Chris and AS,
I'd agree that an event-related design would be much better suited for
the question of linearity of squeeze strength. However, I think it
could be modelled with events as I will explain below.
If you have a block of 30s, you'd model it as 1 event of 30s. Now I
would get the same results if I modelled it as 3 events each lasting
10s with no gap. On the extreme, I could model is as 30 events each
with a duration of 1s. All 3 models would produce the same regressor
and thus the same estimate. Although you've modelled it as short
events, you still have the block regressor. Now, we can add a
parametric modulator that might explain some of the variance in
squeeze strength. The two potential issues are: (1) the variance of
squeeze strength is below the variability of the response during the
block; (2) the parametric modulator is mean-centered per run; and (3)
there might be additional time effects (e.g. habituation in each
block). Thus, if each run doesn't contain enough of the high and low
squeeze strengths, then you might not see much of an effect.
Modeling random events is not the way to go as you are telling the
program that there were no squeezes during the skipped events. This
does not provide any basis for what a true event-related study would
show.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Thu, May 16, 2013 at 2:36 PM, Chris Watson
<[log in to unmask]> wrote:
> I wasn't surprised because you are trying to create an event-related model
> with trials that are too close in time. What you plan on doing in the future
> sounds fine; just be sure to include a random jitter (e.g. sometimes the ISI
> is 3s, sometimes it is 5s, etc.)
>
> On 05/15/2013 04:51 PM, fMRI wrote:
>
> Dear Chris,
>
> Can you please tell me why you were not surprise as this is what I want to
> understand?
>
> Can you please recommend a event related design for me to get what I want in
> the most efficient way.
>
> I will share with you what I am thinking to do next time and I will
> appreciate your comments:
>
>
> I will do an 8 min experiment with a TR of 2.5 s. I will make it event
> related such that the average ISI will be ~ 4 s. I will try to have ~ 100
> events. Do you expect that I get something better using this design? Any
> comments will be appreciate it ?
>
> Thanks
>
> AS
>
> On 15 May 2013, at 21:18, Chris Watson
> <[log in to unmask]> wrote:
>
> 1) That doesn't surprise me that your results were not as expected.
> 2) I still don't think this should be modeled as an event-related design.
> The stimuli are too close together. I don't know how you could justify
> selecting a random subset of stimuli to include in your design and then run
> the analysis.
>
>
> On 05/15/2013 04:06 PM, fMRI wrote:
>
> The design is like this:
>
> 1) 48 blocks, 24 task and 24 rest
> 2) the subject was squeezing in the active task. Each block has a different
> target so that the strength in each block is different.
> 3) the isi was 0.6 second between each squeeze.
> 4) I had around 320 volumes and the TR was 3.1
> 5) each block lats for 20 seconds
>
> I design it in two ways:
>
> 1) the condition was the onset time of each squeeze and the duration was
> zero so as a delta function. Then I added the grip strength values for each
> onset in the parametric modulation.
>
> The result from this one was not as I expected as the linearity was not high
>
> 2) I thought that if I want to see the parametric effects , I should use an
> event design. So before doing this I tried to select randomize onset with
> its grip strength, such that the average isi is 4 seconds. I used the same
> number volumes without changes. I got here very nice results.
>
> Any comment will be appreciate it,
>
>
> Thanks
>
>
> On 15 May 2013, at 19:03, Chris Watson
> <[log in to unmask]> wrote:
>
> I think we'll need to know more about your design, e.g. stimulus duration,
> inter-stimulus interval, and so forth.
>
> On 05/15/2013 01:01 PM, fMRI wrote:
>
> Hi all,
>
> I want to understand basic thing about spm. What is really more important to
> spm the active or task volumes or the specification of a condition.
>
> For example, I have a block design and then defined it as event by randomly
> selecting a number of trials among the active and rest blocks. I get ~ same
> activations using the exact same volumes but changing the condition from ~
> 1000 onset to 100 onsets. The reason why I did that was because I did an
> experiment to see the linear effects of using parameters as a mixed block
> design. The result was not similar to what other did. Then I found that it
> is better to design an experiment such this as an event design. Since I did
> not have a new experiment, I thought I can play with what I have. In terms
> of the linearity, I got really nice result as what I want. I just want to
> understand how and why although I used the same volumes. Does spm ignore the
> other activations that I do not defined? Can you comment please ?
>
> Thanks
>
> AS
>
>
>
>
>
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