What you Need
A computer screen
About five minutes time
What to do
1. Start this experiment by staring at the centre of the rotating spiral
2. Press play on the video
3. KEEP your focus on the centre of the spiral as it rotates for about 90 seconds.
4. When the video ends you should see something very peculiar!
What may happen
My face growing outwards towards you! (donít worry, my face isnít actually doing this, and this strange effect will disappear soon, it's just a temporary visual illusion)
Why does it happen?
Which part of the brain is responsible?
The visual cortex, right at the back of your brain.
Why does this happen?
Being able to process information from your eyes is important business - it allows you to form a picture of the world around you. This processing happens in the visual cortex, which is right at the back of your brain. Visual information comes in through your eyes and is processed in the visual cortex - before being relayed to the relevant parts of your brain to help you react to these visual cues.
In order to do this job properly, the visual cortex requires A LOT of brain space Ė itís the largest system in the brain with about 280 million nerve cells. In amongst this large party of visual cortex cells, there are specific nerve cells that have evolved to process motion Ė we have different motion detecting cells to detect movement from all different directions.
When there is nothing moving, ALL of the motion detecting cells are being activated equally at low levels. The cell activation balances out to mean that you perceive everything as being still.
When staring at the rotating spiral for, even a short period of time, your inward motion detecting cells are working hard to process this information. The cells are effectively starting to get tired. Itís a bit like your brain is doing some heavy exercise just by staring at the movement, with your nerve cells starting to act like a tired muscle and starting to ache.
After staring at the centre of the rotating spiral for a while, your inward detecting motion cells have effectively been fatigued. They have adjusted their activity to dampen down their response. However, your outward detecting motion cells are still very active, so they give a much bigger signal than the tired inward detecting cells.
This unbalances the signals, so you perceive my face as having an outward motion even though it really hasnít. So, by staring consistently at movement in a particular direction for even a short time, subsequently viewed stationary scenes appear to move in the opposite direction, leading to an illusion and change in perception. You can try this type of illusion again, the next time you are looking out from a car or train window.
You can also have a go at the opposite direction rotating spiral, by watching the movie below. What do you think will happen to my face here?
Why do scientists study this?
This type of illusion was first reported in detail by a Scottish rambler - a Mr. Robert Addams who, in 1834, decided to go out for an afternoons stroll in the highlands. Whilst out walking he stumbled upon a waterfall and was mesmerised by the downward cascade of water. After some enchanted minutes he managed to drag his eyes away from the waterfall. But when he looked away, the world appeared to be very different. His world had changed. His new world, and everything in it, now appeared to be moving upwards!
Scientists have looked to see if other species are also affected by this motion after effect. For example, do flies get it? In 1978, two Australian neuroscientists, Srinivasan and Dvorak looked at blow flies - they usually live on sheep. They made the flies walk on a ping pong ball and monitored the electrical activity of the flies' brains as they turned the ball. Their results suggest that the flies are able to adapt to motion perception as well! How flies feel when they land on a stationary leaf or a sheep after having travelled at high speeds in the air must really be something to imagine!
Neuroscientists are still intellectually stimulated by these visual illusions. They help us understand how brains process information and how we perceive the world. So, illusions are important tools to neuroscientists trying to understand perception. On the flipside, illusions are also important tools for understanding what happens when perception goes wrong, which is what happens with schizophrenia. People with schizophrenia experience delusions or hallucinations Ė they see and hear things that arenít necessarily real. This is a change in perception that can be incredibly scary for the person who experiences it. If neuroscientists can understand more about how we perceive the world around us, they will hopefully be able to use this understanding to improve the treatments for schizophrenia.
The Risky Part: what to be aware of and how to keep the science safe:
An account of a peculiar optical phenomenon seen after having looked at a moving body. Addams, R. (1834). London and Edinburgh Philosophical Magazine and Journal of Science, 5, 373Ė374
Srinivasan, M.V., & Dvorak, D.R. (1979). The waterfall illusion in an insect visual system. Vision Research, 19, 1435-1437.
Neuronal adaptation to visual motion in area MT of the Macaque, A. Kohn, J. Movshon Neuron, 39 (2003), pp. 681Ė691