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> > 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 >> between) >> a trial does not apply, as you cannot derive information regarding >> any >> 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 >> theoretically >> 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 >> increasing >> the validity that the change in BOLD response is due to the >> experimental >> events. >> > > >> 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 >> defining >> the conditions Anticipation, Outcome and Rest, in that order. The >> following >> 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 >> changes >> present in both conditions >> >> name: Anticipation minus Outcome >> weight: -1 1 0 >> changes in activation during Anticipation only, excluding BOLD >> response >> 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 > > Andreas > > >> Julie >> >> 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 >> immediatly >> 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 >> anticipation >> phase could bleed into the activity recorded in the outcome phase. >> Thus an >> interpretation of the outcome phase is limited. But can the neural >> activity >> in the outcome phase also influence the preceeding activity in the >> first >> (anticipation) phase? >> >> Thanks for any suggestions! >> >> Cheers >> >> Andreas >