All of us are prone to hallucinations. Sometimes we'll think we hear or see things that nobody else does when we're tired or anxious, or we might smell or taste things that aren't actually there. But up until now, little has been known about the mechanisms that give rise to these experiences.
While research on auditory hallucinations dominate studies of psychosis, according to researchers of a new paper published Monday in the journal Proceedings of the National Academy of Sciences, little work has focused on how our brain produces visual hallucinations.
"The normal brain doesn't really perceive anything as it really is, but adds things [interpretation] based on prior experiences and knowledge," Paul Fletcher, senior study author and psychologist at Cambridge University, told me.
The researchers of the study aimed to find out both how widespread hallucinations were, and figure out if certain people were more prone than others to superimposing their prior experiences and knowledge of the world onto their immediate surroundings.
"So we hypothesized that perhaps what's happening with hallucinations is that this phenomenon is being exaggerated—people are applying too much of what they expect rather than processing the world as it is," Fletcher said.
For their experiment, the researchers worked with 18 people suffering from the early signs of psychosis, and 16 volunteers with no mental health issues. They showed both groups incomplete black and white images, and asked them to make sense of the ambiguous pictures. They then showed both groups a more detailed color picture which the black and white images were based on, before presenting them with the black and white photographs again.
The researchers found that people with or prone to hallucinations were able to read more meaning into the black and white imagery, and perform the task better, after they had seen the color image, than the other group.
"This suggests that under some circumstances, people prone to hearing voices or seeing things might actually be applying their prior experiences more strongly," explained Fletcher.
For the second part of their experiment, the researchers involved a larger group of 40 healthy volunteers, and presented them with the same black and white and colored pictures.
"We looked at whether certain markers for how prone they were to hallucinations would predict how good they would be at the task," said Fletcher. "We found that healthy people who seemed to show some proneness to hallucinations are actually better at the task than people who aren't prone to [hallucinations]."
According to Fletcher, we are all susceptible to thinking we hear a voice or seeing something that's not actually there. This only become problematic, he said, when these experiences are consistent and make the person feel trapped.
"A lot of people in the population will have experienced voice hearing, or seeing something that nobody else sees, but nobody really has a handle on the mechanisms that causes them, and that's really what we're trying to get at here," Fletcher said.
Fletcher admitted that this current research wouldn't help people suffering from psychosis directly. However, he said that understanding the mechanisms that give rise to hallucinations would put them in a better position to think about how they could intervene—whether that be through psychological or medical treatment.
"Ultimately there's a possibility that we can use measures like this to identify people who are at risk of becoming mentally ill," said Fletcher.