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Introduction

 

This report
will investigate optical- and auditory illusions. It will explain how the eyes
and ears work, and how the illusions trick them

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Theory

 

Light and
Sound

To be able
to understand how an illusion works, it is essential to understand how light
and sound is perceived.

 

Vision depends on our
brain, as much as it depends on our eyes. Our eyes main job is to detect
patterns of light, and then it works with our brain to turn these patterns into
images.

 

The way it
does this, is that the light rays enters your eyes by going though the cornea,
the aqueous, the pupil, the lens, the vitreous, and then striking the light
sensitive nerve cells in the retina. The light sensitive nerve cells are often
referred to as the rods and cones (Wonderbaby.org, 2013).  In the retina the visual process begins. The
energy created from the light source creates a chemical change in the light
sensitive cells, placed in the retina. These cells produce electrical activity.

The nerve fibers from these cells then connect to produce the optic nerve, at
the back of the eye.  The optic nerve
from the two eyes meets each other at the optic chiasm. The overlap of these nerve
fibers creates depth perception (Wonderbaby.org, 2013).

 

Sound
however, is changes in pressure over time, transmitted in a medium such as air
or water (Yantis and Abrams, 2015).  

Yantis and
Abrams state in their book Sensation and Perception that:

“The three most
important physical dimensions of sound are frequency, amplitude and waveform.

They are most important because each of these dimensions is closely related to
one of the major perceptual dimensions: pitch, loudness and timbre” (2015).

 

The
frequency is, what this reports auditory illusions will focus on. Frequency
refers to how often the air particles vibrate. 
A high pitch sound is what is referred to as a high frequency. Tones
with high frequencies sound high pitched, and tones with low frequencies sound
low pitched (Yantis and Abrams, 2015), see figure 1.

Figure 1 – The difference between high and low frequency tones.

 

 

 

 

Optical
illusions:

 

The Penrose
Stairs:

The Penrose
Stairs, named after Lionel Penrose, is an impossible staircase. It is a two
dimensional staircase with four 90 degree turns. It is often referred to as the
impossible staircase, since it cannot exist in real life. It tricks our mind to
think that there is never ending upwards or downwards staircase, depending on
which way one would walk on it. The illusion tricks the way we perceive depth.

Figure 2 –
The Penrose Stairs.

 

The Penrose
stairs cannot be built in the real world, but an illusion of it can be built.

Figure 3 and figure 4 demonstrate the same staircase from two different
perspectives and lightings. By using the same principles as used in the
illusion “Ames Room” (Yantis and Abrams, 2015), the observer will be tricked
into thinking that each step on the stairs are the same size, when in reality
they keep getting larger. However this illusion only works when looking from a
certain viewpoint, as you can see on figure 3. 

 

Figure 3
& 4 – Penrose stairs from different viewpoints

 

The Penrose
Stairs works by leading the observer to perceive that the last step is beneath
the first step. The observer is also required to look at the Penrose Stairs
with one eye at the viewpoint in order to eliminate the binocular depth cue.

Just like Ames Room, all the monocular depth cues do lead the observer to an
incorrect perception of the staircase (Yantis and Abrams, 2015).

 

Auditory
Illusions

Where an
optical illusion is an illusion that tricks our eyes, an auditory illusion
tricks our ears. An auditory illusion can trick your mind into thinking there
is a sound, which is not present, or that it hears an “impossible” sound. Auditory
illusions highlight areas where the human ear and brain, as biological survival
instruments, differ from perfect audio receptors.

 

 

Shepard
Tone illusion

The Shepard
tone, which is named after its creator Roger Shepard, is made out of several
sine waves that play a trick on the brain. What it does is continually rise in
pitch and then every so often one of the waves drops down an octave, see figure
2. The brain does not perceive that, because the other sine waves are still
rising, and it then tricks people into believing it keeps getting higher in
pitch. This auditory illusion appears to keep rising in pitch indefinitely
(Shepard, 1964). The Shepard tone, does what the old barber poles does, just in
sound, see figure 3.

 

 

 

Figure 2 & 3 – Example of a Shepard tone Illusion, and a barber pole
(check link for full visual experience of the barber pole: https://openclipart.org/download/282100/BarberShop-Pole-RedBlue-2.svg
)

 

Discussion

 

The Shepard
Tone It was often used in old movies to portray the sound of an UFO1,
but is also commonly used to portray tension in newer films. World famous
composer Hans Zimmer uses the Shepard tone in many of the soundtracks he
creates for film, such as “The Prestige” and “Dunkirk” (Guerrasio, 2017).

Christopher Nolan, which is the director for the film “Dunkirk” stated in an
interview to BuisnessInside that:

“It’s an illusion
where there’s a continuing ascension of tone. It’s a corkscrew effect. It’s
always going up and up and up but it never goes outside of its range. And I
wrote the “Dunkirk” script according to that principle. I interwove
the three timelines in such a way that there’s a continual feeling of
intensity. Increasing intensity. So I wanted to build the music on similar
mathematical principals.”

Besides
film it has also been used in videogames. It has been used in Super Mario 64,
which was created in 1996. It is used in a stage of the game that is called
“The Infinity Stairs”. In this stage the main character Mario has to climb the
stairs, but it is impossible since the stairs are infinite, see figure 4. The
Shepard tone assists the atmosphere, and creates the illusion that Mario keeps
climbing higher.

 

Figure 4 – “The infinity stairs” Also known as “The Endless Stairs” in
Super Mario 64. See video example here: https://www.youtube.com/watch?v=70J8SOX5w64

 

 

Summary and conclusion

 

 

Biblography:

 

Guerrasio, J. (2017). Christopher Nolan explains the ‘audio
illusion’ that created the unique music in ‘Dunkirk’.

Nordic.businessinsider.com. Retrieved 11 January 2018, from
http://nordic.businessinsider.com/dunkirk-music-christopher-nolan-hans-zimmer-2017-7?r=US=T

 

Shepard, R. (1964). Circularity in Judgments of Relative
Pitch. Asa.scitation.org. Retrieved 11 January 2018, from
http://asa.scitation.org/doi/abs/10.1121/1.1919362

 

Wonderbaby.org. (2013). How The Eye And The Brain Work
Together. online Available at:
http://www.wonderbaby.org/sites/wonderbaby2.perkinsdev1.org/files/eye-brain.pdf
Accessed 21 Dec. 2017.

 

Yantis, S. and Abrams, R. (2015). Sensation and Perception.

2nd ed. New York: John Wiley & Sons.

 

 

1 UFO – Unidentified flying object 

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