Let's look at some examples.

Look at the image on the right. There are two panels, a white and grey. In actual fact, both panels are the same shade of grey. It is an example of how the brain uses shadows to make decisions of the world.

The brain is sent an image of both panels having the same shade, however, it interprets that the bottom panel should be in shade and therefore should be darker than the top. Since it is not, the brain perceives the bottom panel as being of lighter colour than the top. The transition between the two panels is drawn to give the impression that the bottom panel is in shade. If you cover the transition between the two panels, using your finger, you will see that they are exactly the same.

Another example of the brain using shadows to make decisions is shown on the right.

The ball always moves diagonally from bottom left to top right. By using the position of the shadow we can make the ball bounce or take-off.

In a world with only one light source the brain has evolved to use shadows as a foolproof way of gauging the movement of objects.

Colour is also a construct of the brain. Colour is constructed by the brain and does not exist in nature. We know this from studying people who have sustained brain damage to key areas of the brain and have lost the ability to interpret what the eye sees as colour.
Click to find out more about colour and the brain.

Perception of motion can also prove to be very tricky. Are objects traveling clockwise or anticlockwise? Can we be sure? Or does it depend on how we view the rotating objects. Well let's see the video on the right.

Look at the two tables on the right.

Which table is thinner and longer? The one on the left or the one on the right.

You would think I was crazy if I said both table tops are exactly the same.

Click to see that they are.

The brain interprets the receding lines of table A as being long. While the horizontal lines of table B are not interpreted as being long.

If you stare at the spiral and then avert your eyes, a stationary object will appear to be moving in the opposite rotation.

Try it. Stare at the spiral for 20 seconds then click on the spiral to a see a picture of clouds.

This happens because in the visual cortex of the brain there are cells that respond to specific directions of motion. So, when a person looks at an object moving downward such as a waterfall, that person's downward receptors are in action. Let's say he stares at the downward motion long enough for those cells to become fatigued and then looks at a stationary object, like the rocks on the side of the waterfall, the rocks appear to be moving upwards. The upward receptors dominate as a result of fatigue of the downward receptors. This phenomenon is known as an after affect.

A combination spiral is made up of two spirals with lines going in opposite directions. When this combination spiral is rotated, apparent expansion and contraction occur.

Try this: Stare at this spiral for 30 seconds and then look at a stationary object. The after affects in the stationary object are opposite to the spiral.

Contrast
This grid, known as Hermann's Grid, is an example of how contrast affects color perception. The area at the corners of the black boxes appear gray. This happens because of something called lateral inhibition. In the retina, when some photoreceptor cells are activated others around them are inhibited. This tends to increase the sharpness of the image sent to the brain.

You will notice that where the white lines intersect, there is black on four sides, whereas the lines themselves are surrounded by black on only two sides. When you look at the intersections, the cells in the retina are surrounded on four sides by other cells that are also receiving light. They are therefore more inhibited than the cells focused on the lines. It is their inhibition that causes the dark spots to appear.
Click to see an image of orange bricks. The orange bricks look darker with a black background and lighter when the white background is applied. Contrast is one factor that enable us to see millions of colors in a world with only 400 wavelengths of visible light. Click to see how contrast influences the colour.

Here are some more illusions

Consider the two rectangles shown on the right. Are they parallel to each other?
Or are they?
Click to see two rectangles that are parallel to each other.
Or are they?

Is the brown square pointed to by the black arrow, and the orange square, in the shaded part of the cube, different?
Are they?

Click to try one more
Are squares A and B the same?
Are they?

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Continue with "The optic nerve and brain interpretation"

How does the brain interpret converging lines?

Why is the shadow used by the brain to give an accurate description of an object's motion?

How does the brain treat vertical lines as compared to horizontal lines?

With only 400 different wavelengths in the visible spectrum our choice of colour seems limited. However the brain can distinguish over one million different colours. Explain how.

What is lateral inhibition? What is the advantage of lateral inhibition?

Explain why after looking at a rotating spiral still images appear to move in the opposite direction to the motion of the rotating spiral.

Read my lips. Do we really hear with our eyes? An example of how dominant the sense of sight is over the sense of hearing view the video on the right. Look at the person speaking and try to make out what he is saying. Now close your eyes and listen to the sound. It is totally different than the sound you hear when you look at the person speaking. Sight can alter the perception of clear sounds when the visual inputs are mismatched with the auditory inputs.

One tends to think of speech perception as an auditory process, however, as this demonstration shows, we use more information than we are aware of to make sense of sound. Often the many inputs to the the brain are processed in an immediate and unconscious manner. Visual information of facial movements coupled with touch and audible signals are used to interpret the spoken language. So when recognising speech the brain cannot differentiate whether it is seeing or hearing the incoming signals.

This phenomenon increases the ability to decipher heard speech in a noisy environment.