Optical Perception

What color is the central square on each visible surface ?

Your brain has evolved to see what’s useful…not reality! Although you see the central square on the top of the cube as brown, and the central square on the side of the cube as orange - amazingly they are both physically identical. Suppose you or your distant ancestors were searching for ripe red fruit in the shadow of a tree. The actual light reflected from the red fruit would be different from in sunlight. However, to eat and survive, you need to still see the fruit as red and therefore ripe.

They’re the same size! Can you believe these tables are in fact the same size? If you were to rotate the red table by 90 degrees, you would see both tables have identical dimensions.

What do you see?

It’s not moving! The rollers in this image appear to move – even though in reality they are utterly stationary. The illusion is so strong that many people feel nauseous if they look at the image for more than a few moments. Notice that only those parts of the image in your peripheral vision appear to move. When you look directly at one of the rollers, you can’t ‘catch’ it moving!

Stare at the + in the center

What happened to the dots? Most people find after seconds of looking at the black cross, the pink dots disappear. However, as soon as you move your eyes or look directly at the dots, they will reappear! This illusion is happening because of the light sensitive cells in your retina called rods and cones. In your periphery you have far fewer cones to detect color. The pink dots stimulate these few cones and soon they become tired. They stop sending signals to your brain so you no longer see the pink dots. When you move your eye to look directly at the pink dots, you are stimulating different ‘fresh’ cones – which send signals to your brain and suddenly you see the pink dots.

Waddle or hop?

How many legs?

Who’s there?

Turn away if you get dizzy!

No, it’s not moving! For many years scientists have known we have specific neurons in our brains that fire when we look at something moving in a particular direction. These are called direction-selective neurons. When scientists examined this illusion they found something quite remarkable. Direction-selective neurons in the visual cortex begin firing when looking at this illusion. This is particularly exciting because up until now, scientists thought this type of brain neuron could only be activated by seeing something that was really moving (either on-screen or in the real world). They think that the cells that detect motion lower down the visual system have actually interpreted the rollers as if they were really moving.

Focus on the dot in the centre and move your head towards and away from the screen.

What happened? Notice that the circle only appears to move in your peripheral vision. When you move your head whilst looking directly at a part of the circle, that part does not move. At first glance, this illusion appears to be made up of grey shapes arranged in a circle, on a grey background. However, if you look more closely at each tiny ‘tablet’ making up the circles, you will see that it is actually made up of a white edge, a grey inner and a darker grey edge.

Shapes or a Word? Did you see the word LIFT? At first, most people just see blocks of black and white. Then suddenly they see the word lift. Everything you see is your brain’s best guess. Once you have made sense of this image it is impossible not to see it again. This is because once your brain has interpreted this image as a word; your brain will always favour this hypothesis.

How many black dots can you count?

What black dots? It’s impossible - as soon as you focus on the black dots, they disappear! Your eye and brain have very special mechanisms for seeing edges clearly. This allows you to see a sharp boundary between an object (e.g. a person or a building) and the background. lateral inhibition. Your mechanism for sharpening edges is called lateral inhibition. It works by the light- sensitive receptors in your eye switching their neighboring receptors off. This makes an edge look more pronounced.

Is this a triangle?

NO The triangle on your screen is 2D, yet your brain automatically tries to picture it as a 3D object. To do this, your visual system is making assumptions based on what it has seen before. For this triangle, most peoples’ visual systems make the false assumption that the triangle is in one plane – i.e. it would lie flat on a table. But if this is true, the triangle would be impossible to build. When you cover up the top of the triangle, you get an idea of what a model of this shape would really look like. It is not a triangle at all – in fact one part of the ‘triangle’ actually sticks up into the air.

Can you get to the centre? Start at the red dot and try to follow the spiral to the centre. Where do you end up?

Who’s chasing whom?

Although this is only a two-dimensional drawing, your brain looks at the contours and sees it as a three-dimensional tunnel. Unconsciously, you are applying the rule that an object must get smaller as it gets further away. Since the monster doing the chasing is not smaller, you conclude it must – in reality – be a bigger monster. When you look down from the top of a 100m tall building, the people below look noticeably small. But when you look 100m down the street, you don’t comment on how small the people look. The reason is you have learned the ‘rules’ for scaling people at a distance, but not from a height.

Stare at the blue dots while you count slowly to 30.

Close your eyes and tilt your head back. A circle of light will slowly appear. Focus on it, and look into it. What do you see? Try this again, but look at a white wall after 30 seconds. Do you still see it?

Stare at the blue dots for 30 second again

Stare at a white wall Most people see an ‘apparition’ of a man – perhaps Jesus. When you look at a white wall, the image appears much bigger. Staring at the bright white areas of the picture tires out some of the light-sensitive detectors in your retina (at the back of your eye). When you then closed your eyes - or looked at a white wall - you see a negative after- image. The areas that were white now look black.

Eyewitness

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