Dreams can be fascinating, compelling, inspiring, or scary.
We attach such characters to our dreams because we can experience and feel our dreams through our visual and auditory senses. I'm sure nightmares will be less scary if we can't see nor hear the content.
We perceive the events that happen in our dreams through our sight and hearing, and that makes perfect sense because we rely on these two senses far more than all other senses.
Then how do people with visual impairment dream?
Well, just as we rely on our two primary senses, the blind rely on their other senses (for instance the sense of smell) in their dream.
A team of Danish scientists recruited 50 adults - 11 blind from birth, 14 who became blind sometime after age 1, and 25 non-blind controls - to investigate how the blind dream.
None of the blind-from-birth participant experienced a visual dream. About 26% of them tasted, 40% smelled, 67% touched and 93% heard in at least one dream.
As for the non-blind group, all of them reported at least one visual dream. Only 7% tasted, 15% smelled, 45% touched, and 64% heard in their dream - so the blind perceive their dream just like how they perceive the real world, and it proves the importance of the sense of hearing to the blind.
And the researchers also discovered one major difference between the blind and the non-blind: the blind had a lot more nightmares - 25% compared to only 6% of non-blind. The reason for this discrepancy is unknown, though the scientists suggested that it may have to do with evolutionary theories on the existence of nightmares.
“.... nightmares can be seen as threat simulations, as a mentally harmless way by which the human mind can adapt to the threats of life..... nightmare gives an individual an opportunity to rehearse the threat perception and the avoidance of coping with the threat.” -- The sensory construction of dreams and nightmare frequency in congenitally blind and late blind participants, Amani Meaidi;
Ron Kupers, Sleep Medicine (2014).
Go ahead and read the full paper here.