> Hi Julie,
> thanks a lot for your reply!
> I will give some further insights:
> Am 01.07.2008 um 22:22 schrieb Julie McEntee:
>> A few questions/points:
>> 1. How did you design the experiment: block or event-related?
> Mixed: It is a bit more complicated: The paradigm I'm talking about
> is some kind of wheel of fortune game: 1. Starting with an epoch
> with variable duration, where a choice for a bet is made. 2. After
> 1-3 seconds an anticipation phase, where the wheel of fortune
> spinns for 10 seconds. 3 The wheel of fortune stops and the outcome
> is displayed for 3 seconds 4. Variable blank screen 8-10 s.
> I modelled: the choice(event) , anticipation phase(block 10s),
> outcome(block 3s).
>> 2. Are you sure that you jittered the scan acquisition time? (it's my
>> understanding that TR is fixed, but this may be incorrect ...)
> I meant by jittered, the onsets of stimuli are not synchroneous
> with the onsets of the TR. Sorry, I did write that in a confusing way.
>> 3. If it is a block design, jittering any of the events within (or
>> a trial does not apply, as you cannot derive information regarding
>> specific event during each trial (i.e., to do that you would need an
>> event-related design, in which case ...
>> 4. When constructing a trial for an event-related design, keep in
>> mind that
>> the BOLD response is, on average, about 20 s, with a latency of
>> about 8 s.
>> Therefore, in order to obtain BOLD response information at a
>> particular time
>> for a given duration during each trial (e.g., stimulus onset,
>> response), the
>> design must allow time for any change in BOLD response to
>> "recover", i.e.,
>> return to baseline. If the BOLD response is not allowed to recover,
>> questions regarding a specific event within each trial is not
>> possible; BOLD response may even disappear after high-pass
>> filtering. The
>> purpose of jittering (typically applied to the inter-trial
>> interval and/or
>> inter-stimulus interval, for example) is to de-convolve stimulus
>> onset with
>> HRF and overcome the correlation between regressors, therefore
>> the validity that the change in BOLD response is due to the
>> To answer your question, if you have taken the above into account
>> in an
>> event-related paradigm, and created a complete model of the
>> experiment at
>> the first (subject) level of analysis (i.e., the model must
>> account for the
>> duration of every event during each trial, as opposed to only
>> those that you
>> are "interested" in) you can ask questions of the model by defining
>> contrasts. For instance, I'll assume you modeled the experiment by
>> the conditions Anticipation, Outcome and Rest, in that order. The
>> contrasts would produce:
> I think I have taken this into account. But my big mistake was,
> that I did not vary the duration of the anticipation phase. Like
> this the regressor of the immediatly ensuing outcome phase is
> always corelated to the regressor of the anticipation phase. As far
> as I understand, a contrast of the outcome phase against the rest
> ( e.g.: 0 1 0 below) can not be interpreted, because it is unclear,
> how much of this "activity" is due to neural activity in the
> anticipation phase and how much is due to the outcome phase...
>> name: Anticipation
>> weight: 1 0 0
>> all changes in BOLD response during Anticipation
>> name: Outcome
>> weight: 0 1 0
>> all changes in BOLD response during Outcome
>> name: Outcome minus Anticipation
>> weight: 1 -1 0
>> changes in activation during Outcome only, excluding BOLD response
>> present in both conditions
>> name: Anticipation minus Outcome
>> weight: -1 1 0
>> changes in activation during Anticipation only, excluding BOLD
>> changes present in both conditions
> I will try this. But I'm not sure, if this really can separate
> activity between the two phases...
>> Creating masks using contrasts and applying them to other
>> contrasts is
>> another approach that is used to isolate BOLD response- perhaps
>> someone else
>> can contribute their expertise in this area, if applicable.
> Would that be something like this?
> To detect activity unique to the outcome phase:
> Create a contrast for the anticipation against all other regressors
> (your 1 0 0), then exclusively masking the outcome contrast (0 1 0) ?
>> I hope this is helpful-
> Thanks again for your suggestions and nice ideas!
> Kind regards
>> Julie E. McEntee, M.A., C.C.R.P.
>> Senior Research Program Coordinator
>> Department of Psychiatry and Behavioral Sciences
>> Division of Psychiatric Neuroimaging
>> Johns Hopkins University School of Medicine
>> 600 N. Wolfe St./ Phipps 300 (office: room 317)
>> Baltimore, MD 21287
>> Phone: 410-502-0468
>> Fax: 410-614-3676
>> -----Original Message-----
>> From: SPM (Statistical Parametric Mapping)
>> [mailto:[log in to unmask]] On
>> Behalf Of Andreas Pedroni
>> Sent: Tuesday, July 01, 2008 3:36 AM
>> To: [log in to unmask]
>> Subject: [SPM] influence of non-jittered block-event
>> Dear SMP list!
>> I have made an fmri experiment with an anticipation phase and an
>> following outcome phase. The scan aqcuision time is jittered in
>> time, but
>> the durations of the two phases are (regrettably) fix (10 sec for
>> anticipation, 3 sec for outcome phase).
>> To me it makes sense, that the neural activity elicited in the
>> phase could bleed into the activity recorded in the outcome phase.
>> Thus an
>> interpretation of the outcome phase is limited. But can the neural
>> in the outcome phase also influence the preceeding activity in the
>> (anticipation) phase?
>> Thanks for any suggestions!