Sensation or perception?

Psychologists split what happens when you see a face, hear a song or taste a lemon into two jobs.

• Sensation is the first job. Specialised cells called receptors pick up a form of physical energy (light, sound waves, pressure, chemicals) and turn it into electrical signals the nervous system can use. That conversion is called transduction. In the eye, the receptors are the rods and cones of the retina. There are about 120 million rods and about 6 million cones in each eye.
• Perception is the second job. The brain organises and interprets those signals, so that a pattern of light and dark becomes "my friend, ten metres away, waving".

Sensation alone is a swarm of signals. Perception gives it a meaning.

Perception runs in two directions at once.

• Bottom-up (data-driven) processing starts from the signals. Edges, colours and movements are detected first and combined into bigger things.
• Top-down processing starts from what you already know and expect. Context, memory and goals shape what the signals are taken to mean.

Read this: "I lvoe reading tihs". You probably read it without trouble even though the letters are scrambled. The raw shapes (bottom-up) were ambiguous, but your expectations (top-down) filled in the sentence. Both directions work all the time, and illusions are often what you get when the two disagree.

Illusions are not mistakes in your eyes. They are the rules your brain uses to guess the most likely scene, shown in a case where the guess is wrong. In everyday life these rules work extremely well, which is why you can catch a ball or recognise a face in a second.

Many scientists now describe the brain as constantly making predictions about the world and then checking them against the incoming signals (the framework is called predictive processing). It is an active research idea, not a settled fact, but it fits a lot of what illusions show.