When the senses are fooled

In a 1976 experiment, people who heard the syllable "ba" while watching lips say "ga" reported hearing "da".

Sensation and perception

OpenStax draws a careful line between two words. Sensation is the activation of receptor cells by a stimulus. Perception is what the brain makes of those signals: the meaningful pattern you are aware of. Perception depends on sensation, but not every sensation is perceived, and the brain fills in, guesses and corrects. Most of the time its guesses are good. Illusions are the cases where we can catch it guessing, and scientists use them to learn how perception works.

Lines that are the same length

Franz Müller-Lyer described this figure in 1889. The lines are identical, yet the one with fins pointing outward usually looks longer. Psychologists still debate the exact reason. A reference work on cross-cultural psychology points out that researchers have known for more than a century that the strength of the illusion is not the same for everyone; a Cambridge expedition to the Torres Strait tested it on local people at the end of the 19th century. The figure is a reminder that what you see is a judgement, not a ruler.

The hole you never notice

You found your blind spot in the first lesson. Each eye has one, yet you do not walk around seeing two dark holes. The American Academy of Ophthalmology explains that the blind spots of the two eyes do not line up, so each eye covers the other's gap. With one eye closed, the brain fills the gap with the brightness, colour and texture of the surroundings, which researchers call perceptual filling-in. In the test card the dot disappears, but you do not see a white hole: you see the same background as around it. The brain is filling in, not seeing.

Rocks that move

Seeing what you hear

Senses also influence one another. In 1976 Harry McGurk and John MacDonald published a short paper in Nature called "Hearing lips and seeing voices". They had dubbed a recording of the syllable "ba" onto a film of a woman's lips saying "ga". Adults watching the film reported hearing "da", a sound that was in neither the audio nor the video. This McGurk effect shows that speech perception is not purely a matter of hearing: the brain combines what the eyes see with what the ears hear.

You met another everyday example in the lesson on smell and taste. Food seems to lose its taste when your nose is blocked, although the taste buds are working normally. Without the aroma, the brain's flavour picture is missing a large piece, and it reports the whole thing as bland. Perception is the brain's best account of the world, built from every sense it can use.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.