Olfaction and the Brain: Study Reveals How Expectation Creates Real “Olfactory Hallucinations”

In sensory science, the classical model suggests that olfaction is a linear process: chemical molecules stimulate receptors in the nose, which send signals to the brain. However, a study published in Scientific Reports challenges this view, demonstrating that olfactory perception can be generated entirely by the brain without the presence of any physical odorant.

Research conducted by scientists at the University of Helsinki reveals that the belief that a scent is present is sufficient to produce detailed olfactory sensations, a phenomenon explained by the theory of Predictive Processing (PP).

The Experiment: Deceiving the Senses with Virtual Reality

To investigate whether the mind can “hallucinate” smells, researchers designed a controlled behavioral experiment using Virtual Reality (VR). Thirty participants were exposed to an immersive 360-degree Amazonian rainforest environment.

The group was divided into two distinct conditions:

• Control Group: Watched the video knowing it was solely an audiovisual experience.
• Manipulation Group: Was led to believe that the experience included advanced olfactory simulation technology. To reinforce this belief, researchers presented a fake apparatus—an “odor machine” with a diffuser and visible cables—and instructed participants that vapors would be released into the room.

It is crucial to note that, in both cases, no olfactory stimulus was released. The air remained chemically neutral.

Results: Real Scents, Imaginary Origin

The data showed a statistically significant difference between the groups. While the control group reported no smells, participants in the manipulation group described vivid olfactory sensations and rated the realism of the scent much higher than the control group.

The descriptions were not generic. Participants reported specific aromas congruent with the visual scenario of the forest, such as “mud,” “freshness,” “dirty water,” “flowers,” and “wet earth”. This proves that short-term expectation can effectively modulate sensory experience.

The Science Behind It: The Brain as a Prediction Machine

This phenomenon validates Predictive Processing (PP) applied to olfaction. According to this theoretical framework, the brain does not passively process sensory information; it acts as a probabilistic “prediction engine” that generates top-down models of the environment.

The process occurs in three stages:

1. Prediction: Upon seeing the forest and receiving the verbal suggestion of the “odor machine,” the brain anticipates the olfactory stimulus.
2. Controlled Hallucination: In the absence of actual sensory data (molecules in the air), the brain fills the gap using its internal models and past memories to construct perception.
3. Perceptual Confirmation: The individual “really” smells the scent because the olfactory cortex is responding to the internal prediction rather than the external environment.

The study suggests that what we perceive as reality is often a “controlled hallucination” tuned by our expectations.

Future Implications: Rehabilitation and Technology

The discovery that manipulating expectations can induce olfactory sensations has profound implications for medicine and technology:

• Anosmia Treatment and Post-COVID Rehabilitation: The use of VR combined with expectation induction can aid in the rehabilitation of patients who have lost their sense of smell (anosmia or hyposmia), helping the brain reactivate neural pathways through memory and prediction, offering a theoretical path to sensory restoration.
• Understanding Neurological Disorders: The study provides a safe model to investigate olfactory hallucinations (phantosmia) common in psychiatric and neurological disorders, such as Schizophrenia, Parkinson’s, and Lewy Body Dementia, where the brain’s predictive system may be generating perceptual “errors” without external control.
• Technological Immersion: For the development of virtual experiences, the results indicate that sensory fidelity can be achieved through visual and narrative suggestion, without the imperative need for complex hardware to emit chemicals.

This study reaffirms that olfaction is a complex sensory modality, where the boundary between environmental chemistry and mental construction is much thinner than science previously believed.

Source:
Berg A, Henttonen P. Believing in simulated virtual scents produces reported olfactory sensations. Sci Rep. 2025 Nov 24;15(1):41583. doi: 10.1038/s41598-025-25418-1. PMID: 41286141; PMCID: PMC12644582.