The lab

The blind spot

Where the optic nerve leaves each eye there are no light receptors, so each eye has a hole in its field of view about the size of a fist at arm's length. You never notice it. The usual explanation is that the brain "fills in" the gap. Dennett argued for something subtler. The brain does not paint anything into the hole. It simply never registers that anything is missing, so there is no absence to notice. Find your own blind spot below.

Try the striped version. When the dot falls into the blind spot, most people see the band continue unbroken. Is the brain drawing in the missing stripe, or just failing to notice a gap? Dennett's point was that from the inside you cannot tell, and that the "drawing in" picture quietly assumes an inner screen that needs touching up.

Change blindness

In the 1990s Ronald Rensink, Kevin O'Regan and James Clark showed that people can look straight at a picture for many seconds without noticing a large change, provided each change is masked by a brief blank, like a blink. Find the change below and time yourself.

Change blindness bears directly on the argument between Dennett and Ned Block. If we see every detail of a scene, why can't we spot a large change? Dennett took it as evidence that the richness of visual experience is partly an illusion. We have the gist and a sense that detail is available, and we take that sense for a detailed picture.

Libet's clock

In 1983 Benjamin Libet asked volunteers to flex a wrist whenever they felt like it, and to note the position of a fast-moving spot on a clock face at the moment they first felt the urge. Electrodes on the scalp recorded the "readiness potential", a slow build-up of brain activity before movement. On average it began several hundred milliseconds before the moment people reported deciding. Try the clock.

Many people concluded that the brain decides before "we" do, and that free will is an illusion. Dennett disagreed in Consciousness Explained and Freedom Evolves (2003). The experiment assumes there is a precise moment when a decision enters consciousness, to be timed against a clock seen in the same inner theatre. But your judgement of where the spot was is itself a product of the brain's drafting, and can be revised. Looking for an exact instant of conscious will is looking for the Cartesian Theatre again. Later work also suggests that the readiness potential may reflect background fluctuations in brain activity rather than a decision already made.

Every figure on the site

• Cups and ballsDennett The audience's view against the magician's method.
• The Cartesian Theatre and multiple draftsDennett One screen, or many competing drafts?
• Colour phiDennett Kolers and von Grünau's moving, colour-changing dot, with the Orwellian and Stalinesque stories.
• And then what happens?Dennett Follow a red tomato through the brain.
• The flag after-imageDennett A red stripe that is nowhere at all.
• Three stancesDennett Physical, design and intentional.
• The Müller-Lyer illusionFrankish An illusion that survives knowing the truth.
• The impossible triangleHumphrey Gregory's sculpture and the ipsundrum.
• Attention and its schemaGraziano The process, and the brain's simplified model of it.
• Video feedbackHofstadter A camera pointed at its own screen.
• The simulated selfJoscha Bach Neurons, the simulated world, and the person inside it.
• Colour spaceThe Churchlands Colours as points in a space of cone activity.
• The axeAristotle Matter, form and function.
• The reflex and the pineal glandDescartes A machine, plus a soul.
• The millLeibniz Walk inside a thinking machine.
• The bundleHume Look for the self.
• The steam whistleHuxley Epiphenomenalism, and Dennett's question.
• Where is the University?Ryle The category mistake.
• The imitation gameTuring Which one is the machine?
• LightningPlace and Smart One event, two descriptions.
• Multiple realisabilityPutnam One role, three kinds of hardware.
• EcholocationNagel What physics records, and what it leaves out.
• The Chinese roomSearle Outside, inside, and the systems reply.
• Mary's roomJackson Four replies, including Dennett's blue banana.
• The nation of ChinaBlock Neurons, or people with radios.
• A two-tone faceSeth Prediction changes what you see.
• Zombie twinsChalmers The inner light, and Dennett's zimboes.
• The combination problemGoff Do little minds make a big one?
• Popperian creaturesPopper Let the hypothesis die in your place.
• The jump to universalityDeutsch Four machines, or one universal computer.