A few questions/points:
1. How did you design the experiment: block or event-related?
2. Are you sure that you jittered the scan acquisition time? (it's my
understanding that TR is fixed, but this may be incorrect ...)
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
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:
weight: 1 0 0
all changes in BOLD response during Anticipation
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
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.
I hope this is helpful-
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
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
Thanks for any suggestions!