How Do Optical Visual Illusions Work?
Optical Visual Illusions
When we look at an optical illusion we are viewing a stationary image, a pattern of shapes and lines, colours and contrasts, in a two-dimensional form. However, these images appear to move.
In some illusions, shapes appear to switch dimensions before our eyes. In others where patterns are involved, the more we stare at them the more lines begin to swirl, circles start to rotate and dots seem to shiver and dance.
We know logically this cannot be, but our eyes tell us it can. Exactly how optical visual illusions work is still partly a mystery. However, scientists just love illusions and have been piecing together just what is going on with these illusionary images.
How Do Our Eyes See Optical Illusions?
When we stare at an image, that pattern is kept in our retina as an after-image for a short amount of time in case it is needed by the visual system after we have looked away.
Our eyes are always moving and they make regular quick small movements called microsaccades.
It is possible that these microsaccades may be causing this retained image to overlap with the actual image on the page.
When this happens, the lines and boundaries of the shapes and patterns do not match up exactly, therefore what we perceive is an overall image that moves, like ripples in water.
How the images move depends on the shapes and lines within the image. The more high contrast the colours or shades are the stronger this effect will be.
Optical Visual Illusions and Visual Perception
When we look at something, our eyes take in this information into our visual system. This is due to the light reflecting onto the retina, a light sensitive membrane at the back of the eye.
- It is the receptor cells in the retina that translate the light into images
- The optic nerve can then take this information up into the brain’s visual cortex
- We detect motion through fast changing light patterns into our eyes
- The strong contrasts and sharp sudden boundaries between them provide visual information suggesting movement.
The Science of Illusions
Backus and Oruc (2005) suggest neuron firing rate within our brain adjusts once the brain has got used to the stimulus it is receiving. Neural activity for high contrasting regions adapt faster than for a low contrasting regions.
This results in the firing rate of neurons slowing down much quicker in response to the high contrasting areas within an image. This change in activity as the eye moves over the different regions may be detected by motion sensors within the primary visual cortex giving rise to the perception of movement within the image, even though it is not actually moving.
The Rotating Snakes Optical Visual Illusion
Can you see the 'snakes' rotating?
“Rotating Snakes” is one of the best and most well-known optical illusions available.
For most people, when looking at the rotating snakes image, sections appear to move in a circular motion, similar to disks slowly turning or rotating.
If you look around the image, the snakes continue to rotate. However, if you fixate on one section of the image, after around 5 seconds, the motion stops and the image appears stationary again.
This suggests that eye movements are an important part of how this illusion works. Certainly the more you look away or to the side of the image and back again, the more movement appears within the image.
What do you see when you look at the “Rotating Snakes” image?
Illusions: What if they don't move?
There are some people who do not see any movement within this illusion and others like it.
This is not unusual and has been demonstrated in many of the scientific studies which have been carried out into such illusions.
- Fraser and Wilcox (1979) tested 678 participants using their staircase illusion and 28% reported no movement within the image
- Faubert and Herbet (1999) tested five participants using their adapted staircase illusion and half reported seeing no movement
How do Optical Visual Illusions Work? – Patterns Within The Image
The “Rotating Snakes” illusion is made up of small blocks of colour arranged in multiple overlapping circular rings.
The patterns and shades of colour used have been shown to be crucial to how we perceive these images as moving.
The sequence of colour used in the original image is Black-Blue-White-Green. This sequence repeats in a pattern around each circular snake ring in an anti-clockwise direction (right to left) making up the image.
How do Optical Visual Illusions Work? - Directional Rotation
What is also unique about the "Rotating Snakes " illusion is that the rings all rotate as complete circles and not individual sections of the rings moving separately.
According to Backrus and Oruc (2005), the visual system has sensors which operate in the background and seek out larger pattern areas of movement within the image. Therefore, multiple small areas within the ring that are seen as movement can be perceived collectively giving the impression of the entire ring (or ‘snake’) moving in one piece.
It is important that the contrast sequence is in the same direction throughout the ring. For example, in the colour version, if the contrast sequence is a repeated pattern of Black-Blue-White-Green from left to right throughout, this is the direction the ring will be perceived to move. If however, some of ring has this sequence but some has the sequence in the opposite direction, there will be very little perceived motion at all.
Colour vs Grey-Scale in Illusions
The colours themselves are not the important factor giving the perception of motion. A grey-scale version of the image also gives a sense of movement with the same properties however, the illusion may not be perceived as strongly.
Conway et al (2005) conducted the first full scale study on the influence of colour and strength of such illusions and found no significant effect. It appears therefore that it is the shading and direction of shading that is important within the repeating sequence and not the actual colours.
How Do Optical Visual Illusions Work? - Contrast
Contrast appears to be more significant in the perception of motion within the image than colour.
No one really knows why a grey-scale version of an illusion is not as strong as a colour version. Backus and Oruc (2005) highlight in the Rotating Snakes illusion, that black against white would be high contrasting, generating a higher level of neuron activity within the visual cortex, compared to the low contrasting areas of blue and green.
They suggest this adaptation of the neuron firing rate may be involved, where the firing rate does not slow down as fast for grey-scale regions as it does for colour regions resulting in a less powerful illusion.
Eye Movement Saccades and the Rotating Snakes Illusion
Otero-Millan et al (2012) studied the "Rotating Snakes" illusion and found a strong relationship between when subjects perceived the illusion to start to move and micro-saccades, our involuntary small and fast eye movements.
- Eight subjects eye movements were monitored
- Subjective reports were collected of when they saw the image move and when they didn't through button presses
- The researchers found that just before subjects reported movement within the image, blinks and micro-saccades were detected via eye tracking
- Just before they reported the snakes stopped moving, their eye movements were stationary on one area
The purpose of these rapid eye movements appear to be to ensure we obtain refreshed data from the image at a constant level even when we have consciously chosen to fixate on one part of the image.
Generally our brain is able to distinguish between things that are actually moving in the real-world and a perception of movement due to the movement of our eyes.
On a basic level the boundaries between regions of colour within these illusions are strong and the shapes are repetitive which confuses this system, activating motion sensors within the visual cortex and movement of the image is perceived when it is not really moving.
Illusions and Our Brain
Optical illusions are an area that we may never fully understand. The "Rotating Snakes" illusion is a strong example of how our visual system confuses what it is really seeing with internal representations, most probably as part of our visual processing networks.
Our visual system, neural activity and internal representations are complex. While patterns, colours and contrasts appear to be very important in how optical illusions work, there are still many questions unanswered.
Such mystery does serve to add to the interest within visual illusions and in the meantime, while researchers continue with the science, we can enjoy the magic and marvel at the ever increasing examples of optical illusions that are now being produced.
© 2014 Fiona Guy