The Big Idea: how do our brains know what's real?

The Guardian

10-02-2025

When did you last hallucinate? 'The visionary tendency is much more common among sane people than is generally suspected,' wrote the 19th-century psychologist Sir Francis Galton. Setting aside the vivid, often emotive, cinema of our dreams, we are all more vulnerable to 'seeing things' than we might at first suppose.
Around four fifths of people who have recently been bereaved report an encounter with their loved one: most commonly a lively sense of their presence, but some hear, see or speak with them. Up to 60% of people who lose sight in later life see things that aren't there, sometimes extravagant images such as the 'two young men … wearing magnificent cloaks … their hats … trimmed with silver' who appeared in the first reported case of Charles Bonnet syndrome, as this phenomenon is known, before 'dissolving' away. A 20-year-old woman blindfolded for 12 hours saw 'cities, skies, kaleidoscopes, lions and sunsets so bright she could 'barely look at them''. After losing a limb, most people carry a 'constant or inconstant phantom of the missing member', as Weir Mitchell, the American neurologist who coined the term phantom limb after studying 90 cases from the American civil war, put it. Pilots on long flights, travellers through snowstorms and deserts, prisoners and hostages held in darkness; their restless brains are all prone to see the things of which they're being deprived.
These examples relate to vision, our dominant sense modality, but other senses can also generate compelling hallucinations: around one in 10 of us will at some point hear a voice that seems to be coming from outside but proves to be self-generated. For one in 100 – excluding people who go to see psychiatrists because of their voices – this happens regularly.
How do our minds and brains conspire to create these deceptive experiences? Studies of deliberate imagery – where subjects are told to 'visualise an apple in your mind's eye' – and of hallucination show that they are cut from the same cloth: both involve activity in sensory regions of the brain. This can be strikingly similar to the activity that occurs when we perceive the real world around us. Such similarities, between brain activity during imagination and while actually perceiving, have a profound implication: that perception itself is a kind of imaginative act.
The notion is an ancient one, but has been given a new lease of life by the idea, from psychology, that prediction is integral to perception, and by evidence from neuroscience that our experience depends absolutely on the work of our sugar- and oxygen-hungry brains. In other words, perception is far more dependent on prior knowledge – painstakingly created internal models of the world – than we usually take it to be. The contemporary expert Anil Seth puts it nicely: 'We tend to think of perception as occurring outside-in, but it mostly occurs inside-out.' One hundred and fifty years earlier, the French historian and psychologist Hippolyte Taine wrote presciently in the same vein: 'External perception is an internal dream which proves to be in harmony with external things; and instead of calling hallucination a false external perception, we should call external perception true hallucination.'
If perception is a kind of true hallucination, a potential problem looms: how can we distinguish what we imagine from what we perceive? The examples above show that we don't always succeed; we can mistake our imaginings for reality, usually transiently, but sometimes, in psychosis for example, more persistently. The opposite also occurs: in the 'Perky effect', people fail to detect that objects they are imagining are being shown to them for real. Mostly, though, we get things right. Some rules of thumb are helpful – high levels of vividness and detail, effortlessness and consistency with context suggest that we're looking at the real world – but not always. Daydreams can be effortless and vivid; hunting for a destination in thick fog can be effortful and the resulting experience indistinct. Somehow, though, the brain weighs up the odds, and generally gets the right answer.
How does it achieve this? Research in AI provides some interesting clues. In 'generative adversarial' models, two elements combine to learn about some aspect of the world: the 'generative' bit aims to predict it as precisely as possible; the 'adversary' does its best to decide whether what it is looking at is the real world or the output of the generative model. The generative model constantly ups its game to masquerade as the real McCoy; the adversary keeps honing its connoisseurship to distinguish the authentic from the fake. Something similar happens in the brain. The 'adversary' in the human brain, charged with reality checking, keeps watch from our huge frontal lobes: Area 10, in particular, at the tip of the frontal cortex, becomes active in tasks requiring us to decide whether items were seen or imagined. It is smaller and less active in people with psychosis than in healthy people, especially so in people with psychosis who hallucinate.
It is fascinating that the apex of our highly evolved frontal lobes should be charged with the vital 'metacognitive' task of distinguishing the imagined from the real. But that doesn't mean those same imaginings can't be rich sources of insight into the world around us. The chemist Friedrich Kekulé famously described the circular structure of the benzene molecule after dreaming of snakes, one of which 'seized its own tail … As if by a flash of lightning I awoke … I spent the rest of the night working out the consequences of the hypothesis.' A dream assisted Dmitri Mendeleyev in the formulation of the periodic table. Einstein imagined how the universe would appear to someone travelling on a beam of light.
The author Malcolm Bradbury said: 'All writers hear voices. You wake up in the morning with the voices … and try and trap them before they run away.' Overlapping processes in our dynamic brains enable us to perceive, to imagine and to fashion anew. We need to know which is which, but as Kekulé advised: 'Let us learn to dream, gentlemen, and then perhaps we shall learn the truth.'
Adam Zeman is the author of The Shape of Things Unseen: A New Science of Imagination (Bloomsbury Circus).
Sign up to Inside Saturday
The only way to get a look behind the scenes of the Saturday magazine. Sign up to get the inside story from our top writers as well as all the must-read articles and columns, delivered to your inbox every weekend.
after newsletter promotion
Being You: A New Science of Consciousness by Anil Seth (Faber, £12.99)
The Experience Machine: How Our Minds Predict and Shape Reality by Andy Clark (Penguin, £10.99)Your Brain on Art: How the Arts Transform Us by Susan Magsamen & Ivy Ross (Canongate, £10.99)