EyeStonishing Notes for Teachers and Home Educators

The activity starts with a consideration of the biology of the eye; how it functions and how our brains interpret the messages. Understandably this is kept brief but interested children can easily research this topic themselves. Firstly, it is important to emphasise that the medium being used – the internet – might not show illusions to best effect. If viewers are using a tablet or mobile phone, the screen will be too small for peripheral vision effects. If the screen resolution is low, again effects might not be demonstrable. We therefore include all the images in high definition for you to download and examine. Experience shows that simply copying these high-resolution images into a simple App like ‘Paint’ and displaying them on a big screen is effective. Now, some science …… Our brain and eyes work so well together that we believe what we are seeing is correct but our visual organs (eyes and brain) can sometimes be fooled into seeing things that aren’t there or incorrectly seeing things that are there. These effects are called optical illusions. Many illusions are created because we don’t see objects on their own. Instead we see them in context and surrounded by other objects which can influence our vision. By playing around with the context of colours, shapes, and tones and by changing the viewpoint of the audience you can create a wide range of illusions that will have you questioning what is real and what is not real.

Video 1 Here an introductory section talks about the eye and vision. It moves on to consider: Scintillating Grids • Dark circles / blobs appear at the point where the white lines cross on the grid. Staring at one central white dot will leave your peripheral vision seeing black dots. • The mysterious blobs only appear when the crossing lines are straight, vertical and horizontal. When this happens the contrast between the lines and the black spaces creates a lateral inhibition effect. This occurs when signals from photoreceptors in your eyes conflict with each other, creating the impression of dark blobs where the white lines cross • Your eyes and brain use contrasts between light and shade to define the shape of objects. Your ability to see in 3D depends on how light and shadow are laid out. When you see a surface that has differently lit areas your enhances the contrast to see it more clearly. This is called lateral inhibition and it works so well that it can create a lot of powerful visual illusions. Motion • Your eyes are so sensitive to light and motion that, sometimes you see movement even when there isn’t any. This is because your eyes make tiny movements that you are not aware of. When you look at an image you do not actually look at it in a steady way. Instead your eyes constantly jerk around, locating interesting parts of the scene to builds up a mental ‘map’ of the whole picture. Scientists call these random eye movements ‘’. • As your eyes try to follow the patterns, they make tiny movements, switching between the lines and the colours. This tricks the brain into seeing movement where there isn’t any. • This effect will not be apparent on a small or low-resolution screen.

• Pulsing Star o The pulsating (right below) may be caused by differences between your central and peripheral vision. Your central vision is sharper and more sensitive to colour while peripheral vision is more sensitive to motion. The difference can create illusions of movement. • In a spin o As your eyes try to follow the patterns, (left below) they make tiny movements, switching between the lines and the colours. This tricks the brain into seeing movement where there isn’t any.

3-D Stereo images • Stereograms (also known as ‘autostereograms’ or ‘magic eye’) are 3D images hidden within a 2D pattern. In order to see the 3D image, move your face nearer to the screen. At this distance, your eyes cannot focus on the image and they focus somewhere behind the image. Now slowly move away from the image while trying to keep your eyes out-of-focus until you see the hidden image. Seeing a stereogram is tricky and you may have to be patient because it can take a little time. • The first stereogram is of a butterfly, then a DNA helix. In the garden it looks like a crocodile! While a bear is centrally located in this one. The bear looks like it is sat in a box or on a window sill. • These 3D images have all been easily visible on a computer screen – with a bit of practice. Surprisingly, they have been hardest to see when the You Tube videos have been shown in full screen mode.

Video 2 Impossible shapes and Impossible Art • Impossible Cube Note how parts of the cube seem to be simultaneously in front of and behind other parts of the cube. You will see something which appears physically possible yet which you know is not. • Impossible triangle Follow the shape of the triangle starting at the top point; note how the left side seems to extend away from you, and the right side seems to extend towards you, yet they seem to lie on the same plane when they reach and are connected by the bottom vertices. You will see something which appears physically possible yet which you know is not. The shape of the triangle might at first appear feasible, but upon reflection one can see that it would be physically impossible to construct such a shape. • The Ambiguous Ring is one of many impossible figures (or impossible objects or undecidable figures): it depicts an object which could not possibly exist. It’s impossible for the Ambiguous Ring to exist because in order for it to exist rules of Euclidean geometry would have to be violated. Look at the green surface: is it the front or the back surface of the ring? You should experience a sense of perceptual confusion as you note the paradoxical nature of the depicted ring. • The Penrose Figure was created by Lionel Sharples Penrose (1898 -1972), a British psychiatrist, geneticist, and mathematician, and his son Sir (1931 -), a British mathematician, physicist and philosopher of science. It was first published in the British Journal of Psychology in 1958 The Penrose Stairs is an impossible figure (or or undecidable figure): it depicts an object which could not possibly exist. It is impossible for the Penrose Stairs to exist because in order for it to exist rules of Euclidean geometry would have to be violated. For example, if one were to complete a circuit of the stairs, one would end up back at the same level that one began, even though each flight of the stairs continuously rise (or fall, depending on the direction of travel). Note how the stairs appear to continuously ascend when tracing round their path in one direction and descend in the other, yet if one were to complete a circuit one would end up back at the same level that one began. You will see something which appears physically possible yet which you know is not. • Belevedere is a painting by Maurits Cornelis Escher (1898 - 1972), a Dutch graphic artist that contains many impossible figures and impossible elements. The print was created in 1958, and the original version is now in the National Gallery of Canada, in Ontario. Look at the paradoxical placement of many of the pillars - look at where they begin at the top and where they end at the bottom, and yet they appear vertical. Look also at the impossible placement of the ladder. And notice the impossible cube that the boy sitting outside of the building is holding. You should experience a sense of perceptual confusion as you note the paradoxical nature of the building and other features in the painting.

Bigger or Smaller? And Lining Up: How long? How far away? How much? We measure distance and estimate the dimensions of objects all the time usually without thinking about it. We can usually tell the difference between the actual size of an object and its apparent size. The actual size of an object cannot be observed (but it can be measured), because it must be viewed from a distance so that your eyes can focus on it. The apparent size of an object depends on it size and how far away it is. Objects that are closer will appear larger than if they are further away. Perspective and size constancy also play a part in how we see things. They obey these simple rules: • For two objects with the same ACTUAL size, the one that looks bigger is thought to be closer. • When we see two objects with the same APPARENT size, the one that appears to be further away is thought to be larger. • Muller-Lyer Illusion: Look at the two horizontal lines; note the apparent difference in length difference. The top horizontal line (with arrow heads pointing outwards) should appear to be longer than the bottom horizontal line (with arrowheads pointing inwards). However, they are in fact the same length. The Müller-Lyer Illusion is named after its creator, Franz Carl Müller-Lyer (1857 - 1916), a German psychiatrist and sociologist, who first published the illusion in the physiology journal Archiv für Anatomie und Physiologie, Physiologische Abteilung in 1889. The Müller-Lyer Illusion is one among a number of illusions where a central aspect of a simple line image – e.g. the length, straightness, or parallelism of lines – appears distorted in virtue of other aspects of the image – e.g. other background/foreground lines, or other intersecting shapes. These are sometimes called ‘geometrical-optical illusions’. There have been a number of attempts to explain how the Müller-Lyer illusion works. The following discussion will focus first on the original arrowhead version, before moving on to other versions. The three most discussed attempts to explain how the illusion works are: (i) ‘misapplied size constancy scaling’ – the arrowheads engage the part of the visual system that deals with depth cues in retinal images, and results in the line with the outward-pointing arrowheads being perceived as longer because it is processed as being further away (Gregory 1997); (ii) ‘conflicting cues’ – the arrowheads are perceived as contributing to the length of the lines, and the longer overall shape of the line with the outward-pointing arrowheads causes the appearance of greater length of that line (Day 1989); (iii) ‘confusion’ – the inter-tip distance between the relevant arrowheads influences perceived length. So, for the line with the inward-pointing arrowheads, the distance between the tips of the arrowheads at opposite ends of the line is shorter than the distance between the tips of the outward-pointing arrowheads of the other line, thus causing the illusion (Sekuler and Erlebacher 1971). • The Look at each of the two orange discs surrounded by the grey discs and try to decide if one is larger than the other. The orange disc on the right appears larger than the one on the left, but both discs are precisely the same size. • Sarcone’s Cross Illusion, Obonai’s Square Illusion The cross and the squares are the same size, but people think that one is bigger than the other. What’s going on: They show that the apparent size of an object can be affected by the objects that are placed close to it, although the effects can vary • Lining Up Paggendorff’s Illusion The is a geometrical- that involves the misperception of the position of one segment of a transverse line that has been interrupted by the contour of an intervening structure (here a rectangle). It is named after Poggendorff, the editor of the journal, who discovered it in the figures Johann Karl Friedrich Zöllner submitted when first reporting on what is now known as the Zöllner illusion, in 1860. In the adjacent picture, a straight black and red line is obscured by a grey rectangle. The blue line, rather than the red line, appears to be a continuation of the black one, which is clearly shown not to be the case on the second picture. Instead there is an apparent position shift of the lower portion of the line. The magnitude of the illusion depends on the properties of the obscuring pattern and the nature of its borders. Another example of a Poggendorff illusion is also given. Let the children decide which broom handle goes with the broom and then let them use the ruler to see if they are right.

Contrast and Confuse Your eyes and brain use contrasts between light and shade to define the shape of objects. Your ability to see in 3D depends on how light and shadows are laid out. When you see a surface that has differently lit areas your visual system enhances the contrast to see it more clearly. This is called lateral inhibition and it works so well that it can create a lot of powerful visual illusions. • Kanizsan Triangle What shapes do you experience in the figure? What shapes are there in the figure The figure is often experienced as a solid triangle pointing upwards that is lighter than the background, which occludes an inverted triangle pointing downwards, and a set of black discs which are also occluded by the solid bright white triangle that points upwards. Surprisingly, none of these shapes are actually present in the figure. The Kanizsa triangle illusion makes us realise the way our visual systems work—which we do not notice in our everyday experience. Looking at the figure, most people will have the visual experience of an apparent brightness contour defining an upright triangle which is occluding three black discs and a second, inverted triangle outlined in black. Of course, these triangles do not in fact exist, and we are not perceiving occluded discs but rather ‘Pac-Man’-like fragments of discs. (‘Pacmen’ is now the standard nomenclature for such inducing elements). A similar illusory ‘filling-in’ of colour that we experience in the upright triangle, such that the figure appears filled with a solid white that is brighter than the rest of the figure, is also highly evident in the Kanizsa square (Fig. 1 – not shown). Note that both the Kanizsa triangle and the Kanizsa square create an illusion of depth – the central figure appears to sit in a higher plane than the inducing pacmen (or the occluded downward pointing triangle). • The Pac Man Illusion Laeng and Endestad used these illusions, in which configurations of these Pac-Man shapes give an illusion of contours around a white square. When the Pac-Man shapes are rotated, the illusion disappears. • Ehrenstein Figure Look at the white space between the lines where the lines would intersect if they increased in length. What objects do you seem to see there? Are they in the foreground or the background? You will likely experience illusory discs where the lines would intersect. These will appear 'filled-in' with a brighter white than the surrounding area. You may also experience these discs as 'figures' which sit in a higher depth plane than the lines and so occlude them. The Ehrenstein figure was first introduced in 1841 by German psychologist Walter Ehrenstein (1899-1961) of the early 20th century Berlin school of Gestalt psychology. For most people, the Ehrenstein figure will produce the visual experience of apparent contours defining white discs. Of course, these discs do not actually exist as part of the figure – we are really perceiving only fragments of lines. Most individuals will experience an illusory ‘filling- in’ of colour such that the apparent discs are filled with a solid white that is brighter than the rest of the figure. A third effect is one of illusory depth – the illusory discs are perceived as figures which occlude the undefined ground which continues behind them in a different plane. • ‘Morning Sunlight and Evening Dusk’ These two illusions are made according to a design by the psychologist and artist Akiyoshi Kitaoka. The figure on the left is called “Morning sunlight”, and appears to have a bright light at the centre. The other one is called “Evening dusk”, and it seems to have a dimmer interior and a glowing corona. Both figures are actually equally bright, inside and outside. (Figures: Bruno Laeng, Tor Endestad and Akiyoshi Kitaoka) This is more to do with light than the brain (liked the illusion and put it in here)

• Dark spot, Bright spot This was presented at the 2019 Jamboree as an activity for children to detach circles, compare them, and replace them on a colour gradient. The resources to do this are provided. For the virtual Jamboree we have made a simple animation to show the same effect. What’s going on: This phenomenon is described as ‘simultaneous brightness contrast’. A colour always seems brighter when surrounded by dark colours or darker when placed on a light background. This is related to ‘lateral inhibition’, an effect that enhances the contrast of the outline of an object compared to its background. It is called lateral inhibition because each photoreceptor in our eyes tend to inhibit the response of the ones next to it.

Ambiguous Images The duck and rabbit illusion is one of the oldest intentionally ambiguous figures created for psychological tests. What’s going on: this illusion is a good example of what scientists call ‘rival- schemata ambiguity’. In other words even though the image is ambiguous there is no dominant shape as both images can be seen at the same time. In 1915, Danish psychologist Edgar Rubin created a more figurative . This now iconic illusion is known as "Rubin's Vase," though the image could also be accurately described as "Rubin's faces." Rubin's Vase utilizes the ambiguity of flat colour and two-dimensional negative space but it is also possible to create ambiguous figures with illustrative lines and shading. Another widely known classic dates back to at least 1888. This elegant and humorous version was drawn by William Ely Hill in 1915. It's titled "My Wife and Mother-in-Law." Can you see the donkey? What about the seal? What's interesting is that once you are aware there are two different ways of seeing the same thing, you are able to appreciate both perspectives equally.

Video 3 ‘Pencil Spinners’ were used with children at the 2019 Jamboree. These are a variation of a thaumatrope: A thaumatrope is a disk with a picture on each side that is attached to two pieces of string. When the strings are twirled quickly between the fingers the two pictures merge into one due to the . For the virtual Jamboree we try to give the same effect by flipping between two images very quickly. Further videos show the classic bird in a cage illusion and many other ideas that we hope will be a stimulus for home or school projects. A ‘pencil spinner’ works in a similar way to the thaumatrope disks. A double-sided image will be attached to the top of a pencil (or kebab stick) and children will make the pencil spin using their hands. What’s going on? The two separate images pass by your eyes so quickly that you are still processing one when you see the next, and you merge the two together to see one image. We have one pencil spinner that demonstrates this - seeing a face inside a TV Thaumatropes can also provide an illusion of motion if the two sides each depict a different phase of motion. We have four pencil spinners that are examples of motion; bat flying, rocket flying, hands waving / facial expression changing, fish in bowls