Brain activity reveals why some people tend to hear voices easier than others

What makes some people more vulnerable to hallucinations than others? A first response has now been given by an experiment in the brain scanner. It reveals that in people who often hear non-existent voices, the cerebellum is less active. This, however, acts as guardian against false perceptions. If this examination is weakened, excessive expectations can lead to hallucinations, as the researchers in the specialist magazine "Science" report.

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They let us see ghostly phenomena, let us hear voices, or even smell scents that are not really there: in a hallucination we perceive things that exist only in our head. This is made possible by the fact that our brain does not simply represent stimuli to nature. Instead, it interprets them and balances them with our expectations, previous experiences and our knowledge. Only then does the perception reach our consciousness.

In the case of hallucination, this chain of the processing step becomes independent - it runs off without a stimulus signal having impelled it. This is often the case with people with psychoses or high fever, but can also be provoked by healthy people, for example, by longer persistent irritation.

Test with chess board and sound

But why do some people tend more to hallucinations than others? What's different in their brain? To find this out, Albert Powers from Yale University and his colleagues have invited four different groups of volunteers to an experiment: healthy and psychotic people who regularly hear voices, as well as healthy and psychotic people who have never had any hallucinations.

All the participants had to look at a screen, where a short chessboard flashed. At the same time a one second long sound - but not always: At first the chessboard was always accompanied by the sound, later the sound was sometimes quieter and sometimes not present at all. Whenever the subjects thought they heard the sound, they should press a button - the longer the more certain they were. During the experiment, the researchers recorded the brain activity of the subjects by means of functional magnetic resonance imaging (fMRI).

Tricked brain

The result: The initially constant combination of chessboard and sound produced so-called conditioned hallucinations in almost all subjects: after a while, they thought they could hear a sound even if no one sounded. The reason: Because initially both stimuli always occur together, the brain learns this and therefore expects this combination.

One takes then what was expected and not what our senses actually tell us, explains Powers.

In other words, the processing chain in the brain combines the visual stimulus with the expectation of the acoustic. Because the chessboard is supposed to be a sound, our brain is complementary to it, even if it does not happen.

Five times more frequently in participants with hallucination background

The exciting thing: not all subjects were equally susceptible to this conditioned hallucination. These acoustic hallucinations occurred five times more frequently in the participants who had previously and regularly heard voices before. They were also 28 percent more confident that the sounds were really there.

The healthy volunteers with no hallucination background, however, noticed in the second half of the experiment that the acoustic signal was increasingly missing. They seldom pressed the "Yes" button, and they were also more unsure about whether they had heard a sound or not.

Expectations prevail

This could explain why some people are more susceptible to hallucinations: Normally, our brain is capable of changing once-placed expectations. It constantly checks them by means of the actual sensory experience. If expectations and stimuli no longer match, it adapts to their expectations accordingly.

Not so in people with psychoses or healthy people who tend to hallucinations: Here the review of expectations works worse. Their brain usually evaluates the internally stored expectations more strongly than the sensory stimuli from the outside. This imbalance between expectation and sensory stimulus can then produce the hallucinations, according to Powers.

Cerebellum as hallucinatory guardian

The researchers also found evidence in the brain scans of this connection of overstated expectations and hallucinations. The more frequent and stable the subjects had the conditioned hallucinations, the less active was their cerebellum. This, however, plays an important role in the planning and coordination of movements and therefore must constantly reconcile its information with the sensory stimuli from outside, as the researchers explain.

However, in humans with psychoses and people who are prone to hallucinations, this comparison is inhibited and their cerebellum is therefore less active.

This suggests that the cerebellum is one of the key guard posts against such flawed perceptions, Powers says.

A further cerebral area reacted differently to the voices: the hippo-campus. It usually resembles sensory stimuli with memories and experiences. This brain area plays a role in the review of pre-eminence, as the researchers explain. In the experiment, the more uncertain the subjects were, the higher the activity in the hippo-campus.

Help with early detection and therapy

Thus this experiment provides valuable insights into the mechanisms that cause hallucinations - which make some people particularly prone to this phenomenon.

As Powers and his colleagues explain, these findings could one day help identify vulnerable individuals earlier.

At the same time, the knowledge about the brain regions involved could perhaps even lead to the development of targeted therapies for voice-listening.

Source: (Science, 2017; doi: 10.1126 / science.aan3458)