Fanny,
You have a 10 second trial. The 1s video is independent of this
initial statement since its entirely enclosed.
You will want to model the thermal stimuli, the video, and the rating
period as 3 different conditions. Thus, you will end up with 12
conditions. I would recommend that you have 30 of each condition (120
10s trials).
Although, you don't need fixation, you will want more than what you
have suggested for several reasons: (1) having only short 1 second
interval will be stressful for the subject; (2) you are wanting to be
able to accurately model the thermal, video, and rating periods, so
more gap will allow the response to come back to baseline before the
next trial.
I'd personally aim for inter-trial-intervals of 5-9 seconds (1 sec.
increments would be fine). Begin the experiment with 21 seconds of
fixation (you'll discard the first 2 TRs and then have 5 more to get
an estimate of the baseline values. Also, make sure you have some time
after the trial to measure the last trial response coming back down to
baseline (15 seconds)
The minimum ITI ever needs to 1-2 seconds, any shorter and the events
won't be separable and you can non-linear effects.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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2011/6/23 Fanny Eugène <[log in to unmask]>:
> Dear SPMers,
>
> I need some advice on timing parameters for an event-related design. Each of my trials lasts 10 seconds, with a 1-second video clip presented 4 s. after the start of the trial. I’m only interested in brain activation during the clip and how it varies across 4 conditions (4 types of video clips). I plan to present a fixation cross of varying durations between trials.
>
> From what I understand, since I plan to contrast activation across conditions and I’m not interested in contrasting each condition with a baseline (null events), the duration of my fixation cross does not need to be very long, but only needs to shift the timing of my events so that they do not always start at the beginning of a TR. Is this right?
>
> With a 3 sec TR, would it be ok for me to use a fixation cross with durations that vary randomly from 500ms to 2500ms in steps of 500ms?
>
> Any advice would be very helpful, thanks!
>
> Fanny
>
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