According to the latest research, auditory hallucinations are dependent on expectations.
A number of psychiatric conditions, such as schizophrenia, involve hallucinations. Often, these hallucinations are auditory - that is, in the form of voices - and they can be incredibly disturbing for the affected individual. However, it is not only people with a psychotic illness that hear voices; in clairvoyants, for instance, they are welcomed.
Although the exact mechanisms behind hallucinations are not understood, there are theories that attempt to explain them. Some psychologists believe that expectation plays a large part: when a strong relationship exists between a particular sensory input and an experience, if the sensory input occurs without the experience, it can trigger a perception of the experience.
As researcher Albert Powers, a clinical instructor in psychiatry, explains, "Hallucinations may arise from an imbalance between our expectations about the environment and the information we get from our senses. You may perceive what you expect, not what your senses are telling you."
A team of scientists led by Yale University in New Haven, CT, set out to investigate this theory in more detail. They wanted to get a better understanding of auditory hallucinations and tease apart the factors that make them worrying for some and a welcome experience for others.
For instance, in earlier studies, the Yale team found that self-described voice-hearing psychics experienced similar auditory hallucinations to people with schizophrenia. The crucial difference is how these hallucinations are experienced: the psychics tended to perceive them as a positive experience.
The researchers ran the current experiment on four groups: voice-hearers (psychotic and non-psychotic) and non-voice-hearers (psychotic and non-psychotic). The non-psychotic voice-hearing group included clairaudients, which are people who claim to be able to hear voices from the spirit world.
Their findings are published this week in the journal Science.
For the study, the researchers used a procedure designed at Yale in the 1890s. Participants are presented with a visual stimulus (a checkerboard) at the same time as an auditory tone. To bring the procedure into the modern world, the participants underwent a brain scan at the same time. The tone varied in volume and sometimes it was not played at all.
The participants were asked to report when they heard the tone by pressing a button. And, by holding down the button for a longer period of time, they could signify how confident they were that they had heard the sound.
Over time, due to expectation, the individuals would report hearing a tone when they saw the checkerboard, even if the tone was not presented.
Individuals from all four groups succumbed to the hallucinations, but the effect was much more pronounced in the voice-hearing groups. Additionally, they were likely to be more confident that they had heard the tone when it was not there.
Toward the end of the trial, the researchers presented the participants with a higher percentage of no-tone trials. They found that patients with a psychotic illness, whether a voice-hearer or not, were much less likely to notice this change. Conversely, individuals without a psychotic illness were much quicker to update their expectations and beliefs about the association.
Brain scans during hallucinations
During the trials, particular attention was paid to an area of the brain known to produce auditory hallucinations when artificially stimulated. It was shown that when the individuals reported hearing the tone when it was not played, this part of the brain lit up, confirming it to be the genuine perception of something that was not there: a hallucination.
The researchers also used computational modeling of the participants' behavior, which showed that the voice-hearers strongly believed that the visual cues were linked to the tones and that these "prior beliefs drove the tone hallucinations."
Computational modeling gave them the ability to differentiate people with a psychosis diagnosis from those without. They found that individuals with psychosis found it difficult to accept that they had not heard the tone, and brain regions associated with psychosis were triggered.
"In both clinical and non-clinical subjects, we see some of the same brain processes at work during conditioned hallucinations as those engaged when voice-hearers report hallucinations in the scanner."
Senior study author Philip Corlett, Yale School of Medicine
The team hopes that the findings might help to design an objective way to discern people who need treatment and those who do not.