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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