It's difficult to work out what is going on in your own mind, let alone anyone else's. That's one of the reasons why people with aphantasia, or who do not have an inner monologue (anendophasia), may not realize that their minds work differently from other people's.

Aphantasia is a difference in the way the brain generates mental imagery. When the majority of people are asked to picture a horse, for example, they really do see a horse clearly in their mind's eye. You may see details of the horse you are picturing, its long, flowing mane, or its horrendous white sweat quite clearly. For around 1-4 percent of people worldwide, or around 82-380 million people on Earth, that is not the case.

"Aphantasia is the inability to voluntarily visualize mental images – a neurological variation that affects how people think, dream, remember, and learn. When someone with aphantasia tries to visualize an apple, a loved one's face, or a childhood memory, they experience no mental pictures," the Aphantasia Network explains.

"Instead of mental pictures, your mind works with facts, concepts, and knowledge. You know what a horse looks like – four legs, mane, tail – you just can't see one in your head."