Now you see it, now you... oh...

You may have heard of blindsight, where the eye functions perfectly, the brain processes the signals... but doesn't deliver them to the conscious mind. In such a state, people can avoid obstacles whilst being unable to see them. I've often wondered how much this goes on normally, i.e. when we start imagining things we don't start walking into walls. Turns out there's an even more subtle aspect that bridges the gap between full blindsight and normal consciousness.

A magician sat at a table in front of a group of schoolchildren and threw a ball up in the air a few times. Before the final throw, his hand secretly went under the table, letting the ball fall onto his lap, after which he proceeded to throw an imaginary ball up in the air... what was truly surprising is that more than half of the children claimed to have seen an illusory ball leave the magician’s hand and disappear somewhere midway between the magician and the ceiling. This was clearly an illusion because on the final throw, no ball had left his hand; the children had perceived an event that never took place.

My intuition about the role of my gaze was correct, for the illusion was far less effective when I looked at my hand that was concealing the ball. These findings reveal some interesting insights into the illusion. They illustrate that the illusion is mostly driven by expectations, rather than perceptual afterimages. More recently, we have shown that even when you simply pretend to throw a ball up in the air without ever having thrown the ball for real, more than a third of people still experience the illusion. In some ways, we are behaving like dogs who run after the stick their owner simply pretends to throw.

So you see things because your brain is expecting to see them and fills in the gaps. This is weird enough, but we're more or less familiar with this from optical illusions. What's even stranger is that consciousness is operating on different levels with regard to perception. Some levels are indeed fooled by the expectation illusion, but others aren't.

 Although most of our participants experienced an illusory event, the eyes were not tricked. When the ball was thrown for real, most of the participants managed to look at it when it reached the top of the screen. During the fake throw, participants claimed to have seen the illusory ball at the top of the screen, but they did not move their eyes there, which suggests that our eyes are resilient to the illusion. This result took us by surprise, but it dovetails with several other findings and highlights another truly amazing feature of visual illusions.

Richard Gregory and colleagues have shown that if participants are presented with a hollow mask and are asked to point to the nose, they point to a location outside the mask. This is because they consciously perceive the face as being solid. However, if you ask them to quickly flick the nose, their hand moves inside the mask and touches the correct location. This is because flicking is a visually guided action that is driven by the dorsal stream, the visual system that requires reliable spatial information, and thus is not fooled by the illusion. Our eyes are also driven by the dorsal stream, so even though your conscious perception has been fooled by the illusion, your eyes have not.

What's perhaps most impressive of all is that your perception isn't what's happening right now. All the processing required takes time - about a tenth of a second. The brain, amazingly, is able to extrapolate all the information it receives to correct for this by extrapolating into the future. Everything you see is your brain's best guess as to what the world is likely to look like right now. It's accurate enough to catch fast-moving balls and other objects... in a crude but literal sense, we're living in our own simulation.

Neural signals are initiated in the retina and then pass via different neural centers to the visual cortex and higher cortical areas, which eventually build a mental representation of the outside world. Neural processing is not instantaneous because neural signals are passed along neurons at a finite speed. It takes about a tenth of a second for the light registered by the retina to become a visual perception in the brain.  Let me put it in context: if you are walking at a modest speed of about one meter per second, a tenth-of-a-second delay will result in you perceiving the world as lagging 10 centimeters behind you. This is quite hard to believe because you simply do not experience the world as lagging, and such a perceptual error should certainly result in many early-morning collisions. 

It is only once you start thinking about some of the huge day-to-day challenges our visual system constantly faces that the true wonders of the brain start to emerge. Our brain uses a really clever and almost science-fictional trick that prevents us from living in the past: we look into the future. Our visual system is continuously predicting the future, and the world that you are now perceiving is the world that your visual system has predicted to be the present in the past.

A Magician Explains Why We See What's Not There - Issue 70: Variables - Nautilus

Norman Triplett was a pioneer in the psychology of magic, and back in 1900, he published a wonderful scientific paper on magic that, among many other things, discusses an experiment on an intriguing magical illusion. A magician sat at a table in front of a group of schoolchildren and threw a ball up in the air a few times.