Lol, sorry this is very humorous to me. I must say, it's somewhat strange that you wouldn't know that the soul exists completely independent of the physical body. Of course the soul entered that body within the womb of its mother and will then experience everything through the perception of that body but once the physical body dies the soul is released.
So because of the reality that the soul exists independent of the body.... once the soul is released from it that soul is also released from every single problem the body suffered so the soul itself is completely unaffected by it as it exists as one observing the body, but it is NOT the body. The body is simply a vehicle, and just like when you drive any such vehicle everything that vehicle undergoes you will notice while driving it however you still exist independent of that vehicle. So if you car gets a flat tire you can't drive it normally of course, so you have to fix it to regain normal operation but it is of no consequence to the driver other than having an external problem.
Dementia is a process of brain function breaking down, and as brain function begins breaking down you will experience that process. But it is merely your perception alone, because you are the one observing that situation (you aren't the actual situation though). If you could picture yourself looking through a sheet of perfectly clear glass as you navigate this planet and that sheet of glass is your window to viewing this particular world (but you are not that glass). Now imagine the glass being marred or scratched, no matter where you look you will see that scratch. Now the glass has become warped, and everywhere you look you observe a warped image....now the glass has shattered and is no longer useful at all and so you leave that window behind.
Obviously the soul and body are the same way, once the brain has become "warped" and begins to lose normal function so your perception and view through that brain are warped as well. Once you entered the womb of the physical body during conception your soul (you) was intertwined within the functions of that body which include the nervous system, the development of the brain and all that body uses to experience this world. It is much like being trapped within a cage, your soul is contained within this vessel and your brain acts as a component that confines your experience to this physical world. Since the soul is energetic, electrical and alive also the body which is a conduit for the soul must be an active medium capable of being a vessel for the soul.
This is why I compare the brain to an electrical panel....it's simply an "electrical" component that regulates the flow of consciousness (soul)...and the physical body nothing more than a circuit board. If you compared the experience of the soul to a house obviously the house would be the physical body and the circuit panel where all the power is regulated and controlled would be the brain. However neither the house nor that electrical panel create electricity, it merely confines it to that house so the house can become something to be inhabited in a useful way.
The actual mind is something of another subject really (one I would have to write much more about). The mind is a projection of consciousness and is basically nothing short of a storage area in which becomes useful for the conscious soul to store memory, categorize and access information. The soul has mind but the soul is the one observing the mind, and both the soul and the mind are subject to whatever a brain experiences but only while they exist within a body.
Because both the soul and the mind exist independent of the physical body it renders the brain and body nothing more than flesh and bones, cells, blood and tissue. Without a soul the body is stillborn. And just the same once the soul leaves the body it regains normal perception of anything it observed or experienced through the human body.
Anyways, I hoped this helps you conceptualize what it means to be soul. Hopefully you won't just ignore it and reject it because you're an atheist. Think about what I wrote here and if it makes sense incorporate it into your data base. If you need me to elaborate on anything just ask questions.
Look up the term subtle body (spirit body) if you get time, basically the soul inhabits bodies for each plane of existence. Once you leave a particular world or plane you are present within the preceding one. Or when the soul is placed within an alternate world, it must have a vehicle useful for navigating that specific world. So each vessel depending on which place within creation is evolved to withstand that environment, it must have a body that correlates with each world or plane of existence. If the soul were to leave creation entirely it would have no embodiment and would basically be pure consciousness and mind. That is the state in which the Creator resides eternally though just like energy, consciousness pervades all of creation....in other words exists independent of form as well as within form.