The Brain’s Grand Illusion: How We All Hallucinate Reality

What if reality is not something we see, but something our brain creates? Modern neuroscience suggests that our perception of the world is not an exact reflection of external reality. Instead, it is an internal construction, shaped by our senses, expectations, memories, and emotions. In a very real sense, everyone hallucinates their own version of reality and we only call it “reality” because we agree on parts of the hallucination.

The Predictive Brain

According to neuroscientist Anil Seth, the brain operates as a prediction machine. It constantly generates models of the world, predicting what we will see, hear, and feel next. Our senses then send feedback to correct these predictions when they are wrong. This process is called predictive processing, and it means that perception is not a passive receipt of information, but an active inference about what is most likely happening.

Seth explains that perception is “a controlled hallucination.” When the brain’s predictions match incoming sensory data, we experience a stable reality. But when those predictions are off, we experience illusions or in extreme cases, hallucinations.

Perception as a Personal Filter

Each person’s predictions are shaped by their unique history, beliefs, and emotional state. This means no two people perceive the same event exactly the same way. For example, studies have shown that anxiety can make people interpret neutral facial expressions as threatening, while happiness can make them see the world in brighter colors. The brain literally alters sensory input to match the emotional tone of the moment.

This also explains cultural and individual differences in perception. Language, for instance, shapes how we categorize colors and emotions. Memory, too, plays a role. Our past experiences provide a framework that influences how we interpret current sensory input.

Hallucination and Reality on a Spectrum

Neuroscientist Karl Friston describes perception and hallucination as points on the same continuum. Both involve the brain’s predictions, but in hallucinations, those predictions are not corrected by sensory input. Under normal conditions, the brain balances prediction and evidence. Under certain conditions such as in dreams, sensory deprivation, or with psychedelic use the brain’s predictive influence increases, allowing internal imagery to dominate.

Psychedelics and the Perceptual Model

Psychedelics like psilocybin and LSD provide a vivid example of how our sense of reality can shift. Research using brain imaging has shown that psychedelics decrease the activity of the default mode network (DMN), a system involved in maintaining our sense of self and internal narrative. With the DMN’s control reduced, sensory and emotional areas of the brain communicate more freely, blending perception, imagination, and meaning into a unified experience.

This state highlights how flexible perception really is. It reveals that what we call “normal” consciousness is simply one possible interpretation of sensory data, held together by prediction and agreement.

The Beauty of Our Shared Hallucination

Understanding perception as a controlled hallucination can be both humbling and liberating. It shows that reality is not fixed or fully objective, but a co-creation between the brain and the world. Each of us contributes to the shared illusion we call life, shaped by our biology, culture, and consciousness.

By realizing this, we can begin to question the rigid assumptions we hold about the world and ourselves. In doing so, we move closer to a more fluid, compassionate understanding of reality one that honors the mystery of perception itself.

Sources:

• Seth, A. K. (2021). Being You: A New Science of Consciousness. Dutton.

• Friston, K. (2010). “The free-energy principle: a unified brain theory?” Nature Reviews Neuroscience, 11(2), 127–138.

• Carhart-Harris, R. L., & Friston, K. J. (2019). “REBUS and the anarchic brain: Toward a unified model of the brain action of psychedelics.” Pharmacological Reviews, 71(3), 316–344.

• Barrett, L. F., & Bar, M. (2009). “See it with feeling: Affective predictions during object perception.” Philosophical Transactions of the Royal Society B, 364(1521), 1325–1334.

• Hohwy, J. (2013). The Predictive Mind. Oxford University Press.