Then they measured how much people relied on this prior sensory knowledge when shown the visual stimulus without the sound.Īt first, at least some members of all of the groups heard the sound even when it wasn’t there. They paired a visual stimulus of a checkerboard on a computer screen with a brief 1-kilohertz tone, presenting the light and sound repeatedly until participants learned to associate the two. The latter group included both people who had been diagnosed with psychosis and healthy adults. The team introduced this new information-in the form of a Pavlovian learning task-to the psychics, patients diagnosed with psychosis, and others in a control group who had not heard voices before.
AUDITORY HYPNAGOGIC HALLUCINATIONS SERIES
Next the researchers designed a series of experiments to introduce new beliefs about sensory information. “They sounded the same in terms of how loud the voices they heard were, the frequency of occurrence, where they were hearing them in space-within or outside their heads-and length and complexity of what the voices uttered,” Powers says.
Almost immediately the two noticed that the psychics’ descriptions of hearing voices were remarkably similar to the experiences of their patients diagnosed with psychosis. They vetted individuals using forensic psychiatry techniques to ensure that people were not simply pretending to experience auditory hallucinations. The team visited a local Connecticut organization for psychics and began interviewing people. To test the idea that hallucinations are the result of an over-expectant brain, Powers and fellow Yale University psychologist Philip Corlett decided to study a diverse group of people who reported hearing voices on a regular basis-including those who had been diagnosed with psychosis, along with self-identified psychics who had not been diagnosed with any psychiatric illness. Culture and religion may also play a role in interpreting what individuals perceive, and whether the voices they hear are helpful or disruptive. One theory posits hallucinations arise when the brain relies too strongly on these expectations, filling in details even when an actual auditory input does not exist. For example, we are able to hear the sound of running water or the murmur of a friend talking across the room and then react in an instant, Abi-Dargham says. “It is constantly scanning the environment and relying on previous knowledge to fill in the gaps what we perceive.” Because our expectations are usually accurate, the system generally works well. “The brain is a predictive machine,” explains Anissa Abi-Dargham, a psychiatrist at Stony Brook University School of Medicine, who was not involved in the new work. Normally when the brain receives sensory information, such as sound, it actively works to fill in information to make sense of what it hears-its location, volume and other details. “We wanted to understand what’s common and what’s protecting people who hallucinate but who don’t require psychological intervention,” he says. Even in the absence of these predisposing factors, approximately one in 20 people hear voices or see visual hallucinations at least once in their lifetimes, according to mental health surveys conducted by the World Health Organization. Whereas most researchers have focused on the brain abnormalities that occur in people suffering at an extreme end of this spectrum, Powers and his colleagues have turned their attention to milder cases in a new study.
Drugs, sleep deprivation and migraines can often trigger the illusion of sounds or sights that are not there. Healthy people also experience hallucinations. But increasing evidence over the past two decades suggests hearing imaginary sounds is not always a sign of mental illness. Some patients report hearing voices others hear phantom melodies. Auditory hallucinations are the most common type experienced. The condition is often a hallmark of psychosis, occurring in an estimated 60 to 70 percent of people with schizophrenia, and in a subset of those diagnosed with bipolar disorder, dementia and major depression. At his psychiatric clinic in the Connecticut Mental Health Center, Albert Powers sees people every day who experience hallucinations.
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