that helps,
Jeanette

On Mon, Dec 5, 2011 at 4:53 PM, Jeremy Elman <[log in to unmask]> wrote:
Hello,

I have a question regarding experimental design if we are looking to isolate delay activity during a memory encoding experiment. We would like to have subjects encode a word (2.5s or so), followed by a variable length fixation period (.5-8s). Our question of interest is whether a given region of the brain which tends to have low (or even suppressed compared to baseline) levels of activity during the stimulus encoding phase may come on-line given enough time during the following fixation period. We have a few questions about the best way to examine this though as the encoding and fixation regressors would be correlated.

Assuming the stimulus presentation (encoding period) is of a set length, the fixation onset is completely correlated with the stimulus onset. Is there any good way to isolate this fixation activity? My impression was that orthogonalising wrt to the stimulus regressor wouldn't be sufficient to get around this problem.

Does the variable duration of the fixation phase help to disentangle these two events or are the correlated onsets still the main issue? We could vary the stimulus encoding duration independently, but this isn't ideal behaviorally as it would be unclear whether the same processes are occurring for the duration of the encoding event at longer times.  Any other ideas on how to approach this problem?

Thank you in advance for your help,
Jeremy