Aphantasia is one of the strangest neurological conditions in which the individual can't visualise...well anything.
In normal functioning, one is able to think of a box or shape when the fancy takes them. But this thinking can take on visualisation, in which the mind's eye can see the object.
In aphantasia this ability is missing.
Let's demonstrate this, If I ask the reader to imagine a bright sunny day, maybe you're walking the dog and throwing a ball. Maybe the ball is green and as it bounces along the ground, the dog's tail wags vigorously with excitement.
The reader most likely is able to picture this scene without much difficulty, even if they don't own a dog.
But with aphantasia there would be nothing but an empty vacuum where the image should be.
According to https://aphantasia.com/what-is-aphantasia/ an "estimated 3 to 5% of the population suffer with aphantasia."
Needless to say this can have a large impact on one's life, and the ability to find refuge in the glorious sanctuary of imagination -something a lot of us take for granted.
This would also have an impact on mnemonic techniques such as the method of Loci or the memory palace technique which requires visualisation.