Seeing isn’t believing (Image: Brian Stauffer

Can you tell a snake from a pretzel? Some can’t – and their experiences are revealing how the brain builds up a coherent picture of the world

AFTER her minor stroke, BP started to feel as if her eyes were playing tricks on her. TV shows became confusing: in one film, she was surprised to see a character reel as if punched by an invisible man. Sometimes BP would miss seeing things that were right before her eyes, causing her to bump into furniture or people.

BP’s stroke had damaged a key part of her visual system, giving rise to a rare disorder called simultanagnosia. This meant that she often saw just one object at a time. When looking at her place setting on the dinner table, for example, BP might see just a spoon, with everything else a blur (Brain, vol 114, p 1523).

BP’s problems are just one example of a group of disorders known collectively as visual agnosias, usually caused by some kind of brain damage. Another form results in people having trouble recognising and naming objects, as experienced by the agnosic immortalised in the title of Oliver Sacks’s 1985 best-seller The Man Who Mistook His Wife for a Hat.

Agnosias have become particularly interesting to neuroscientists in the past decade or so, as advances in brain scanning techniques have allowed them to close in on what’s going on in the brain. This gives researchers a unique opportunity to work out how the brain normally makes sense of the world. “Humans are naturally so good at this, it’s difficult to …