Vision as an eye structure & theories.pptx

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1. Vision (Eye) Week 7

2. Vision Outline - The eye - Brain regions involved in visual processing - Perception of colors and forms - Perception of orientation and movement

3. Sensation & Perception Sensation: A simple process in which sense organs, in response to stimuli, collect information about the world with the help of the nervous system.’ Perception: is the conscious experience and interpretation of information from the senses and involves neurons in the central nervous system. SENSATION ATTENTION PERCEPTION

4. Mechanism Receptors are located at the extreme external end of the sense organs. These receptors are stimulated after receiving a specific stimulus and pass on message to the brain by generating electro-chemical neural impulses and then carry back the message from there to various parts of the body. Thus, they in collaboration with the nervous system carry out external and internal functions .

5. Types of Sensation Visual Sensation: sense of sight Auditory Sensation: sense of hearing Olfactory sensation: Sense of smell Gustation: Sense of taste

6. Cont … Cutaneous sensation : Skin senses for pressure, temperature, pain (touch) Kinaesthetic sensation : Sense of posture and movement Organic sensation : a sensation (as hunger, thirst, nausea) arising from internal organs.

7. Humans are generally credited with the following senses: (a) vision; (b) audition; (c) smell; (d) taste; (e) touch (or the skin senses)

8. Visual Sensation The Eyes Our eyes are responsible for 80% of all the information our brain receives. That is why, the eye is called the queen of sensation.

9. The sense organ involved in visual sensation is the eye, and it is stimulated by light. Eyes are encased in orbital cavity and each eye is attached to it with six nerves. Eye is a very sensitive organ. Eyelids, eyebrows and eyelashes protect the eye from perspiration and dirt and keep it intact.

10. Visual System The human visual system consists of the eyes, several parts of the brain, and the pathways connecting them. The first stage in vision is, of course, the eye, which contains two systems: one for forming the image and The other for transducing the image into electrical impulses.

11. Structure of the Eye   Blind Spot

12. An analogy is often made between an eye and a camera. While this analogy is misleading for many aspects of the visual system , it is appropriate for the image-forming system, whose function is to focus light reflected from an object so as to form an image of the object on the retina, which is a thin layer of tissue at the back of the eyeball.

13. The image-forming system itself consists of the cornea, the pupil, and the lens. cornea is the transparent front surface of the eye: Light enters here, and rays are bent inward by it to begin the formation of the image. The lens completes the process of focusing the light on the retina (see Figure 4.9). To focus on objects at different distances, the lens changes shape. It becomes more spherical for near objects and flatter for far ones.

14. The pupil, the third component of the image forming system, is a circular opening between the cornea and the lens whose diameter varies in response to the level of light present. It is largest in dim light and smallest in bright light, thereby helping to ensure that enough light passes through the lens to maintain image quality at different light levels. All of these components focus the image on the retina.

15. Structure and Function of the Eye Human eye consists of three layers: Sclerotic Coat Choroid Coat Retinal Coat  

16. 1. Sclerotic Coat Outermost layer Made up of thick white fibrous material called Sclera. Protective layer. i - Cornea: The front of Sclerotic coat is transparent and is known as cornea. Light enters the eye through the  cornea , the clear, curved layer. It helps focus light on the retina at the back of the eye.

17. 2. Choroid Coat Middle layer Black or very dark grey in colour Contains pigments that absorb excess light. Contains thin blood vessels that supply blood to the eye.

18. Cont.. i . Iris Front of the choroid coat. Located behind the cornea. Gives the eye its characteristic colour. The brown, black or blue colour that we see in the centre of eye is in fact the iris.

19. Cont … ii. Pupil Opening in the center of the iris Black dot in the middle of the eye Controls the amount of light entering the eye so it can be focused on the retina to begin the process of sight.

20. Cont … iii. Ciliary Muscles Located behind the iris. Help the iris to move. They help the iris to contract or dilate to control the amount of light.

21. Ciliary Muscles help the iris to contract or dilate

22. Cont … iv. Lens Attached to the ciliary muscles. Biconvex lens lies behind Iris. Convex lens adjust eye for long sight. Concave lens adjust eye for short sight.

23. Cont … v. Aqueous Humour Watery liquid lying between the lens and the cornea. Keeps the eye wet Cornea

24. 3. Retinal Coat Inner most layer Cup-shaped part. Optic nevers are located in it. Very sensitive part (A little excess light entering the eye can damage it). Optic Nerves

25. Cont … i . Vitreous Humour Thick, sticky, transparent fluid which fills the eye chamber. Keeps structure of the eye intact.

26. Cont … ii. Retina The purpose of the  retina  is to receive light that the lens has focused, convert the light into neural signals, and send these signals on to the brain for visual recognition. Contains two types of cells (Rods and Cones) which make vision possible.

27. Difference b/w Photo-receptors RODS CONES Straight and rod shaped Cone- shaped Responsible for vision in low light conditions and Dark Adaptation. Responsible for vision in bright light and Colour vision. Loss of rod cells causes night blindness. Loss of cone cells causes legal blindness

28. Dark Adaptation is the process by which our eyes adjust to darkness after being exposed to light. For example, when we move from a bright, sunny area outside to a relatively dark room inside, it is difficult to see at first. But gradually our eyes recover and become more sensitive to the dim light indoors. Colour Vision is the ability of the eye to discriminate between colors excited by light of different wavelengths. Explanation

29. Cont … iii. Blind Spot Spot on the retina. No rods and cones are present here. Any light entering this spot goes undecteced and no vision takes place.

30. Cont.… iv. Optic Nerve Detects every image on the retina and carries it to the occipital lobe at the back of the brain. The inverted image focussed on the retina gets upright when the optic nerve carries it to the brain and thus, the process of the vision is completed.

31. Brain Regions Involved in Visual Processing

32. Color Blindness It is the inability to see certain colors, is a hereditary condition in which the proteins of one or more cones either do not function or are inadequate in number. Total color blindness is extremely rare. Impaired appreciation of colour can happen. Red green blindness is the commonest type of color blindness. Individual with red and green color blindness cannot distinguish red from green. Transmission is genetical : X linked recessive

33. Types of Color Blindness Monochromacy : Have only one type of cones and perceive one color. (See blue color only) Dichormacy : Have only two types of cones in the retina and perceive two colors. Trichromate: See red, green & blue color Color Blindness: Protoanopia : A person with loss of red cones Deuteranopia : A person with loss of green cones T ritanopia : A person with loss of blue cones

34. Explaining Color Vision Why some people are color blind? Two processes are involved Trichromatic theory of color vision Opponent process theory of color vision

35. How Color Vision Works One receptor is sensitive to the color green , another to the color blue , and a third to the color red . The combinations of these three colors produce all of the colors that we are capable of perceiving. Researchers suggest that people are able to distinguish between as many as seven million different colors. Photoreceptors also tend to have different sensitivity levels. Blue receptors are the most sensitive and red the least; the ability to perceive color requires interaction between at least two types of photoreceptors. These three colors can then be combined to form any visible color in the spectrum.

36. Two complementary theories of color vision are the  trichromatic theory  and the  opponent process  theory. The trichromatic theory, or  Young–Helmholtz theory , proposed in the 19th century by  Thomas Young  and  Hermann von Helmholtz , posits three types of cones preferentially sensitive to blue, green, and red, respectively.    Ewald Hering  proposed the opponent process theory in 1872. It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. Both theories are generally accepted as valid, describing different stages in visual physiology, 

37. Trichromatic Theory of color vision As early as 1802, Young suggested that the eye contained different photoreceptor cells that were sensitive to different wavelengths of light in the visible spectrum. Breaking down the word trichromatic, tri, which means “three,” and chromatic, which means “colored.” It was later in the mid-1800s that researcher Hermann von Helmholtz expanded upon Young's original theory and suggested that the cone receptors of the eye were either short-wavelength ( blue ), medium-wavelength ( green ), or long-wavelength ( red ). He also proposed that it was the strength of the signals detected by the receptor cells that determined how the brain interpreted color in the environment. Helmholtz discovered that people with normal color vision need three wavelengths of light to create different colors through a series of experiments.

38. Trichromatic Theory of color vision Three kinds of cones in retina. Each of which responds primarily to a specific range of wavelength. Blue-violet colors, green, yellow-red. Perception of color is influenced by relative strength with which each of three kinds of cones activate. E.g ; if we see blue sky, the blue violet cones are primarily triggered & other show less activity.

39. Opponent process theory of color vision The opponent process theory suggests that the way humans perceive colors is controlled by three opposing systems. We need four unique colors to characterize perception of color: blue, yellow, red, and green. According to this theory, there are three opposing channels in our vision. They are: blue versus yellow red versus green black versus white

40. We perceive a hue based on up to two colors at a time, but we can only detect one of the opposing colors at a time. The opponent process theory proposes that one member of the color pair suppresses the other color. For example, we do see yellowish-greens and reddish-yellows, but we never see reddish-green or yellowish-blue color hues. The theory was first proposed by German physiologist Ewald Hering in the late 1800s. Hering disagreed with the leading theory of his time, known as the trivariance of vision theory or trichromatic theory, put forth by Hermann von Helmholtz. This theory suggested that color vision is based on three primary colors: red, green, and blue. Instead, Hering believed that the way we view colors is based on a system of opposing colors.

41. Opponent process theory of color vision Receptor cells for color are linked in pairs, working in opposition to each other There are blue-yellow, red-green, black-white pairing If a light contains more yellow than blue, the cells that respond to yellow will be stimulated while blue ones are inhibited & object will appear yellow. Afterimages: A visual image seen after a stimulus has been removed. Negative afterimage: An afterimage whose polarity is the opposite of the original stimulus - Light stimuli produce dark negative afterimages Colors are complementary: Red produces green afterimages, blue produces yellow afterimages (and vice-versa)

42. Opponent process theory provides good explanation for afterimage When we stare at yellow, our receptor cells for yellow of the yellow-blue pairing become fatigued & are less able to respond to yellow stimuli. In contrast the receptor cells for the blue part of the pair are not tired, because they are not tired, when we look at white surface, the light reflected off it would stimulate both yellow & blue. But fatigue of yellow prevents this from happening

43. Opponent process theory versus trichromatic theory Hering’s opponent process theory clashed with the trichromatic theory that dominated his time. In fact, Hering was known to strongly oppose von Helmholtz’s theory. So which is correct? It turns out that both of these theories are necessary to fully describe the intricacies of human color vision. The trichromatic theory helps to explain how each type of cone receptor detects different wavelengths in light. On the other hand, the opponent process theory helps explain how these cones connect to the nerve cells that determine how we actually perceive a color in our brain. In other words, the trichromatic theory explains how color vision happens at the receptors, while opponent process theory interprets how color vision occurs at a neural level.

44. Perception of Orientation & Movement Research with humans and other animals has helped to reveal the role of the extra-striate cortex in visual perception of movement. Damage to this region severely disrupts a monkey’s ability to perceive moving stimuli. Bilateral damage to the human brain that includes area V5 produces an inability to perceive movement— akinetopsia . Instead of a smooth progression of movement in their environment, individuals with akinetopsia experience a series of still images that appear to refresh periodically.

45. Example Patient L. M. had an almost total loss of movement perception. She was unable to cross a street without traffic lights because she could not judge the speed at which cars were moving. Although she could perceive movements, she found moving objects very unpleasant to look at. For example, while talking with another person, she avoided looking at the person’s mouth because she found its movements very disturbing. When the investigators asked her to try to detect movements of a visual target in the laboratory, she said, “First the target is completely at rest. Then it suddenly jumps upwards and downwards”. She was able to see that the target was constantly changing its position, but she was unaware of any sensation of movement.