[TeMPOraL]

> Nobody has proven that it's not trainable, so I'd assume that it is.

I've never heard of a successful training regime, or even of anyone who successfully trained it, so that's an evidence against it being trainable.

> And I spend most of my free time on visual hobbies, like playing video games or watching movies. But maybe I'm just attracted to those hobbies, because I'm a visual type.

I'd guess the latter, because I also spend a lot of time on visual hobbies, like playing video games or 3D modelling, and it didn't help me in any way. I still don't have a mind's eye when I'm awake.

[Kaiyou]

Absence of evidence isn't evidence of absence. A good training exercise I can think of is to build complex Lego models according to the instructions. It's how I spend lots of my time in my early childhood, before I was allowed to play video games and watch TV. By touching the pieces and rotating them yourself you get good image training of how shapes look when you rotate them. Building things in a 3D space based on drawn instructions from only a single point of view should also help to build mental images.