From my recent experience I believe you can train your mental vision, at least to some extent. I play chess and the ability to imagine a position and moves in your head is quite common among chess players, but I was always struggling with it. I could not see the board clearly in my mind and when I was doing exercises on telling the color of a given square I was checking coordinates parity, as I could not see it in my mind. Only recently I tried to train chess vision starting with 3x3 board, than extending to 4x4 and finally glueing 8x8 with 4 4x4 boards. To my surprise after a while I started seeing the board more clearly and I could memorize some simple positions. I've noticed that my general mental vision improved significantly at the same time. If you don't play chess you can start with playing tic tac toe in your head, focusing on seeing the board and marks. I think such exercise is better than imaging an apple, because you can objectively check if what you see is correct. Any board game would do, but start with a small board, and extend only when you feel comfortable. Imagining horse moves on a 4x4 board, focusing on seeing square colors helped me a lot.