We should probably take this discussion elsewhere, since I'm fine leaving the example either way (we've done the source-space-magnitude contrast elsewhere anyway)... P.S. it says "constrast" instead of "contrast" :)

The first thing I'm not clear on is, once you've done this contrast, aren't all points non-negative like @mluessi said? How do you then know whether condition A was "more active" than condition B?

And then a second issue. Even in the case where you keep in linear by doing `pick_normal` or a fixed inverse to deal with this problem, it seems like there is still A > B or B > A ambiguity. Consider a single dipole in source space. It seems like a huge *negative* current in condition A (with zero current in condition B) cannot be disambiguated from a huge *positive* current in condition B (with zero current in condition A) once you do (A - B) -- in other words, you get the same signed number out from the calculation in each of these two cases. This would be true for sensor-space contrasts, as well as source-space contrasts where linearity is preserved (`pick_normal` or `fixed`). However, in the first presented case (A hugely negative), I'd want to be able to say "A was more active than B", and in the second case (B hugely positive), I'd want to be able to say "B was more active than A". This to me is the motivation behind first going to source space, then comparing the magnitudes of activation, then contrasting conditions A and B.

That being said, the differing number of epochs can be a real problem with this method. I choose to underpower my analyses in this case (`equalize_epoch_counts`), but if there's a better alternative I'm all ears.