>Wouldn't masking [1 1] with [1 0] yield a map of I voxels across the
>entire task that is masked so that only those in the first half appear
>as active? If so, wouldn't this yield the same map as that of the simple
>(unmasked) [1 0] contrast?
Yes and no. Yes, masking [1 1] would yield only those voxels showing a
main effect of I>N across the whole task which were also present in the
first half but this is not the same as the [1 0] contrast. Presumably you
want [1 1] to be the *main effect* over time. But to get this using a
linear contrast you need to make sure that the mean effect that comes from
[1 1] is not driven entirely by either the first or second half of your
design. That is, a big effect in a particular voxel in the second half of
the experiment but no effect in the first half, may still show up in the [1
1] contrast (because it's a mean). By masking with *both* [1 0] & [0 1]
guarantees that the effects are present in both the first and second halves
respectively, at some given threshold.
> From my understanding, If I wanted to find I voxels that were more
>active in the first compared to the second half, wouldn't I mask [1 -1]
>with the [1 1] contrast? I interpret this as asking "of I voxels active
>across the entire task, which are more active in the first compared to
>the second half?"
Not exactly. The contrast [1 -1] will directly give you the voxels which
were more active in the first than second session. Masking with [1 1] in
this case isn't necessarily meaningful because it doesn't guarantee that
the voxel is showing a main effect. If you want to limit your [1 -1]
contrast to only those voxels "activated" during both halves of the task,
then once again you need to make with [1 0] and [0 1].
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]