The problem with this kind of pseud-scientific eso stuff is, that it relies purely on anecdotal evidence — anectdotal evidence by people who want to believe in it.

If we assume the Schuman resonance is a real magnetic phenomenon that can be observed, the questions that these people would need to answer are:

1. If this affects the brain, through what mechanism would that happen? Induction? How would a study look to test this?

2. If such a low frequency oscillation with such a low energy does affect the brain, why don't we go completely cookoo bananas in the presence of magnetic fields that are many magnitudes stronger?

3. What evidence would you need to disprove that this effect happens?

In practise one could "simply" do a randomized double blind study with a high enough number of people that have to do different tasks and you subject some to the magnetic fields and others not. If you are extra good you do it in a way that nobody (even the experimenters) knows whether the magnetic field is on for a given candidate or not.

And then you compare the results of those tasks.

But I think the problem is that they don't even wanna know, believing is enough.