Hallucinations: how the brain tries to thwart them

Do you think you perceive the world around you as it is? Think again, because your senses are not infallible. And, even if they were, the brain, too, is far from trustworthy. The ground is favorable to the development of hallucinations.

But we are able to tell the difference between illusion and reality, confirms a study published this Friday in the journal Science. The brain records events, and analyzes and questions its own expectations when a perception can be misleading. He can be fooled, but also knows how to react.

All concerned

Who has never been convinced that their phone was ringing or, more frequently still, vibrating, before realizing that, in the end, no call or message had been received? This is a hallucination, and we are all subject to it. An experiment carried out in the 19th century at Yale University (United States) had already shown that hallucinations occur when the brain places more trust in its expectations than in the sensory stimuli it receives, such as visual or auditory signals. .

It consisted in viewing an image repeatedly, each time associated with a sound. When the sound is removed, the subjects of the experiment continue to “hear” it. It is in a way the conditioned reflex experience of Pavlov’s dog: by accustoming the animal to hearing a sound when feeding, the salivation reflex occurs even without the presence of food. The stimulus is no longer there, but the dog still expects to receive his food.

Sorting through ideas

Here, the researchers replicated the Yale experiment, incorporating four types of participants. They combined people with or without hearing voices in their lives, with whether or not they might have psychosis. They placed them in front of images, with a specific sound. When they thought they heard the sound, they were asked to press a button; the more sure they had heard the sound, the longer they had to press.

And the hypothesis that the researchers had made was verified: people who had heard voices before showed much more confidence when they thought they heard the sound when it was not playing, and therefore they were victims of a hallucination. Healthy people who had never heard voices had a tendency to spot images without sound, much more quickly. Suggesting that their brain was sorting better between its expectations, its beliefs about the world around it, and reality.

Schizophrenics are less skeptical

Schizophrenics (in the group of psychotics who have heard voices before) were five times more likely to say they heard a sound when there was none. And their certainty in these cases was 28% higher than that of other hallucination victims.

Using imaging, the researchers were also able to observe participants’ brain activity. In people who have heard voices, and especially in schizophrenics, they have recorded abnormal neuronal activity in different regions of the brain, especially those related to the formation of representations of reality.

The stronger the hallucinations, the less activity of the cerebellum. Knowing that this area plays an important role in anticipation and motor coordination by receiving continuous perceptual information, this lack of activity then seems to translate a neglect of the senses in favor of experience.