Thanks for the suggestion, Mark. I guess that's similar to what I had in
mind. But still have some questions for the setup below.
>Hi,
>
>> Model 2:
>> Set up 3 EVs for the levels of ratings (binary 0/1 for off/on) and set up
>> contrasts using the mean ratings for each level as weights.
>
>Model 2 is the way to go. As you noted with model 1, having a single EV
>restricts the relationship between ratings and stimuli. With 3 EVs and
>using different contrasts you can ask a wider range of questions.
>
>> Problem: How? Say, to test the hypothesis that brain activation is
>> modulated linearly with the mean ratings (e.g. 162, 248, 356, for S1, S2,
>> S3 respectively). Any F test needed?
>
>No F test needed.
>The best you can do with only three rating levels is:
>
> EV1(S1) EV2(S2) EV3(S3)
>positive linear -1 0 1
>negative linear 1 0 -1
How is this different from just testing the difference between S1 and S3.
Say if I have parameter estimates of 50, 150, and 100 for S1, S2, S3
respectively, I will get significant map for the positive linear contrast.
But the relationship is not exactly linear among the three levels, right?
So for a brain area to show positive linear modulation by the three levels,
should I get the conjunction between the following two contrasts? I know
that the sum of these two contrasts is equivalent to the above setup. What
I am not clear is whether it should be the sum of the following two
contrasts or the conjunction of them to assert the linear relationship
among the three levels.
EV1(S1) EV2(S2) EV3(S3)
C1 -1 1 0
C2 0 -1 1
Also how am I supposed to use the mean ratings (above) to test their
relationship instead of the categorical levels? Do I just replace 1/-1 to
the actual ratings or the inverse of the ratings (multiplied by 1/-1)?
>
>This can be setup similarly if you had more rating levels, e.g. 5:
>
> EV1(S1) EV2(S2) EV3(S3) EV4(S4) EV5(S5)
>positive linear -2 -1 0 1 2
>
>However, note that this does NOT exclude higher order relationships
>(you could set up contrasts to look for these as well - if there are
>enough rating levels) and is only looking "locally" at the ratings
>strengths used.
>
>Cheers, Mark.
>
>Mark Woolrich.
>
>Oxford University Centre for Functional MRI of the Brain (FMRIB),
>John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
>
>Tel: (+44)1865-222782 Homepage: http://www.fmrib.ox.ac.uk/~woolrich
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