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Illusions

Visual Illusions Exhibits 1-27

From the Illusions Exhibition at Questacon February 14 - August 29, 1999

1. Adjust and Measure

These exhibits allow visitors to view a number of well-documented illusions. Visitors attempt to compensate for the illusion's effect and are then able to measure how effective they were at doing so. Five of the six adjust and measure exhibits are presented in boldly coloured upright boxes that encourage visitors to focus only on the illusion.

The adjust and measure illusions include:

• Arrows - Muller-Lyer Illusion (right). Try out Questacon's interactive Muller-Lyer Illusion.
• Line up the Lines - Poggendorf Illusion (try out Questacon's interactive Poggendorff Illusion)
• Fading Tracks - Ponzo Illusion
• Find the Middle - Judd Illusion
• Find the Right Hole - Ebbinghaus Illusion
• Upside Down T Illusion

The inside story

'The organisational mechanisms of vision selection, distortion, filling in, and omissions are best demonstrated by illusions,' (Kandel et al, 1991, p443).

All of the illusions in the Adjust and Measure series demonstrate that perception is a creative process and the result of a myriad of assumptions that the brain makes when interpreting visual data. In the Muller-Lyer illusion (see diagram above), the lines are perceived to be of different length because experience has taught us to use space and shape as an indicator of size. Even learning that the two lines are the same does not relieve the perception that they differ. The fact that learning does not prevent us from being tricked by an illusion is a characteristic of many illusions.

2. Mirror Maze

Based on the famous Lucerne Labyrinth, this exhibit allows visitors to experience the confusion caused by a mirror labyrinth. Visitors enter the maze and try to firstly find their way out and secondly to remember the path that they took. This exhibit is a favorite with visitors and highlights how easily the brain can be confused.

3. Mirror Wall

The mirror wall is a series of mirrors in a concertina arrangement. Apart from being visually appealing, when visitors walk alongside the mirror they are able to see their images moving in opposite directions.

The inside story

Mirror Maze and Mirror Wall both illustrate the creative process of perception and the assumptions the brain makes when interpreting visual information. It is easy to become lost in the maze as when the brain utilises its set of assumptions required when we navigate our way through an environment, they are ineffective. Instead of seeing the way ahead when walking through the maze, we see where we have come from which is very confusing. The Mirror wall confuses us because we see ourselves moving in the opposite direction to where we are heading.

4. Mirror Kaleidoscope

Mirror Kaleidoscope is a series of 3-sided mirrors that allow visitors to view the symmetries of various objects. Visitors can determine how many folds of symmetry a particular object has by rotating the mirrors around an object resting on well lit domes. The mirrors are made of reinforced acrylic and as such are very durable.

5. Spinning Spiral

The Spinning Spiral is one of the most popular exhibits due mainly to the large effect it has on our visual system.

The inside story

The Spiral is based on what is known as "The Waterfall Illusion", and its effect is caused by the laziness of the movement receptors in the eye. Visitors look at the moving spiral for approximately 20 seconds and then look at a stationary object, for instance, a friend's face. As the visual receptors have minimised the effect of the rotation when viewing the spiral, their reverse action continues for a short time when visitor's look at an object. The result is bizarre convolutions of the object being viewed.

Psychologists refer to this effects as 'adaptation and rebound effects' and it is a feature of all sensory systems. A non-visual example occurs when you place your hands in a container of hot and cold water respectively, for half a minute and then simultaneously place both hands into a container of warm water. The warm water will immediately feel cool to the hand that was in hot water and vice versa. Motion, like that supplied by the spinning spiral can also give rise to temporary adaptation and after effects.

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The biological reason for this effect relates to the functioning of photoreceptive cells in the retina. The photoreceptive cells, as their name implies, respond to light and once they are activated send on information, via a transmitter substance, to the bipolar cells. Bipolar cells are neurons which form synapses with photoreceptive cells. In turn, bipolar cells transmit information to ganglion cells, which are another type of neuron that transmit information across the retina to the optic nerves. Thus, visual information travels through a chain of three cell types to reach the brain:

photoreceptor › bipolar cell › ganglion › brain (see diagram below).

The human retina is comprised of two types of photoreceptors: 125 million rods and 6 million cones, named after their shape. The photoreceptors contain a photopigment, which in the absence of light, is attached to a protein. When a photon (light particle) strikes a deactivated photopigment (one attached to a protein) it splits apart into its constituent molecules. The splitting of the photopigment initiates a transduction signal process whereby a series of chemical reactions stimulate the photoreceptor to send a message to the bipolar cell, and so forth, delivering information to the brain.

A photoreceptor must contain deactivated photopigment in order to be stimulated by a photon of light. Photoreceptor cells are constantly restoring their chemical balance back to its deactivated state with photopigments reattaching to a protein molecule, post-activation. The time it takes to restore this chemical balance takes can cause a delay. The delay only becomes apparent when we stare at an image (for more than 30 seconds), as the photoreceptors become depleted of deactivated photopigment. This phenomenon becomes manifest if we look at a blank area after staring at an image, whereupon we will see a negative of the image stared at initially.

(For more information refer to Carlson (1993), chapter 4 section 2 Vision.)

6. Word Pendulum

This exhibit demonstrates the phenomenon known as 'Persistence of Vision'. Visitors type in a word up to 8 characters in length and watch the swinging pendulum display this word through its arc. Flashing lights within the pendulum time their flashing so that the visitor actually sees the word.

7. Persistence of Vision

Visitors move a slit in a perspex sheet across a number of different shaped images. The persistence of vision phenomenon makes the shapes appear to be different when viewed through the moving slit than they do when viewed as a whole. For example a circle takes on the image of an oval when viewed through the sliding plate.

The inside story

Word Pendulum and Persistence of Vision are based on the same illusionary phenomenon persistence of vision. This occurs when an image is changing at a rate faster than our visual system is able to detect. As stated in the explanation box above, an image is 'burned' onto the retina and there is a delay until the photopigments are ready to respond to more incoming photons. The brain compensates when such a situation arises by blending the information it is receiving into a single composite image. Movies and television rely on this principle to 'animate' a series of still images, which are flashed before us at a rate of 24 stills per second.

8. Vases and Faces

The Vases and Faces exhibit is a variation of a well-known psychology phenomenon known as the figure-ground illusion.

Visitors view a series of spinning vases and are able to see faces in between the vases. The vases are created so that silhouettes of human heads can be seen in between them. The spinning component of the exhibit makes the faces appear to be talking.

The inside story

Most of what we see can be classified as either object or background. Objects are things with particular shapes and particular locations in space. “Backgrounds are essentially formless and serve mostly to help us judge the location of objects we see in front of them. Psychologists use the terms figure and ground to label an object and its background, respectively. The classification of an item as a figure or as part of the background is not an intrinsic property of the item; rather, it depends on the behaviour of the observer. Sometimes, we receive ambiguous clues about what is object and what is background.” (Carlson, 1993, p137)

The vase and face illusion is an example of an image where the figure and ground can be reversed, however, it is not possible to see both the vases and faces (figure and ground) at the same time. Furthermore images like this example still work no matter how well you understand what is going on. These are called 'perpetual illusions' because they stick around even if you try to get rid of them. These types of illusions are also referred to as 'ambiguous' or 'unstable' because they can correctly be interpreted in different ways and can appear to switch from one tothe other.

The switching pattern of the image proves that the brain is constantly reanalysing information, even if it has processed the same information already. Not only does the brain continuously re-examine all the information it receives, it is capable of coming to different conclusions from identical signals.

9. Cup or Plate

Visitors view a series of objects that represent different stages in a transformation from a cup to a plate. Visitors choose either the cup or plate, view the subsequent objects and then decide when the cup changes to a plate or vice-versa.

The inside story

The brain has a tendency to hold onto a given perception once decided. In the case of the cup or plate, visitors find that depending on the initial object chosen, the perception seems to take longer to change than expected.

10. Cafe Wall

Based on a famous cafe wall in Bristol, England, this exhibit show how the brain relies on distinct patterns, regular outlines and borders to understand what it is seeing.

From a distance, the cafe wall appears to be comprised of different shaped black and white squares arranged in sloping rows. However, if each row of squares is viewed independently, we can tell that all of the squares are the same size and the rows are horizontal. The confusion arises because the rows of squares are not aligned in a regular pattern on top of each other and as a result we perceive a crazy pattern on the cafe wall.

11. Classic Illusions

This is a tabletop exhibit that illustrates a number of classical psychology illusions. In addition to the graphical illusions, the table also holds a 3-dimensional Kanisza Triangle and a green banana that illustrates how background colour can influence our perception of an object's colour.

Those classical illusions displayed include:

• Ehrenstein-Orbison Illusion Impossible Triangle
• Penrose Stairs Impossible Colonnade
• Impossible Frame Impossible Temple
• Schroder Stairs Herman Grid
• McCullough Effects I & II After Effects I, II & III
• Kanisza Polygons Fishes Illusion
• Bear Illusion

The inside Story

Cafe wall and Classic illusions are similar to exhibits 1 and 8. They all demonstrate that perception is a creative process, the organisation of different elements of visual stimulation which involves the brain making assumptions about the incoming information, as well as, the processing of the image sensed by the retina.

The Gestalt psychologists devised a number of laws which relate to the organisation of visual elements and our tendency to perceive elements as belonging together. For example, we perceive even an unfamiliar figure when its outline is enclosed. Furthermore, while the presence of a boundary defines most figures we see, the presence of a boundary is not necessary for the perception of form (see Kanisza Polygons). The law of proximity says that elements of similar shape will be perceived as part of the same form. The law of good continuation accounts for predictability or simplicity and the fact that we have a preference to see smooth patterns. The law of closure states that our visual system will compensate for missing information. The final Gestalt law, the law of common fate, accounts for movement and the fact that elements that move in the same direction will be perceived as belonging together. When, for example, an animal is camouflaged the boundaries of its figure are not distinguishable from the background. When such an animal moves, all the elements that comprise its form move together and we can immediately perceive its form.

12. Neon Phenomenon

Visitors take a perspex grid and hold it over a white background. The grid has a combination of black and neon coloured lines on it. When held over the white background, the brain fills in the non-coloured areas to form a number of different shapes. For example, one grid contains a series of coloured crosses but forms a large doughnut shape when held against the white background.

13. Bendable Pencil

Based on previous knowledge we know that pencils don't bend, but this three metre replica appears to bend between two giant 3-dimensional nuts. Careful painting and lighting has created the illusion here. The nuts are actually made of 2-dimensional folded sections but our previous knowledge about nuts causes us to make the assumption that they are 3-dimensional. This 2-dimensionality makes the pencil appear to bend as is passes through them.

14. Rotating Star

Visitors stand below the rotating star with one eye closed. Without the depth cues that binocular vision gives us, the brain is fooled about the star's shape and the direction of its rotation. This is a very startling illusion that produces gasps of amazement as the visitor's perception of the star shifts.

The inside story

Refer to the Inside Story for exhibit 16

15. Dents and Bumps

Visitors stand in front of the Dents and Bumps wall. They decide which shapes on the wall are dents and which are bumps then press the adjacent button to change the lighting. The areas they thought were bumps actually turn out to be dents and vice versa.

The inside story

'The patterns of light and shadow regions in a scene show its shading can provide us with cues about the three-dimensional shapes of objects. Although the cues provided by shading do not usually tell us much about the absolute distances of objects from us, they can tell us which parts of objects are closer and which are further away. Our visual system appears to interpret such stimuli as if they were illuminated from above.' (Carlson, 1993, p153)

As our common perception about light is that it comes from above, we assume that the light in Dents and Bumps is coming from above. This leads us to misinterpret the dents and bumps because the light is actually coming from below. By pushing the button the light source is changed to above and we see the true picture.

16. Stereo Theatre

Visitors are seated in a mini theatre in front of a screen. They use polarised glasses to watch a 3-dimensional movie. Playing the film through 2 projectors at slightly different angles creates the 3-dimensional effect.

The theatre can seat approximately 12 people while the movie depicts some everyday activities and lasts approximately 3 minutes.

The inside story

The information contained in the text box below was taken from Kandel et al, (1991) Principles of neural science, chapter 30 Perception of Motion, Depth and Form. This 'Inside Story' is applicable to exhibits 16. Stereo Theatre, through to 22. Textured Face Map.

'One of the two major tasks of the visual system is to convert a two-dimensional retinal image into three dimensions. How is this transformation achieved? How do we estimate the relative depth of a three-dimensional object in the visual field? Psychophysical studies indicate that the convergence from two to three dimensions relies on two types of cues: cues for monocular depth and stereoscopic cues for binocular disparity.' (Kandel et al, 1991, p454)

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Monocular Cues Create Far-Field Depth Perception

'At distances greater than about 100 feet, the retinal images seen by each eye are almost identical, so that looking at a distance we are essentially one-eyed. Nevertheless we can perceive depth with one eye by relying on a variety of monocular depth cues. The first four of these were appreciated by the artists of antiquity, rediscovered during the Renaissance, and codified in the 16^th century by Leonardo da Vinci.

1. Previous familiarity. If we know from experience something about the size of a person, we can judge the person's distance.
2. Interposition. If one person is partly hiding another person, we assume the person in front is closer.
3. Linear and size perspectives. Parallel lines, such as those of a railroad track, appear to converge with distance. The greater the convergence of lines, the greater the impression of distance. The visual system interprets the convergence as depth by assuming that parallel lines remain parallel.
4. Distribution of shadows and illumination. Patterns of light and dark can give the impression of depth. For example, brighter shades of colours tend to be seen as nearer. In painting this distribution of light and shadow is called chiaroscuro.
5. Motion parallax. Perhaps the most important of these cues, is not a pictorial cue. As we move our heads or bodies from side to side, the images projected by an object in the visual field moves across the retina. Nearby objects seem to move quickly and in the direction opposite to our own movement, whereas distant objects move more slowly.' (Kendel et al, 1991 p454-455)

Stereoscopic Cues Create Near-Field Depth Perception

'Although monocular cues are important for depth perception at a distance, the perception of depth for near objects less that 100 feet away is also mediated by stereoscopic vision. This involves comparing the retinal images in the two eyes. When we fixate on a point, the image of this point falls upon the center of the retina in each eye. The convergence of the two eyes causes that point to fall on corresponding points on each central retina. The point of focus is called the fixation point; the parallel (vertical) plane of points on which it lies is called the fixation plane (see figure below). The distance of an image from the center of the two eyes allows the visual system to calculate the distance of the object relative to the fixation point. Any point on the object that is nearer or farther than the fixation point will project an image at some distance from the center of the retina. Parts of the objects that are closer to us will be farther apart on the retina in a horizontal direction. Parts of the object that are farther from us will project closer together on the retina.

Because the two eyes are 6cm apart, each eye views the world from a slightly different position. Thus, three-dimensional objects produce slightly different images on the two retinas. This can be clearly demonstrated by closing each eye in turn. A vision is switched from one to the other eye, any near object will appear to skip sideways. We adapt to this disparity by sensory fusion by fixating both eyes on one point. By positioning the eyes so that the left and right images of the object fall on corresponding positions on the two retinas, we see one object. Fusion is not perfect, however, when we fix our gaze on a three-dimensional object. The two retinal images of the object do not fall on exactly corresponding positions. The difference in position, called binocular disparity , depends on the distance of the object from the fixation plane. Thus, points on the three-dimensional object just outside the fixation plane stimulate different points on each eye, and the multiple disparities provide cues for stereopsis, the perception of solid objects.' (Kendel et al, 1991, p455).

17. History of Stereo Photography

Visitors are seated in a theatre and use polarised glasses to watch a 3-dimensional slide show. Playing the slide show through 2 projectors at slightly different angles creates the 3-dimensional effect.

See the inside story for exhibit 16.

18. Confusing Cards

Visitors view 3 identical playing cards through an eye piece in the blue box. They are then asked to move the middle card until they believe it's at the same depth as the other 2 cards. When they have done this, they lift the lid on the box and see if they were correct. They are incorrect more often than not.

The reason why it is difficult to place the third card at the same depth is due to the lack of binocular vision. See The Inside Story for exhibit 16.

19. Three Dimensional Pictures I

Visitors can create a 3-dimensional picture by viewing stereoscopic picture pairs through a prism-like piece of perspex known as a Glinty. The Glinty merges the images when held at the correct distance between the visitor's eye and the picture.

See the inside story for exhibit 16.

20. Three Dimensional Pictures II

Visitors can create a 3-dimensional picture by using a stereoscope.

See the inside story for exhibit 16.

21. Red and Green 3D

A viewing stand ensures that visitors see a picture through a red filter (left eye) and a green filter (right eye). This presents two slightly different images of the picture simultaneously. The result is that the brain merges the two images and creates a 3-dimensional effect.

See the inside story for exhibit 16.

22. Textured Face Map

Visitors view a series of horizontal lines with one eye and attempt to perceive an image from the lines. After this the visitor can look at the lines with both eyes and find the lines form contours of a face. This highlights the role of binocular vision when perceiving depth. It also shows how textures gives us visual information about an object.

See the inside story for exhibit 16.

23. Revealing Shadows

A number of famous people's silhouettes are projected onto a viewing surface for a few seconds. Visitors are then asked to guess who the different people are based on this information.

24. Spinning Stroboscope

A large wheel is printed with frames from a picture that show an action sequence. Visitors spin the wheel and focus on a single section in front of them. A strobe light is timed to illuminate each still as it passes and the stills become animated.

See the inside story for exhibits 6 and 7.

25. Disappearing Colours

With this exhibit, visitors are asked to focus on a point in the middle of a coloured picture. After about 10 seconds the picture fades at the edge of their visual field. As they flick their eyes to the edge of this field, the colour suddenly brightens.

When we view an object or image by focussing on a particular point, the image fades away at the edges of our vision. The receptors in our eye need constant stimulation and as such we make minute movements of our eye when we are looking at something.

The inside story

'Eyes make several kinds of movements. Vision, though popularly thought to be a passive experience (eyes being our windows on the world), requires the behaviour of looking, which consists of moving our eyes and head. The eyes have a repertoire of movements that function for visual perception. Experiments with stabilised images show that the small, involuntary movements keep the image moving across the photoreceptors, thus preventing them from adapting to a constant stimulus'. (Carlson, 1993, p111). Many sensory systems respond better to changing stimuli than to constant ones.

26. Blue Lags Behind

A projector shines 3 coloured dots onto a screen and visitors are able to focus the three dots backwards and forwards. When the dots are moved the blue dot seems to move more slowly than both the red and green dots.

It is not known exactly why this occurs but it is believed to be related to the accuracy with which our brain interprets the colour blue.

27. Stairwell

This exhibit is based on the distorted room concept seen in many centres. Visitors step up to a viewing hole and look into a large oddly shaped box. What they see is a descending stairwell. A friend may step inside the box so that it appears as if they are standing at the top of the stairwell. In fact the stairwell is simply painted onto the back wall to create the illusion of a descending staircase.

The inside story

The creation of the stairwell illusion relies on the manipulation of the visual cues used by the brain to perceive depth and perspective. See The Inside Story for exhibit 16.

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