7 Illusions

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The Eye optical illusions and diseases
By Catherine Grenier and Érika Fitsimmons
École Joseph-François Perrault, Montreal, June
2001
Content validation and language revision Leïla
Touta ?Science animée, 2001
Translated from French by Nigel Ward
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Contents
Introduction
Structure of the eye
some diseases of the eye
optical illusions
Bibliography
Click a heading
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The structure of the eye
The cornea, iris and sclera (the white part of
the eye) are the external parts of the eye and
are thus visible.The retina, lens, macula and
optic nerve are inside.
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Transparent parts of the eye

• The transparent media of the eye transmit light
  to the retina. The transparent media are listed
  here in order, from the exterior to the interior
• the cornea
• the aqueous humour
• the crystalline lens
• the vitreous humour

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Structure of the eye

• The membranes are listed here in order, starting
  with the outside of the eye and working inwards.
• the sclera a white resistant membrane which
  protects the eye
• the choroid a membrane with many blood vessels,
  it nourishes the eye
• the retina the light-sensitive membrane of the
  eye, it contains nerve cells that absorb light
  and transform it into nerve impulses.

The optic nerve transmits these nerve signals to
the visual region of the brain.
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Diseases of the eye
detachment of the retina
squinting
Click a heading
styes and chalazions
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Detachment of the retina

• the retina is, as you know, like a cinema screen
  and covers the inner surface of the back of the
  eye. If this screen becomes detached from its
  support we call this detachment of the retina.
• it is a serious condition which can lead to
  blindness if not treated.
• it occurs between the ages of 45 and 60.
• it is a painless condition.

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Detachment of the retina
What causes the detachment ?
short-sightedness dangerous degenerative lesions
of the retina the detachment of the back of the
vitreous humour from the retina aphakia (the
absence of the crystalline lens) ocular
traumatisms personal or family history of
detachment of the retina
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Detachment of the retina
the symptoms

• perception of dark or irregular images, due to
  objects floating in the vitreous humour.
• impression of flying insects and of coloured
  lightning (this is a warning sign but detachment
  has not yet occurred).
• impression of looking through a red veil.
• significant loss of sharpness of central vision.

treatment

• surgery
• immobilisation of the eye before and after the
  operation is important
• treatment of the cause of the detachment may be
  attempted.

Return to diseases
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Squinting(being cross-eyed)
squinting is the deviation of one of the
two eyes which no longer looks in the same
direction as the other. it is
accompanied by visual disturbances.
it is a common disorder (4 of children), which
generally appears before the age of 4.
there is a significant genetic predisposition
65 of squinting children come from families
where squinting is present. it is a
serious condition which in 65 of cases can lead
to the loss of vision in one eye this is
called amblyopia or lazy eye. this
amblyopia must be treated very early. After 4 or
5 years, there is minimal hope of healing.
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Squinting
the causes

• Sometimes no cause can be identified.
• It can be a knock-on effect of ocular problems
  such as a cataract which is an obstacle to the
  visual perception.

evolution

• So as not to see double, the child uses only one
  eye, the one that is not deviated.
• The brain is stimulated only by that one eye.
• If the squinting is not treated then the unused
  eye becomes blind.
• After the age of 6 years old, vision can no
  longer be restored to the blind eye.

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Squinting
Treatment
First stage the wearing of special glasses
to reduce the angle of deviation Second
stage (only performed if a visible ocular
deviation persists) surgical treatment.
Treatment of amblyopia is a very important stage
to obtain the same level of sharpness of vision
in both eyes. This last treatment must be
continued for several years to avoid a relapse of
the amblyopia.
Return to diseases
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Styes and chalazions
These are diseases of the eyelids. In our eyelids
we have, in addition to skin and muscle,
eyelashes and oil-producing glands, called
Meibomius glands. Meibomius glands are a
specialised form of sebaceous gland. Every hair
on your body has a sebaceous gland near its base
which coats the hair with an oily substance
called sebum. The Meibomius glands dont have
hairs, instead they squeeze sebum into the tear
liquid that coat your eye to slow down its
evaporation. A stye is a boil on the eyelid. A
boil is an inflammation of a hair follicle caused
by a staphylococcus bacterium. Chalazion is an
inflammation of the Meibomius gland (see photo)
caused by the blocking of the duct which drains
the gland.
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symptoms
The stye an infection of the sebaceous glands at
the base of the eyelashes. While they produce no
lasting damage, they can be quite painful.
A chalazion a small hard lump under the skin,
not very painful.
evolution
The stye the pus comes out by itself and the
stye dries out.
The chalazion can disappear by itself. But it
can come back or get infected.
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treatment
Stye the local application of hot, humid
compresses several times per day and application
of an antibiotic ophthalmic cream.
Chalazion application of an anti-inflammatory
antibiotic cream for 2 weeks. If the nodule does
not regress it is then necessary to remove it by
surgery.
Return to diseases
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Optical Illusions
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Stare at the black points for 30 seconds and then
close your eyes
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What do you see?
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Musician or woman?
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Are the lines straight or curved?
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What colour are the circles?
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If you cannot see a number in each of the
pictures below then you have some kind of colour
blindness and should have a checkup. (These tests
only work if you are looking at this page in
colour, of course!)
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Shifting gears Afterimages of complementary
colors create apparent movement in our peripheral
vision as our eyes shift across the page.
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The red squares are the same color in the upper
part and in the lower part of the "X.
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Three Streams Apparent movement of the streams
is created by afterimages as our eyes shift to
examine the picture.
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Warped Squares? There are no curved lines in
these figures. You can use a ruler to check it
out. The diagonal patterns created by the tiny
squares distort the perception of the pictures.
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Checkerboard with shadow The squares labeled A
and B are the same shade of gray. The illusion
that B is lighter than A is caused by the
relative contrast of the surrounding dark squares
and by the fact that our vision compensates for
the shadow of the cylinder.
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There are no gray spots at the corners of the
squares.
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WORD COLOUR TEST In this test DO NOT READ the
words, just say aloud the COLOUR of each word.
YELLOW BLUE ORANGEBLACK RED GREENPURPLE YELLOW
REDORANGE GREEN BLACKBLUE RED PURPLEGREEN BLUE
ORANGE   This is a type of psycholinguistic
test that poses some difficulty because the
portion of the brain that handles language has
the conflicting tasks of verbalizing the colour
of the written words while ignoring the meaning
of words representing colours.
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Perpetually ascending staircase How can the man
go up all the time? Can such a staircase be built
as a real object?
If you enjoy optical illusions check
out http//www.scientificpsychic.com/graphics/
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Stare at the black dot, then approach the screen
then move away !!
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Bibliography
BRES, Stéphane, CHAMPIN Pierre-Antoine, HERAUD
Jean-Mathias, HERILIER Vincent, JOLION
Jean-Michel LOUPIAS Etienne. (Page consulted 25
April 2001). Traitement dimages et vision
artificielle, Online http//telesun.insa-lyon.f
r
Kerignard, Philippe, Meyer, Lucie. (Page
consulted 30 April 2001). Illusions doptique,
online http //www.chez.com/kerignard/optical35
.htm
Medisite. (Page consulted 25 April 2001).
Maladies des yeux, online http//www.medisite.f
r/pathologies
BERTORELLO, Serge. (Page consulted 15 May 2001).
Techniques dastronomie, online
http//serge.bertorello.free.fr
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Bibliography
Cercle d'Action pour le Dépistage des Troubles
visuels. (Page consulted 26 April 2001).
Amblyopie et Strabisme, On line
http//perso.wanadoo.fr/strabismecadet/somm2
Rossant, Lyonel, Jacqueline Rossant- Lumbroso.
(Page consulted 22 April 2001). le detachement
rétinien, On line http//www.doctissimo.fr/ht
ml/sante/encyclopedie/sa_1572_decol_retine.htm