Hi Jeanette,
Thank you for the reply! We are actually more interested in what is
going on during the fixation period, so would it make sense to model
this phase and leave the stimulus events unmodeled? If so, then maybe
it's more important that the ISI of fixation conditions (long vs.
short) are varied rather than the time between stimulus and fixation
onsets?
Our hypothesis is that not only should there be a flip in activity
within our region of interest from stimulus to fixation (low to high),
but that longer duration fixations should be considered different types
of events than short fixation events. This is because we think
processes occurring in this region are only beginning to kick in when
people have time to disengage from the externally attended stimulus.
Hopefully that makes sense.
Thanks for your help, it's much appreciated.
Jeremy
On Monday, December 05, 2011 3:14:39 PM, Jeanette Mumford wrote:
> 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]
> <mailto:[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
>
>
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