Hi!
I'm interested in comparing two groups on a certain contrast (task 1 vs. task 2) and have been running conjunction and difference anlayses on it:

3dcalc -prefix file \
-a 'anova_grp1+tlrc[7]' \
-b 'anova_grp2+tlrc[7]' \
-expr 'step (a-4.3) - step (b-4.3)'

for the difference, or

-expr 'step (a-4.3) * step (b-4.3)'

for the conjunction analysis.

From my understanding, the step function will transform any effect that is larger than F=4.3 into 1, and any effect that is smaller than that into 0. However, it will not consider whether this effect is due to an activation or a deactivation. Is this true? I guess I've missed something, cause otherwise it will consider an activation (e.g., in a) and a deactivation (e.g., in b) to be an overlap/similarity between the groups, as long as the F values for these contrasts are above 4.3. Is there a differentiation between activations and deactivations in the function?

Any suggestions are appreciated!

All the best,
Hedvig.