Transcript 6 Illusions
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 Click here to begin Contents Introduction Structure of the eye some diseases of the eye optical illusions Bibliography Click a heading 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. 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 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. detachment of the retina squinting styes and chalazions Click a heading 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. 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 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 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. 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. 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 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 don’t 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. 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. 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 Stare at the black points for 30 seconds and then close your eyes What do you see? Musician or woman? Are the lines straight or curved? What colour are the circles? 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!) Shifting gears: Afterimages of complementary colors create apparent movement in our peripheral vision as our eyes shift across the page. The red squares are the same color in the upper part and in the lower part of the "X“. Three Streams: Apparent movement of the streams is created by afterimages as our eyes shift to examine the picture. 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. 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. There are no gray spots at the corners of the squares. WORD COLOUR TEST In this test DO NOT READ the words, just say aloud the COLOUR of each word. YELLOW BLUE ORANGE BLACK RED GREEN PURPLE YELLOW RED ORANGE GREEN BLACK BLUE RED PURPLE GREEN 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. 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/ Stare at the black dot, then approach the screen then move away !! Bibliography BRES, Stéphane, CHAMPIN Pierre-Antoine, HERAUD Jean-Mathias, HERILIER Vincent, JOLION Jean-Michel LOUPIAS Etienne. (Page consulted 25 April 2001). Traitement d’images et vision artificielle, [Online]: http://telesun.insa-lyon.fr Kerignard, Philippe, Meyer, Lucie. (Page consulted 30 April 2001). Illusions d’optique, [online]: http ://www.chez.com/kerignard/optical35.htm Medisite. (Page consulted 25 April 2001). Maladies des yeux, [online]: http://www.medisite.fr/pathologies BERTORELLO, Serge. (Page consulted 15 May 2001). Techniques d’astronomie, [online]: http://serge.bertorello.free.fr 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/html/sante/encyclopedie/sa_1572_decol_retine.htm