Here are some of the most effective visual illusions, in JPEG and PDF formats. In each case you can click on the thumbnail to see a full-screen JPEG, while selecting the other link will download the PDF Postscript version, which can be scaled to any resolution without loss of detail. (Our show includes some A0 versions of the illusions.)
Flick your eyes across the picture to enhance the impression that the coils are rotating: it’s the peripheral vision that experiences this effect more strongly.
Experts are still not convinced about a single explanation for this phenomenon, but the shading of the pattern is believed to be an important aspect. The coloured segments are repeated in a definite order: a reasonably dark area (green), then a brighter one (white), a dimmer area (blue), and then the darkest one (black).
It is where these segments meet that is important. Where there is a large contrast difference between the neighbouring segments (the green-white and white-blue borders), this information is processed more quickly by the brain than the neighbouring segments with a small contrast difference (the blue-black and black-green borders). This slight processing delay is similar to what we experience when we see moving objects, so this makes us believe that we are looking at circles that are actually moving.
Genetics plays a part in determining how people perceive this illusion. Different people see rotation in different directions, but relatives tend to see the same as each other. A significant number of people – about one in every four – see no movement at all in this type of illusion.
This illusion was developed by Professor Akiyoshi Kitaoka at the Department of Psychology in Japan's Ritsumeikan University. Visit his website, to see a huge number of examples of his work, including explanations. (Beneath the images on his page you will find PDF links to papers describing the effects, and possible causes.)
This image consists of black squares surrounded by grey lines, with white circles at the intersections of these lines. As you flick your eyes across the picture, you should see dark spots appearing at the intersections.
Some scientists believe that this illusion occurs because of an effect in your eyes, whilst others believe it takes place in your brain. To examine each explanation in turn:
1. If it's an effect in your eyes:
The cells in your eyes that respond to light form small circular groups, and they only respond to contrasts in light. If a cell in the middle of a group detects light, it creates a signal to send back to the brain. If cells on the outside of the group detect light, however, they prevent that signal from being sent.
The grey lines on each side of the white spots produce enough surrounding light to encourage the cells on the outside of the group to respond. They therefore stop the signal that is created by the bright white spot being sent to the brain, so we see it as being darker.
2. If it's an effect in your brain:
The scintillating effect occurs because the brain actually groups information from the cells that respond to light in larger oval groups, and these do not work by responding to contrasts in light. So if one of these oval groups overlaps the white spot, most of it will still be over the black squares. As a result, the brain assumes that the whole group is darker, and the white spots appear darker.
What you should see in this illusion is the centre of the image bulging out towards you, but this is a flat poster and all its lines are parallel, so how can this be?
It's the smaller contrasting squares in each black or white square that cause this effect, but no one really knows how this happens. One idea centres on the small squares, which are the opposite colour to the main one, and are positioned in opposite corners to each other. The colour and close positioning in the corners tricks the mind into thinking that the lines are not at right angles to each other: they are in fact slanted. Each square towards the middle of the grid contributes to this, and helps to build the overall “bulging” illusion.
Anothr theory is that the smaller squares appear to make their own pattern, separate to that caused by the background grid. This interferes with our perception of the larger square grid, causing the lines to appear slanted, and the centre to bulge out.
This was the first optical illusion to show a rotating motion effect. Stare at the central point, and move your head towards and then away from the image. You should see the rings begin to spin: they will move in one direction as you move towards the image, and the direction will reverse as you move away. The speed of the rotational movement will depend on the speed at which you move your head.
Of course, the image is a flat sheet, and the reasons we see movement are complex. First, our peripheral vision may interpret the apparent increase or decrease in size of the images as you move as circular motion.
The positioning of the shapes and of the dark and light edges is likely to play a part too. The visual system creates motion from the differences in contrast across edges – you will notice the light edges of the shapes in one circle are always closest to the dark edges of the shapes in the larger circle around it.
Neurons (nerve cells) in the visual cortex (the part of the brain where your vision is processed) are organised into subgroups, each of which responds best to lines at different specific angles. These neurons will be activated when they detect lines at their preferred angle, and a further subgroup of these neurons will become excited when those same lines move at right angles (90 degrees) to their direction.
The brain determines the angle of objects by "looking" at which groups of neurons are active, and it also works out the direction of motion through the activity of the same neurons. This doubling up of angle information and motion detection works well if a line is moving at right angles to its direction, but if the line is moving in any other way, the brain gets confused.
When you move towards and away from the image, this causes the shapes to move up and down in your vision, so both the direction- and motion-specific neurons are activated. The brain recognises that these neurons are active and therefore perceives that the shapes are moving in a direction as well. The two motions, added together, create an illusion of circular motion.
The image is usually seen as having a central purple disk floating above a green background, especially when you quickly flick your eyes across the image.
This is, however, a flat poster, so the disk and background are on the same level. The floating effect is caused by the processing of horizontal and vertical movements in different ways.
Due to the colouration and pattern, the brain interprets the apparent movement of the disk and the grid differently. For the background, any vertical changes to the image are much more noticeable than horizontal changes; for the disk, this situation is reversed.
As a result, the two parts of the image appear to act differently to each other, so our brain interprets this clash to mean that the disk and the grid are separate objects, and it treats them independently, causing the floating effect.
Stare into the middle of this image. You may see the coloured rings rotating or small flashes travelling around them.
Tiny, involuntary eye movements are believed to be the cause of this effect. These movements (called microsaccades) are happening almost all of the time (when our eyes are open), and they normally aid our peripheral vision, which is the ability to perceive –or “see” – other details in our visual field that are around the main object(s) on which we are focusing.
Researchers believe that – as we make these little movements with our eyes – we create many images of the same object, ever so slightly different to each other, which creates the illusion of movement. Since each of these eye movements is different in size and direction, you will neither see an image that is completely still nor one which is continually moving, but a mixture of the two.
There are three colours: magenta, light green and white. On first glance, however, there appears to be a darker shade of magenta/red, as well as a darker shade of green running from top-left to bottom-right.
It's the placement of the colours side by side that has this aggregating effect: each of the green and magenta squares reinforce the dark shade of the other square. The fact that the size of the squares is small, relative to our viewing distance, also makes the effect more pronounced.
This merging/enhacing effect is used in colour printing, where only a limited number of colours is used to print a picture (traditionally cyan, magenta, yellow and black), but the close proximity of small dots of different colours leads our eyes and brain to see a much larger range of colours.
The 3D dragon Illusion
Click here for the A4 PDF file with the dragon pattern.
Instructions: the sheet in this PDF file should be printed out (preferably on a colour printer), and the instructions followed to make the dragon. (You'll need scissors and tape or glue.) For the Senses show, we have a giant version, used to illustrate the illusion to our audience.
The following short film clip shows the giant version of the dragon model, in our meeting room.
Once you've constructed the dragon from the instructions above, hold it in one hand, close one eye and look at its face. You should, after a few seconds, see it looking forwards at you. Now move it to the left and right, or up and down): the dragon’s gaze should follow you!
When looking at a face, we expect it to be convex, with the nose sticking out towards us. This assumption is so strong that the brain interprets the ways in which the parts of the dragon's head move as being part of a normally shaped head and so our brain assumes that a mistake has been made, when looking at the unusual concave face of the dragon.
This preference of our visual system to see a “normal” face is much stronger than the smaller visual cues our eyes see, such as the slightly abnormal shadows, so we believe that we are indeed looking at a “normal” face.
We make this effect even more vivid by closing one eye: we lose the sense of depth in the picture – one of the important cues we have in looking at the world around us. All of this leads to the strange head movements we experience when we move the dragon (or ourselves) around.
This model, inspired by the work of Jerry Andrus, was created by Binary Arts (now Think Fun) as part of the January 1998 Gathering for Gardner, to celebrate the work and life of the science writer Martin Gardner. The PDF for an A4 sheet (including instructions) can be found at this page.
