Z-score in Human Brain Atlas


I’m working with the allen human brain atlas gene search. I have already downloaded the expression data from the gene I’m researching (the DAOA gene).
The thing I don’t understand is what the expression is compared to. Take for example the frontal lobe. There are multiple substructures from which the z-scores are calculated. But from what data are the z-scores calculated, I mean: the expression in the structure was measured, but to what average expression was it compared?
Also: can I calculate the average z-score from e.g. the frontal lobe by using all the known z-scores? Or do I have to make a difference between positive and negative z-scores? Or isn’t it possible to calculate the average z-score of a topstructure at all?

Hopfully someone has an answer for me.
Thanks already!

These topics have been addressed in a few other posts (here, here, and here). If you can’t find your answers there, please let me know.