Hi Jack
nope, the design is not orthogonal - but that should not stop you from
estimating A and B.
The (1,1) contrast in option 2 is not easily interpretable and gives
not what you think it does: (0,1) clearly is attention+movement (B) and
(-1,1) is movement only, therefore (1,0) is attention only (A) -
unfortunately this is not very efficient as you're estimating the
attention effect size from A only... Option 1 is probably what you want
cheers
christian
On 3 Sep 2004, at 08:46, Jack Grinband wrote:
> Hi Joe,
>
>> If I understand the design correctly, the two are equivalent but in
>> either
>> case there is a problem of rank deficiency when trying to model
>> attention
>> (since it is present in both of the two conditions). Are there also
>> blocks
>> of baseline such as rest, fixation, whatever that are not being
>> modeled? If not, the effects of attention are not distinguishable
>> because
>> there is no experimental variation.
>
> Sorry for not being clear. The stimuli are on for 20s, off for 20s.
> So
> there is always a baseline period between A and B where the subject
> simply
> fixates.
>
>> Another way to think about it is if there are only two blocked
>> conditions,
>> you only need one binary variable to model them (ie 1 EV).
>
> So given this stimulus:
>
> A -- baseline -- A & B -- baseline -- A -- baseline -- A & B -- etc
>
> my understanding is that A and B are in fact orthogonal. A voxel that
> responds to both A stimuli and B stimuli should have a larger amplitude
> during the A & B block. I think one problem here is that it is
> impossible
> to tell if there is an A/B interaction, since we never see B by itself.
>
> But the other problem is the interpretation of the "A&B" regressor in
> Option #2. What does that activation mean if it has two cognitive
> factors
> responsible for the activation? In Option #1, the meaning of the
> regressors, I think, is clear.
>
> thanks,
>
> jack
>
--
Christian F. Beckmann
Oxford University Centre for Functional
Magnetic Resonance Imaging of the Brain,
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
Email: [log in to unmask] - http://www.fmrib.ox.ac.uk/~beckmann/
Phone: +44(0)1865 222782 Fax: +44(0)1865 222717
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