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2014 The Bad Science Show - Melbourne School Shows & School Incursions

The Mystery Of The Growing Head

The Motion After Effect

Deceptology - The Neuroscience Of Deception

Who are you gonna believe? Me or your own eyes?

Joni Mitchell

*This resource is intended for audiences who have seen Deceptology. 


What's The Point?

To learn about neural adaptation and the motion after effect. You'll not only understand the effect as an optical illusion but also in a broader neurological context. You may also throw up.


The Trick

In Deceptology, the audience stared at a spinning spiral and then at my big fat head. Depending on the direction the spiral spun my head appeared to shrink to half its size OR blow up like a balloon.


The Explanation

This occurs because of the Motion After Effect.


The Motion After Effect occurs when we observe at a stationary object after staring for an extended period time at a repetitive, moving stimuli.


The stimuli might be rushing river, a passing train, a spinning ceiling fan or even a game of Guitar Hero. Aristotle observed the effect as far back as 350 BC although he did not provide much in the way of detail. We can assume he wasn't playing Guitar Hero.

It wasn't until 1820 that Austrian physiologist  Jan Evangelista Purkyně experienced the effect after watching a calvary parade that the phenomena was studied scientifically. A few years later, London Chemist Robert Addams reported the effect after staring at a Scottish waterfall.


However, it is only in recent years that psychologists and neuroscientists have figured out the possible cause of the effect.

Scientists believe that different neurons are responsible for encoding different types of movement. When we experience the same type of motion over and over again, those particular neurons become less receptive. This might be because the brain is filtering out the unimportant information or because the neurons are becoming fatigued.

So if the spinning spiral appears to be shrinking, the neurons responsible for encoding things getting smaller will become less receptive than those responsible for those getting larger. When you stop looking at the disk and start looking at my head, those 'growing' neurons overcompensate and my head appears to grow.


When you stare a shrinking spiral, the parts of your brain responsible for seeing things shrink respond less, making anything else you look at grow.


1) Have you ever experienced the motion after effect in your day to day life?

2) What other "after effects" have you experience? ​

3) What other repetitive stimuli could you stare at to produce the effect?

4) What does this phenomena tell us about our sense of sight?



1) Print out this image


2) Carefully cut out the spiral and glue it to an old CD or LP (You finally have a use for them). The red dot needs to be in the centre of the CD hole. 

3) Poke a hole through the red dot and thread a piece of string through.

4) Tie a toothpick to the end of the string beneath the CD and tape it to the CD. 

5) You can now hold the string at one end and have the spinning disk hang at the other end. The CD should be parallel with the floor.

6) Carefully spin the disk and look down at the spinning spiral. You now have your very own spinning disk.


7) Experiment to find the best conditions to achieve the Motion After Effect. Does it work best near the disk or far away? Low light or high? Spinning fast or slow?

Further Reading

Motion After Effect - The Surfing Scientist

George Mather - Motion After Effect

Nicholas J. Johnson


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