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SPM  May 2011

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Subject:

Re: Modelling Categorical Responses

From:

"MCLAREN, Donald" <[log in to unmask]>

Reply-To:

MCLAREN, Donald

Date:

Fri, 6 May 2011 13:26:43 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (108 lines)

You entered onsets times in seconds and told the program they were in
scans. This means that your design doesn't reflect what the subject
was doing.

Change Scans to Seconds and they should be similar.

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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On Fri, May 6, 2011 at 1:19 PM, shaza zaghlool <[log in to unmask]> wrote:
> Hello,
> I'm working with a dataset that has the following task:
>
> STOP SIGNAL TASK: A task in which an external stimulus signals the
> participant to interrupt an already-initiated motor response.
>
> These are the task conditions:
> -Go trial
> Trials on which a go stimulus is presented and the subject is expected to
> respond
> -Stop trial
> Trials on which a stop signal is presented at some delay (stop-signal delay)
> following the go stimulus, and the subject is expected to stop
> -Successful Stop Trial
> A trial on which a stop signal is presented at a delay following the go
> stimulus, and the subject successfully withholds their response
> -Unsuccessful stop trial
> A trial on which a stop signal is presented at a delay following the go
> stimulus, and the subject fails to withhold their response
>
> I have 15 subjects which I specified a 1st-level model for and then
> estimated the model using the classical method. The cluster sizes and
> locations I got weren't consistent for the 15 subjects at all though. I'm
> not sure where I could have gone wrong...
>
> I attached a sample of behavioral files for one of the sessions for one of
> the subjects.
>
> The behav data columns are:
>
> Onset: onset ime of the trial in seconds
>
> TrialType: 0=go trial, 1=stop trial
>
> Stimulus: right versus left arrow
>
> SSD: stop signal delay (for stop trials only)
>
> Response: response code (0=no response)
>
> RT: response time
>
> CorrectGo: 1=correct go response, 0=incorrect response
>
> SuccStop: 1=response successfully withheld, 0=response executed (stop trials
> only)
>
>
>
> During my fmri model specification, under "timing parameters" and "units for
> design" I chose "scans" as opposed to "seconds".
>
> I specified 3 conditions (go_onsets, stopfail_onsets, and
> stopsuccessful_onsets). I took the onsets for those 3 conditions from the
> first column of task001_run001_go_ons.txt, task001_run001_stopfail_ons.txt,
> and task001_run001_stopsucc_ons.txt respectively. (I wasn't sure what to do
> with the other 2 columns in each of those files)
>
> I also entered "0" for all the durations under all the conditions. Finally,
> under "Multiple Regressors" I entered the realignment parameters I got from
> the realignment stage in the preprocessing.
>
>
>
> After I estimated the model, I used the contrast manager to specify 3
> T-contrasts. The contrasts I specified were Go>NoGo, Go>Baseline, and
> Go<Baseline.
>
> None of these contrasts gave me similar activation pattern results
> throughout the 15 subjects. (I didn't mask these contrasts with anything,and
> used a p value of 0.001 and an extent threshold of 0 for all experiments.
>
>
>
> Any help would be greatly appreciated.
>
> Regards

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