A hallucination is a sensation that does not come from any external stimulus . Therefore, why do they occur? When we think about the perception of the senses, we do so from the idea that it is a passive process by which a stimulus arrives and is interpreted by our brain. But it is more complex than that.
In fact, perception has more to do with preconceptions or assumptions of reality. That is, the way we perceive an event is the result of the combination of an external stimulus, of course, and our pre-established beliefs.
Some scientists believe that hallucinations occur when an imbalance occurs between the external stimulus and pre-established beliefs , that is, when this internal fact check fails. To verify this hypothesis, the scientists carried out an investigation: they created a game that caused auditory hallucinations, and they monitored the effect on the brain and behavior.
A game that causes auditory hallucinations
The experiment puts a twist on classic Pavlovian conditioning , whereby a group of dogs learned to associate food with a certain sound. Instead, the scientists trained the experiment subjects to associate a vision with a certain sound. In the game, a chessboard is projected at the same time as a tone , 1000 Hz in frequency.
As the game progressed, the intensity of the tone got lower and lower, until it died away. After enough repetitions, the subjects began to imagine hearing the sound as the board appeared in front of them, even when the sound was not emitted. Despite the absence of sound, the participants continued to “hear” the tone: they were experiencing a hallucination.
A similar auditory hallucination occurs in everyday life: when you think you are hearing your phone ring or buzz, only to find that it is off. “People come to expect sound so much that the brain hears it,” says Albert Powers, a psychiatrist at Yale University and author of the new study.
In addition, to test the effect of pre-established beliefs about hallucinations, the participants had to press a button every time they thought they heard the tone , whether it was sounding or not, and were even asked to press with greater intensity the more confident they were to hear the tone. sound.
During the experiment, there were four groups: a control group, with no history of psychosis or hallucinations. One second, just with a history of psychosis; a third, from hallucinations; and the last, with a combination of both.
Despite the psychiatric history of the participants, all of them experienced conditioned hallucinations during the game. Of course, people with a history of hallucinations were more susceptible to the effect of the experiment, and they were the safest to press the button even when no sound was emitted.
A scan connected to the participants’ brains showed which areas of the brain were activated during hallucinations . Thus, the study has allowed researchers to create a model of the influence that primary beliefs have during the experience of a hallucination.
Hallucinations are strongly linked to pre-established beliefs, and participants with psychosis found it most difficult to change those beliefs, which corresponded to intense activity in the hippocampus and cerebellum , which control memory and our motor system. On the other hand, people without a history of psychosis, although at the beginning of the game they were strongly convinced that the sounds were emitted at the same time as the chessboard, progressively changed their beliefs: they showed more flexibility to modify pre-established thoughts.
The hope of the researchers is to be able to diagnose the extent to which hallucinations lead to psychosis, and to determine effective treatments in the future.
