Neither, they imagine that they see something and lack the language to convey the imagined unperceived object, the notion of phantasia or aphantasia is merely a by-product of the term "picture it in your mind's eye" or the like, leading many ignorant people to believe that something is literally rendered within the individual's head.

Realising this is not the case is waking up, but then some attention seekers gave it the name aphantasia to make themselves feel special.