Just for absolutely clarification, a positive PPI resulting from condition
A PM on the left and 0 on the right (with contrail set to PM) is
interpreted as greater connectivity with increasing PM values?

>>> Correct.

And a negative PPI would be the opposite?

>>> Correct.

Can you specify multiple conditions on the left hand side and 0 on the
right to look for an average difference from 0?

>>> Correct. I would be careful about doing this though. You need to make
sure that the PMs are scaled in a similar manner. For example, if one PM is
trial number and the other PM is post-scan rating, then I wouldn't average
those because they are scaled differently.


NOTE: SPM only displays positive values, so if you want to review the
negatives in
SPM you'd need to create the reverse contrast.

>>> This is for any contrast in SPM - not specific to PPI. If you want the
negative for the second level, you have to build a negative contrast.