@controversial...
Do I have a proof of Atma?
> You cannot grab Atma like you grab objects. You cannot see Atma like you see the material world. Atma is subtler than the space. Atma has to be directly realized. How do you know you exist?
> Why it is apart from brain activity?
Atma is not something apart from brain activity like you are understanding but it pervades everything becoming itself a witness consciousness. When you see movie in a cinema hall, there is so much activity in the screen, but the screen doesn't move and the whole movie cannot exist without the screen. Similarly, the substratum for brain activity is Atma but Atma transcends all activity. It is peace and bliss. Brain activity is Prakriti's Rajasic Movement. Atma is Purusha which is an immanent reality. The intellect borrows Atma's light for understanding. Atma when expands becomes Brahma. It is immeasurable, unfathomable, unthinkable, not an ordinary consciousness. It is beyond time and space and this has to be directly realized.