Download Motion Aftereffect Illusion

Teacher Demonstration 21
Motion Aftereffect Illusion
Materials:
• LP Record
• Matchstick/toothpick
• Record player (optional)
• String (eg wool, twine)
• Printout of spiral (page 3)
Instructions
This amazing motion after-effect illusion can be demonstrated with a record player or
on a piece of string. Students will want to, and can easily repeat the illusion at home
to impress their families and friends.
1 Print out the spiral on page 4.
Note: use a photocopier to make further copies in
order to save printer ink cartridges
2 Cut the spiral out along the dotted line.
3. Use a pencil to centre the spiral onto a LP record and
secure with sticky tape.
Teacher Demo 21 – Motion Aftereffect Illusion | Page 1
© 2007 Ruben Meerman | ABC Science Online
4. Tie a matchstick or toothpick to the end of a piece of
wool (the wool should be about 40cm long).
5. Thread the matchstick through the hole in the record.
6. Suspend the record on the string and spin it gently so
that it is rotating horizontally at roughly the same
speed as a record spinning (ie roughly 33rpm). The
record will become stable due to the gyroscopic
effect
Note: the record becomes very stable and stays
horizontal due to the gyroscopic effect
7. Stare at the centre of the spiral for at least 10 seconds.
For demonstration, have students standing in groups
of up to six around the record.
Note: the longer you stare at the spiral, the stronger
and more long-lasting the motion aftereffect
8. Now look at the skin on the back of your hand – it
will appear to be squirming! You will notice the
motion is in the opposite direction to that with which
you spun the record.
Note: try spinning the record in the opposite direction
and the squirming effect will also reverse
Teacher Demo 21 – Motion Aftereffect Illusion | Page 2
© 2007 Ruben Meerman | ABC Science Online
Teacher notes
The motion aftereffect illusion has been known since ancient Greek times and was
reported by Aristotle. It was reported in 1834 by Robert Addams who noticed the
effect after staring at the Falls of Foyers in Scotland for a prolonged period. When he
averted his gaze to the rocks beside the waterfall, he perceived a motion in the
opposite direction of the water and so the phenomenon is often referred to as the
Waterfall Illusion. The spiral motion after effect was reported in 1849 by the Belgian
physicist Joseph Plateau who invented the stroboscope.
Since then, the motion aftereffect has been studied extensively by psychologists
however the cause is still not fully understood. Some researchers attribute the effect
to the adaptation by specific neurons in the visual cortex responsible for the
perception of motion. It is believed that specific neurons perceive motion in specific
directions. When staring at a stationary scene, the outputs from all motion perceiving
neurons in all directions are balanced. Staring at a particular motion in one direction is
believed to lead to adaptation (possibly simply due to fatigue) resulting in a decreased
output by neurons in that specific direction. If the moving stimulus is now suddenly
removed, the output remains low in that specific direction for a brief period. But
because the output from the opposite motion neurons remains higher by comparison
during this brief period, there is now an imbalance and a perceived motion in the
opposite direction to the original stimulus.
Although the effect is not yet fully understood, one thing is definitely clear: the eyes
alone are not responsible for our sense of sight. Our perception of the world we live
in is the result of visual stimuli which are interpreted by our brain (specifically the
visual cortex). It is also clear that our perception can be “tricked”.
Online versions of motion aftereffect illusions can be found at:
Prof Michael Bach’s Website
University Eye Hospital – Freiburg
www.michaelbach.de/ot/mot_adaptSpiral/index.html
Prof George Mather’s Website
Experimental Psychology – University of Sussex
http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/MAE.HTML
The Exploratorium
www.exploratorium.edu/exhibits/depth_spinner
Teacher Demo 21 – Motion Aftereffect Illusion | Page 3
© 2007 Ruben Meerman | ABC Science Online
Print then cut out and stick to an LP record
Teacher Demo 21 – Motion Aftereffect Illusion | Page 4
© 2007 Ruben Meerman | ABC Science Online