>Thus, if the main effect across the entire task was not being driven by
>one half over the other, the [1 1] map masked with [1 0] should
>essentially be the same as the [1 1] map masked with [0 1], correct?
Yep, except you can't know this without doing the masking.
>if there are differences, then it suggests that the global main effect
>across the task is not a great indicator of what regions are activated
>as a function of time. In essence then, [1 1] represents a mean of [1 0]
>and [0 1] activation, not a simple addition, correct?
That's right. Linear contrasts are means -- not additions.
>So masking [1 -1] with [1 1] is wrong for my purposes because only
>voxels commonly active (on average) across both halves will be used in
>the mask (assuming my understanding of [1 1] as mean and not summed
>activation is correct). If instead, I mask [1 -1] with both [1 0] and [0
>1] simultaneously, then it will mask the [1 -1] contrast with I>N voxels
>that appear in both halves, correct? And this yields a map of I>N
>voxels that were more active in the first half compared to second?
That's right. This will yield a map of I(first) > I(last) only in voxels
where there is an I>N effect in the first and last half of the experiment.
Joseph Devlin, Ph. D.
FMRIB, Dept. of Clinical Neurology
University of Oxford
John Radcliffe Hospital
Headley Way, Headington
Oxford OX3 9DU, U.K.
Phone: +44 (0)1865 222 494
Fax: +44 (0)1865 222 717
Email: [log in to unmask]