Hi Lin,
In general, everything you don't model (i.e. have an EV for) ends up in
the implicit baseline. When you set up a contrast like [1 0], you are
contrasting your first Ev vs. this implicit baseline. So in your case,
if all you don't model is fixation only, your implicit baseline is your
fixation condition, and your last 3 contrasts are
correct. The condition for which the EV is weighted 0 is simply not
taken into account in this contrast.
Best,
Stephane
Lin Nga wrote:
> Does that mean that if I have more than one EV, I cannot contrast a single
> EV to baseline (i.e. my fixation condition)? For example, I have:
> Task - Control [1 -1]
> Control - Task [-1 1]
> Task - Fixation [1 0]
> Control - Fixation [0 1]
> All - Fixation [1 1]
>
> Are the last three contrasts wrong? Assuming they are correctly set up, I
> would think that something like "Task - Fixation [1 0]" would contrast task
> to both fixation (baseline) and control. I'm sure I'm misunderstanding this.
> Could anyone clarify it for me?
>
> Thanks a bunch,
> Lin
>
> On Fri, 23 Jan 2009 07:23:34 +0000, Steve Smith <[log in to unmask]> wrote:
>
>
>> If you have two tasks, i.e. "task" and "control" then yes you put them
>> as two EVs, and contrast them with (e.g.) [1 -1].
>> If you have just one task, and want to compare that to baseline, then
>> you have one EV, and contrast the task with baseline using [1]
>> Steve.
>>
>>
>>
>>
>> On 22 Jan 2009, at 22:02, Lin Nga wrote:
>>
>>
>>> Hi everyone,
>>>
>>> Sorry if I'm treading old grounds here but it isn't exactly clear to
>>> me how
>>> FSL compares the EVs to fixation condition if it isn't explicitly
>>> set as an
>>> EV. For example if there are two EV's, task and control, I know that
>>> if I
>>> wanted to contrast task to fixation I would use [1 0]. Wouldn't task
>>> be
>>> contrasted to both control and fixation in this case, since control
>>> is set to 0?
>>>
>>> Thanks again,
>>> Lin
>>>
>>>
>> ---------------------------------------------------------------------------
>> Stephen M. Smith, Professor of Biomedical Engineering
>> Associate Director, Oxford University FMRIB Centre
>>
>> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
>> +44 (0) 1865 222726 (fax 222717)
>> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
>> ---------------------------------------------------------------------------
>>
>
>
>
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Stephane Jacobs
Postdoctoral Research Associate
Human Neuroimaging and Transcranial Stimulation Lab
Department of Psychology
1227 University of Oregon
Eugene, OR 97403 - USA
Email: [log in to unmask]
Tel: (1) 541-346-4184
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