Meditation while trying to fall asleep for several years. Originally this was just noting exercises to help process things my autistic little brain had maybe missed that day - as well as examining feeling that were perhaps suppressed while more lucid. Eventually the wake/sleep began blurring, and I figured out how to visualise piece by piece,. At first, it was just very basic shapes, which I could make move when tied to emotions. Then it became repeating, shifting, full-field patterns/shapes, initially monochrome, but becoming colorful over time when made to move quickly or with purpose.

Then suddenly I wasn’t doing it manually anymore, and it got a lot stronger. It took several days for it to calibrate and tune its self into a useful, non-overwhelming sense. I assume most people learn to access that subconscious layer when young, and settle on a baseline they find acceptable to their mind? I heavy a heavy suspicion that autism and dissociative affect can be linked with aphantasia.