Dreams and psychotic hallucinations do have things in common. They both feature perceptual sensations that seem real, but which are conjured up by our brains.
However, there are also differences. While dreams are known to be highly visual, psychotic hallucinations are primarily auditory. They generally involve hearing things that aren’t real rather than seeing things that don’t exist. And this difference is an important reason why “the idea of dreaming as a model for psychosis has remained speculative and controversial,” write Roar Fosse of the Vestre Viken Hospital Trust, Norway, and Frank Larøi of the Norwegian Centre of Excellence for Mental Disorders Research at the University of Oslo.
However, to date, there’s been very little investigation into perceptions of sounds in dreams, the pair reports. And now they have data suggesting that, in fact, auditory perceptions are common. If this is the case, the links between psychotic experiences and dreams may be stronger than has been recently supposed.
Fosse and Larøi used social networks and a psychology group on Facebook to recruit 13 people who reported having good dream recall, but no psychiatric disorders or any other condition. Neither did they report taking any medication that might affect their sleep. The group consisted of 12 women and one man (with a mean age of 28).
They were asked to keep a dream diary: to write down any dreams they could recall that had taken place just prior to awakening from sleep, and to keep doing this until they each had records for 10 dreams. They were told to record the detail of any dream, whether or not it featured sounds. But at the same time, they were asked to focus on any auditory content, and describe this in detail.
The researchers discovered that sounds featured often, being reported in 80% to 100% of each participant’s dreams. Most often, the sounds consisted of other people speaking. (There were even five instances described as speech in a foreign language that the dreamer did not understand.) But there were also 122 instances of the dreamer speaking and 59 instances of other types of sound, such as glass breaking, gun shots, people walking or running, and a radio playing.
“Considering that dream reports include experiences from only the last minutes of sleep before awakening, and that dreaming normally takes place throughout REM sleep as well as in light NREM sleep, the available evidence suggests that normal, healthy people usually experience internally generated auditory sensations an array of times every night,” the researchers conclude.
There are certainly some limitations to the study. This was a small group of almost all women who self-reported having good dream recall and who volunteered for the study — so may not represent the broader population. Also, rather than being regularly woken and asked what they were dreaming about, they only reported dreams when they woke spontaneously. Earlier work has found that such dreams tend to be more dramatic. So the content that they reported may not be typical of dreams experienced throughout the night.
However, the findings are intriguing, and hint at all kinds of interesting work to come.
There are some parallels between the normal sleep wake cycle and theories about how psychotic hallucinations come about. As someone falls asleep and moves through early sleep stages into REM, there’s a general decrease in reflective, directed thought and a gradual increase in internally-generated perceptions. Theories of psychotic hallucinations, meanwhile, propose that an impairment in higher order, “top-down” regions (typically in the prefrontal cortex) allows for more internally-generated, sensory-like hallucinatory experiences.