Hi again,
There is one thing that I still find unclear in this expression:

<No, it seems that -expr 'step (a-a_threshold) - step (b-b_threshold)' would <give you the following:

<if the value at a voxel is 1, activation in group 1 but no activation in group 2;
<if the value is -1, activation in group 2 but no activation in group 1;
<if the value is 0, both groups are either active or inactive at the same time.

Since the step function will replace any value <0 with a 0, this means that for example a t-value of +4 and a t-value of -4 (actually minus any number) will both be transformed into 0 if the threshold is for example 4.3. The consequence of this is that a deactivation of any size and a small activation below the threshold will be considered to be the same thing although they are very different. So the difficulty, then, would be to distinguish between a lack of activation and a deactivation, unless I've misunderstood something.

Thanks,
Hedvig.