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User:Emmap12/The stepping feet illusion

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The Stepping Feet illusion[Note 1] is an optical illusion that is based around strong contrasting colours. This illusion occurs when there are two squares of different colours, traditionally a light and dark grey square, moving together at a constant speed across a background of stationary black and white stripes. The blocks alternatively change from high to low contrast as they move over the stationary background, and as a result, the blocks appear to alternate between high and low speeds.[1] This illusion demonstrates the concept that speed varies with contrast, as the lower the contrast, the slower the blocks of colour are perceived to be moving.[2] Therefore when the dark square is placed on the white stripe, it has high contrast and when it is placed on the black stripe, it has low contrast. As a consequence of these contrasting colours, the two squares appear to speed up and slow down alternatively, which resembles a pair of walking feet.[1] This strong resemblance now accounts for the name of the stepping feet illusion. As further research was conducted, it was found that an important factor in the stepping feet illusion was the contrast of the leading and trailing edges of the grey squares.[1] These edges were found to be responsible for the appearance of walking feet, as initially it was thought that the illusion was produced from the lateral edges.[3]

This type of illusion originally stems from one of the most primitive forms of vision, known as motion perception. This form of vision allows us to keep track of moving objects as well as our own movements.[3] 

Common Types of Illusions edit

There are many different types of illusions that make use of the five human senses. These illusions manipulate the senses and as a result they manipulate how the brain normally organizes and interprets sensory stimulation.[4] Illusions may occur with any of the human senses but visual illusions are most commonly experienced and understood.[4] There are many ways that an illusion can be interpreted, the most common interpretation is that the brain makes assumptions during perception. These assumptions allows us to perceive visual illusions in the way they are intending to be represented. 

List of Common Illusions edit

Below are some of the common types of illusions that make use of human senses:

  • Optical
  • Auditory
  • Tactile
  • Temporal

Optical Illusion edit

An optical illusion is defined as, "a perception, as of visual stimuli that represents what is perceived in a way different from the way it is in reality."[5] This concept can be confusing to interpret as the image that is being presented to the eyes, differs to how the brain interprets the stimulus. The brain interprets the image that is visually seen by the eyes and produces an image that is a false representation of what the reality of the object is.

Common Optical Illusions: edit

 
An example of a literal optical illusion, called "All is Vanity" by Charles Allen Gilbert.

There are three different types of optical illusions that are commonly used. These illusions are commonly used to allow people to have different perceptions of visual stimuli.

They are:

  1. Literal Optical Illusion
  2. Physiological Illusion
  3. Cognitive Illusion
 
An example of a physiological illusion, called "Rotating Snakes" by Akiyoshi Kitaoka.

A literal optical illusion create images that are different from the images that create them. A very common example of this type of illusion is from Charles Allan Gilbert who produced the illusion named, “All is Vanity” in 1892, which at first glance shows a human skull until closer inspection where a lady can be seen sitting in front of a mirror.[6]

The physiological illusion has effects on the eyes and brain, due to excessive stimulation of a specific type, such as brightness, inclination or colour. A common example of this illusion is the “Rotating Snakes” illusion, originally created by Akiyoshu Kitaoka. This image gives the illusion of rotation of ‘wheels’ which is observed in the peripheral vision and the near-peripheral vision.[7]

 
An example of a cognitive illusion, called "The Wall" by István Orosz.

The third illusion type is the cognitive illusion, which is the result of unconscious inferences. These illusions rely on knowledge about the world such as depth in order to interpret these images. A common example is by István Orosz who created the image called “The Wall”, where the rotation of the wall can be looked at in more than one way which oftern have hidden meanings that elicit a perceptual 'switch' between the alternative interpretations, paradoxical or impossible structures, and undecipherable figures.[8]

The stepping feet illusion is known as a cognitive optical illusion as the brain is attempting to make sense of the image it is processing. This results in the illusion of stepping feet as the two grey squares travelling over the black and white stripped background appears to speed up and slow town alternatively, like the two feet of a walker.[1]

Motion Perception edit

Motion perception is the process of inferring the speed and direction of objects that move in a visual scene given some visual input. It is important for both humans and animals to make use of motion perception as it can be beneficial in many real world situations. Motion perception is thought to be a primitive form of vision and play a crucial role between predator and prey.[9]

Real World Applications: edit

One example of a real world application for motion perception is shown through predators in the wild stalking their prey. In the wild gazelles are commonly sought after as prey by other animals. A Gazelle is a species of antelope which has excellent vision for motion that enables them to see predators who may be moving slowly towards them.[3] Gazelles often freeze as a defence mechanism if they are aware that a potential predator is stalking them.[3] This is a deterrent for other species as it is used to outsmart the predator. Gazelles have better motion perception than lions or big cats, who are predators for this species, therefore they are able to detect the incoming stimuli before the lion is aware of the gazelle’s actions.

The human eye is slightly different to some of the species mentioned above as we have not evolved to be exceptionally perceptive in locating moving objects at certain speeds. This is demonstrated by the time that the moon takes to sail across the sky. It is too slow for our eyes to see, as the fastest speed we can interpret is approximately one thousand times faster than this.[3]

Another real world application of motion perception being used, can be seen during the production and creation of motion pictures and television programs. Producers have used their knowledge of motion perception to enhance the reality of the images produced. These images have been conceived by producers who were aware that human beings have a quirk in their visual system.[10] This quirk allowed them to use the visual system to their advantage and create illusions in their films and television programs. For example when there is a rapid series of still images presented to a person, the mind has the ability to "fill in" the gaps between the image frames and therefore create the appearance of continuous motion. This appearance of continuous motion is called apparent motion which helps it to be distinguished from "real" motion, as real motion is defined as an object moving continuously across a viewer's visual field.[10]

One last real world application for motion perception is that in foggy conditions, moving cars appear to be moving slower. This is an illusion and is due to the low contrast of the clouds. As a result of this low contrast, the speed that individuals perceive the oncoming vehicles is slower than if the conditions were clear.[3]

These examples of real world applications are useful to see have a look into how motion perception is applied and used, and the role it can play in both the animal and human visual system.

History edit

Stuart Anstis in 2001 reported that relative motion is much easier to see than absolute motion.[11] This started multiple experiments where participants were asked to report on strengths of illusions and it was found through these experiments that speed depends on contrast.[12] In further studies, Anstis went on to test out different illusions and theories starting with the "stepping feet" illusion, in 2001.[11]

Initial Experiment edit

In the initial experiment to test the validity of the "stepping feet" illusion participants were required to fixate on a computer-controlled monitor screen in a dimly lit room. The stimuli shown to participants was moved along a horizontal path that lay just above the fixation point. The display comprised of a stationary black and white grating of vertical stripes, with a square wave luminance profile. A horizontal grey bar moved smoothly and repetitively to the right across the stripped background at a set velocity.[11]

The participants task was to rate the apparent smoothness of the bar's motion and if it appeared to move at a constant velocity at all times, the rating given would be a 10. On the opposite end of the scale, if the bar appeared to come to a complete halt, the rating given would be a 0. Therefore if a rating of 10 was given, this represented no illusion present and conversely if the rating was 0, this represented that there was a very strong illusion present.

The results from this experiment found that when a grey bar moved across a black and white stripped background there was an illusion present as participants reported that the bar moving across the screen was accelerating and decelerating. It was concluded that the 'footsteps' illusion is dependent on contrast as when the experiment was manipulated to make the grey bar a different shade, participants reported the strength of the illusion being altered. As the bar appeared to move across the screen like footsteps, this experiment became known as the 'footsteps' illusion.[11]

Notes edit

1.:^ Otherwise known as the 'footsteps' illusion.

See Also edit

Further Reading edit

  • Anstis S M, Rogers B J 1975, “Illusory reversal of depth and movement during changes of contrast” Vision Research 15 957-961.
  • Campbell F W, Maffei L 1981, “The influence of spatial frequency and contrast on the perception of moving patterns” Vision Research 21 713-721.
  • Thompson P 1976 Velocity aftereffects and the perception of movement PhD Thesis, University of Cambridge, Cambridge, UK.

External Links edit

References edit

  1. ^ a b c d Anstis, S. (2004). Factors affecting footsteps: Contrast can change the apparent speed, amplitude and direction of motion. Vision research, 44(18), 2171-2178.
  2. ^ Thompson, P., & Stone, L. S. (1997). Contrast affects flicker and speed perception differently. Vision research, 37(10), 1255-1260.
  3. ^ a b c d e f Anstis, S. (2003). Moving objects appear to slow down at low contrasts.Neural Networks, 16(5), 933-938.
  4. ^ a b Aarts, R. M. (2007, April). The application of illusions and psychoacoustics to small loudspeaker configurations. In Audio Engineering Society Conference: UK 22nd Conference: Illusions in Sound. Audio Engineering Society.
  5. ^ Optical Illusion. (n.d.). Dictionary.com Unabridged. Retrieved June 04, 2016 from Dictionary.com websitehttp://www.dictionary.com/browse/optical-illusion
  6. ^ Robinson, C. (2004). Images in nanoscience/technology. Discovering the nanoscale. Ios Press, Amsterdam, 165-172.
  7. ^ Kitaoka, A. (2006). Anomalous motion illusion and stereopsis. Journal of Three Dimensional Images (Japan), 20(4), 9-14.
  8. ^ "Bridges 2008 Exhibit - Istvan Orosz". www.bridgesmathart.org. Retrieved 2016-06-05.
  9. ^ Walls, G. L. (1942). The vertebrate eye and its adaptive radiation.
  10. ^ a b Ramachandran, V. S., & Anstis, S. M. (1986). The perception of apparent motion. Scientific American, 254(6), 102-109.
  11. ^ a b c d Anstis, S. (2001). Footsteps and inchworms: Illusions show that contrast affects apparent speed. Perception, 30(7), 785-794.
  12. ^ Thompson, P. (1982). Perceived rate of movement depends on contrast.Vision research, 22(3), 377-380.
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