In a new study published in Psychological Medicine, researchers have uncovered intriguing insights into how our brains can sometimes trick us into hearing voices that aren’t there. The research team demonstrated that a robotically-assisted experimental procedure is able to induce auditory-verbal hallucinations in healthy individuals.
Auditory-verbal hallucinations, the experience of hearing voices or sounds that are not actually present, have long fascinated scientists and puzzled those who suffer from them. These hallucinations are most commonly associated with schizophrenia, but they can also occur in other psychiatric disorders and even in people without any known mental health issues.
Previous studies have linked auditory-verbal hallucinations to disruptions in the brain’s self-monitoring processes, where the brain fails to recognize its own actions or thoughts as self-generated. This misattribution can lead individuals to perceive internally generated mental events as external, resulting in the experience of hearing voices.
“Hallucinations are complex experiences that can have a big negative impact on people who experience them,” explained study author Pavo Orepic (@OrepicPavo), a postdoctoral researcher at the University of Geneva, who conducted the research along with his colleagues at the Laboratory of Cognitive Neuroscience at the Swiss Federal Institute of Technology Lausanne.
“Studying hallucinations and the brain mechanisms behind them is challenging due to major methodological limitations, such as the inability to induce them in real-time, repeatedly, and only for a short duration in a laboratory. Moreover, most of the hallucination research has been conducted on patients, which makes it difficult to separate the effects of hallucinations from those of other clinical conditions. This work specifically focuses on ‘hearing voices’ which is the most common and highly distressing psychotic symptom.”
The study involved 48 participants, all right-handed and fluent in French, who were unaware of the study’s purpose. These participants were chosen from the general population, and none had a history of psychiatric or neurological disorders, ensuring a diverse and healthy sample for the research.
To stimulate auditory hallucinations in the participants, Orepic and his colleagues used a clever approach. They recorded the participants’ voices while saying nine one-syllable French words with negative connotations. These words were chosen based on previous research that indicated auditory hallucinations often involve negative content.
The recorded voices were then processed to standardize their sound intensity and duration, ensuring consistency across stimuli. This standardized set of recordings served as the self-voice stimuli in a voice detection task, a critical component of the study.
In addition to participants’ self-voices, the researchers also created equivalent voice recordings of a person unknown to the participants, ensuring that both self-voice and other-voice stimuli were available for the study.
The research team designed a comprehensive experimental procedure to explore the induction of auditory hallucinations in healthy participants. The procedure consisted of several blocks, each serving a distinct purpose.
Sensorimotor Blocks (Assessment of Illusory Effects): In this phase, participants were blindfolded and placed between two parts of a robotic system. They were asked to perform repeated poking movements with their right index finger using the front part of the robot. These movements were replicated by the back part of the robot, resulting in tactile sensations on the participants’ backs.
The key here was that the researchers manipulated the timing of these tactile sensations. In some cases, the touch on the back was synchronous with the participants’ finger movements, while in others, there was a 500-millisecond delay, creating a sensory conflict.
After each session of sensorimotor stimulation, participants filled out a questionnaire to rate the strength of various perceptual experiences, including illusory self-touch, somatic passivity (feeling as if someone else was touching their back), and the presence of hallucinations (feeling as if someone was standing close to them).
Staircase Blocks (Defining Hearing Thresholds): In this phase, the researchers aimed to determine each participant’s individual hearing threshold. They employed a voice detection task combined with a one-up-one-down staircase procedure. Participants continuously heard bursts of pink noise and were instructed to report whether they heard a voice in the noise.
The threshold for hearing voices was calculated based on the participants’ responses. Importantly, this procedure ensured that the voices were presented at a level just at or near each participant’s hearing threshold.
Task Blocks (Combining Voice Detection with Sensorimotor Stimulation): The heart of the study lay in these task blocks. Participants performed the voice detection task while being exposed to sensorimotor stimulation. The key variables were the type of sensorimotor stimulation (synchronous or asynchronous) and the type of vocal stimuli (self-voice or other-voice).
Each participant completed four Task blocks, with the order of these blocks being carefully counterbalanced. The blocks started with 30 seconds of sensorimotor stimulation, followed by the voice detection task. Critically, auditory stimuli were not synchronized with participants’ movements but were presented concurrently with their robotic interactions.
The primary outcome measure was the rate of vocal false alarms, which represent false reports of hearing voices in trials where there were no actual voices present.
The researchers conducted statistical analyses to understand the factors influencing false alarms. They found that the timing of sensorimotor stimulation played a crucial role. When the touch on the back was synchronous with their finger movements, participants reported significantly more false alarms, indicating the perception of hearing voices that weren’t there.
Moreover, these false alarms were more likely to occur when participants heard their self-voices rather than other-voices. This suggests a fascinating link between sensory feedback from their own actions and the creation of auditory hallucinations.
“We developed a robotic procedure able to make healthy participants hear voices when they were in fact hearing only noise,” Orepic told PsyPost. “This was done by taking insights from the two most prominent theories about why voices might occur in the brain.”
The researchers also found that individuals with higher levels of delusional ideation were more likely to experience auditory hallucinations, further supporting the idea that cognitive factors contribute to these phenomena.
“We observed that people who are more prone to delusional thinking were more likely to report lab-induced voices,” Orepic said. “This supports the theory that experiences like hallucinations and delusions are present across the phenomenological spectrum even in the general population.”
While the study sheds light on the fascinating world of auditory hallucinations, it’s essential to acknowledge its limitations. One limitation is that the study primarily focused on vocal false alarms as the primary outcome measure. Future research could explore other aspects of auditory-verbal hallucinations, such as the emotional content of the voices and their perceived reality.
Additionally, the study used a relatively small sample size, and the participants were drawn from the general population. Future studies could benefit from larger and more diverse samples, including individuals with diagnosed psychiatric conditions.
“Our lab-induced voices are still very far from the real clinical experience of hearing voices, which is characterized by a very heterogeneous phenomenology, i.e., the voices people hear differ in many experiential dimensions – the number of voices, gender, voice content, location, emotional valence, etc,” Orepic told PsyPost. “Future work should try to address different phenomenological aspects of the experience.”
“Our goal is to improve our understanding of hearing voices so that we could find better ways to help people who suffer from them,” Orepic added. “Simulating hearing voices in the lab, this work might have wide-ranging implications for the understanding of this detrimental psychotic symptom, whose etiology remains highly unknown.”
The study, “Robotically-induced auditory-verbal hallucinations: combining self-monitoring and strong perceptual priors“, was authored by Pavo Orepic, Fosco Bernasconi, Melissa Faggella, Nathan Faivre, and Olaf Blanke.
