Hooray for Hermann
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’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.
What do you 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.
Schuman’s Square
Print out a copy of the target, then cut it exactly in half. How could you 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).
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Kanisza’s Triangle
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.
Print out a copy of the three black circles and then cut out the segments. Can you arrange these ‘circles’ so as to make a white triangle? (Once you've had a go of it, check out the solution at the bottom of this page.)
Fake Spokes
Print out two copies of the discs, and cut them out. Then, hold the two squares up at eye level, almost at arm's length from your face. Move the right hand square in a small circular pattern. Does there appear to be spokes rotating across the concentric circles on the right hand square? Do they appear on the left hand one, too? It works just as well by moving the left square and keeping the right one still!
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.
Benham in a Spin
Print out a copy of Benham's Disk, glue it onto scrap cardboard and cut it out. Make a hole through the centre. Pass a wooden skewer or pencil through the hole and set the disk spinning. When spinning, is it still grey or do you see colours? Most people do! (It may help to tilt the disk as it spins and dim the lights.)
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.
Kanisza’s Triangle Solution
