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Illusions Exhibition

Activities

Ghost Busters

Movement, colours and shapes that aren’t really there.

The things you’ll need

• Class set of the images on the following pages:
  Parts 1,2 & 3 and Parts 4 & 5 enlarged on the photocopier
• Class set of scissors
• Glue or tape
• Scrap cardboard
• Ruler (helpful but not essential)
• Satay skewers (with points cut off)

The preparation

Make copies and, if you wish, overhead transparencies of the images shown.

The activities

Images for activity 4. Parts 1,2 & 3 and Parts 4 & 5.

Ghost Busters Part One: HOORAY FOR HERMANN

Show your students the copy of Hermann’s Grid and ask what they see. Most people ‘see’ grey spots where the lines of white space intersect, despite the conscious knowledge that there is no grey there at all.

The inside story

Hermann’s Grid is an example of lateral inhibition — a mechanism of our visual system. Light sensitive cells are arranged in rows on the retina and it is possible to stimulate just one cell, say called Cell X, to send a signal to the brain. If, however, Cell X’s neighbours are also stimulated, Cell X’s signal won’t be as strong. Stimulating the neighbours of any particular cell actually inhibits the strength of that cell&rsuo;s response. This means that the strength of any signal sent from the retina is dependent on the signals nearby. The places where the white lines in Hermann’s Grid intersect have white surroundings in four different directions so they appear darker than they actually are. The cells viewing light from those intersections are being inhibited by the nearby cells.

Ghost Busters Part Two: SCHUMAN’S SQUARE

Ask your students to cut out the target, then cut it exactly in half. Now ask them how they could place the half targets in the box so as to make a white square. This is called Schuman’s Square and it is an example of how the arrangement of certain real shapes can create the illusion of other, imaginary ones. (Block & Yuker, 1989).

Ghost Busters Part Three: KANISZA’S TRIANGLE

Ask your students to cut out three black circles and then cut out the segments. Now ask them to arrange these ‘circles’ so as to make a white triangle. They should be able to do this quicker now. Draw a ‘v’ shape on your board and ask your students to see if they can strengthen the illusion using just three of these shapes. The solution (known as Kanisza’s Triangle) is shown on page 16.

The inside story

Human brains are very competent at making assumptions and drawing conclusions based on combining whatever clues are available with past experiences. We use this subconscious educated guesswork every day and figures such as Schuman’s Square and Kanisza’s Triangle are just two examples. Our brains tend to assume the existence of an object (the imaginary square or the triangle) simply because it seems the most likely explanation for the clues presented. In other words, the brain draws conclusions based on nothing but circumstantial evidence. Rather than this being a demonstration of a failing in the brain’s visual system, scientists think it actually proves the efficiency of the system. Humans need to constantly take in, interpret and respond to such huge amounts of information that we need this ability to ‘jump to conclusions’ even if it means we are sometimes mistaken. Discuss this with your students.

Ghost Busters Part Four: FAKE SPOKES

Ask your students to cut around each of the squares surrounding the disks, as indicated and then to hold the two squares up at eye level, almost at arm's length from their faces. Then ask them to move the right hand square in a small circular pattern. Not only will there appear to be spokes rotating across the concentric circles on the right hand square, but these will appear on the left hand one, too. It works just as well by moving the left square and keeping the right one still.

The inside story

Scientists think this illusion occurs because our eyes move in small jerky movements and our visual system becomes confused by so many similar lines moving this way. Of course, there are no spokes in either set of circles and one set of circles is not moving at all. The fact that we ‘see’ spokes and movement where neither exists is an example of our brain making one of its ‘best guesses’ about the information it is receiving.

Ghost Busters Part Five: BENHAM IN A SPIN

Ask your students to glue the Benham's Disk onto scrap cardboard and cut it out. Make a hole through the centre. Pass a skewer* through the hole and set the disk spinning. (Perhaps ask your students what they think they will see before you start spinning). Most people expect to see grey but actually ‘see’ colours (it may help to tilt the disk as it spins and dim the lights)

The inside story

These are called ‘subjective colours’ because they are illusory. Subjective colours tend to be rather pale but interestingly, their patterns can look different if the disk is spinning the opposite way. It is thought that this illusion occurs because the rapidly spinning black and white signals have some similarity to the brain’s own code for colours.

The follow-up

With card or construction paper, ask students to create their own grids. For example, Hermann’s Grid from part 1 can also work when the squares are white and the spaces black, or with other colours. Using the concept of segments cut out of circles, ask your students to create other illusory images.

*The options

You can use opened out paper clips or old plastic lollipop sticks instead of wooden skewers to make a Benham’s Disk.