Vision 1
Slide 2 The obvious analogy for the eye is a camera, and the simplest camera is a pinhole camera: a dark box with light-sensitive film on one side and a pinhole on the other. The image is made by light shining through the pinhole. Rays of light are straight unless they are bent (refracted) by passing through a material like glass: so this makes it obvious why the image is upside down. Slide 3 The pinhole camera passes very little light. To get a brighter image, we can use a lens to form the image. This works by bending the light and we ll see later exactly how this happens. To control the amount of light that gets in, we use the aperture diaphragm. 2
Slide 4 Now we have the same components that we find in our eye. The analogy is made clear in this slide Slide 5 Important structures in the eye 3
Slide 6 A convex lens will focus light onto a point. If light approaches the lens in nearly parallel rays (as light from a very distant object will do) then the distance from the lens to the focal point (the focal length of the lens) is a measure of the power of the lens. The shorted that distance, the more powerful is the lens. The power is measured in diopters (see slide for calculation). Slide 7 So a lens with a high power will focus the light closer to itself. A powerful lens is more convex than a less powerful lens (it s fatter). Slide 8 Summary of where in the eye refraction happens. Note that most of it is in the cornea, with small contributions from the lens. Note also that we can assess the contribution from the front and back of the lens and of the cornea: the back of the lens is more curved than the front and thus refracts more powerfully; the back of the cornea is concave and thus has a negative refracting power. Most significant of all: the cornea is the major refracting element, and this is because the transition from a low to a high refractive index happens at the front of the cornea. Other refracting surfaces have lower power because there is no transition between a low and a high refractive index. 4
Slide 9 Normal visual acuity is defined as shown in the slide. Some people can manage better: 6/6 (or 20/20) vision is simply a useful standard of normal eyesight. Using the simple model of the eye shown here (the reduced eye ), where we ignore the lens for simplicity and put all the refraction in the cornea, we can show that exactly one ray of light from each part of the image will pass through the cornea in a straight line and land on the retina, and that all these straight lines will pass through one point 17mm from the retina. Based on this we can calculate the separation on the retina between two points that we are only just able to distinguish: this separation is about 5 µm, which corresponds to the size of 2-3 photoreceptors (see later slide). Slide 10 There are two common methods of measuring visual acuity, the Snellen chart (made of letters of different sizes) and the Landolt C test shown here. In each case the feature you need to distinguish has a size corresponding to the normal visual acuity (see slide 10) of 1.75mm at 6m (1minute of arc). The test stimuli are designed to be readable to a normal subject at different distances, here 6m and 12m. If you can see at 6m only what a normal subject can see at 12m (and not a smaller one), your visual acuity is defined as 6/12. A Landolt C designed to be seen at 24m would have a gap 7mm wide, and if you could see only that one at 6m your visual acuity would be 6/24 (and so on) Slide 11 Because light rays from close objects are diverging, you need greater refractive power (a thicker, more curved lens) to focus the image of close objects onto the retina. Look at this in conjunction with slide 8: the focal length of the fatter lens is shorter, i.e. its refracting power in diopters is greater. 5
Slide 12 When we focus on near objects, our lens changes in shape like the lens shown in slide 12. This is possible because the human lens, when isolated from the eye, assumes a rounded shape. Most of the time it is held stretched by the sclera, pulling on the suspensory ligaments. Focusing on a near object involves contraction of the circular ciliary muscle, which allows the lens to become more curved. This is the mechanism of accommodation. Slide 13 Accommodation is measured in diopters, just like the power of a lens. If you measure the closest point you can focus clearly (called the near point ), in metres, then the inverse of that is your accommodative power in diopters. Slide 14 Accommodative power declines with age because our lens becomes less able to assume its round shape when relaxed. This is because all proteins in our body become stiffer with age. This decline is not very noticeable up to the late 30s (accommodation ~5 diopters, near point ~20cm) because we seldom want to hold anything closer than that. However in the 40s the decline becomes noticeable as (for instance) we have to hold a book further from our eyes in order to read it. 6
Slide 15 If the eyeball is the wrong shape (or less commonly, the cornea or lens are the wrong shape) we get a refractive error. In hyperopia, the eyeball cannot focus even on distant objects because the image would land behind the retina (even further behind in the case of near objects). More refracting power is needed to focus on the retina; this can be provided by accommodation (slides 12-16), but is tiring over long periods and the decline in accommodation in age makes hyperopia an increasing problem in an older person. Myopia makes it impossible to focus on distant objects (they are focused in front of the retina) although near ones can be focused on the retina. Slide 16 Refractive errors can be corrected by using lenses to increase or decrease refractive power. In myopia we want to focus further back, and thus to have less refractive power. We thus use a concave lens which has a negative refracting power in diopters. In hyperopia we need to focus further forward, and thus to have more refractive power. This can be done with a convex lens, like those shown in slides 7 and 8, which has a positive refracting power in diopters. Astigmatism is too much or too little refacting power in one axis (horizontal or vertical) relative to the other axis. It can also be corrected with additional lenses (see Seeley if you are curious about astigmatism but I won t be asking you about it in the exam; you should however understand myopia and hyperopia). Slide 17 Aqueous humour is produced behind the iris and flows forward through the pupit into the canal of Schlemm and thence into a vein. If the canal becomes blocked glaucoma can result because pressure approaches arterial pressure. Blood flow through the retina can be blocked leading to blindness. 7
Slide 18 The amount of light entering the eye is controlled by the pupil, the round opening in the iris. It gets smaller in bright light, and larger in dim light (just like you d open or close the aperture of a camera to let in more or less light). The iris is controlled by the autonomic nervous system, the sympathetic nerves causing it to dilate and parasympathetic to constrict. Slide 19 Now we move from the optical and muscular components of the eye to the neural components in the retina. This slide shows the neural components of the eye: the retina and the optic nerve, and some associated features Slide 20 This slide shows the basic structure of the retina. The fovea (or fovea centralis, the same thing) is the point on the retina that corresponds to what you re looking at (i.e the point on which your gaze is fixed). The rest of the retina is called the peripheral retina. Note that the photoreceptors (the light-sensitive cells) are on the back of the retina as far as the light is concerned, i.e. light has to go through the retina to reach them. 8
Slide 21 A view of the retina through an ophthalmoscope. This shines a light into the eye and makes an image of the retina. Note: (1) the fovea; (2) the entry point of the optic nerve, which is also where the blood vessels enter the eye. All the blood vessels you can see are in front of the photoreceptors, i.e. the might has to go around them in order to reach the photoreceptors. Note that the peripheral retina has many blood vessels, the fovea few. Slide 22 Sections cut through the retina in the periphery and the fovea. Light has to pass through a lot of structures in the peripheral retina before it reaches the photoreceptors. In contrast, in the fovea, everything else is moved aside so that light reaches the photoreceptors directly. 9
Slide 23 Two types of phtoreceptor cell, rods and cones. We will go into their different functions in later slides. Where the optic nerve enters the eye, the optic disc, is clearly visible because the blood vessels enter there too. Note that there are no photoreceptors at all in the optic disc. This is the basis for the blind spot phenomenon. If you haven t done the online blind spot experiments (on Blackboard), go and do them. Slide 24 Differences between rods and cones. Make sure you understand these. 10
Slide 25 The colour sensitivities of the cone pigments. Slide 26 Deficiencies in distinguishing colours (colloquially, colour blindness ) are caused by missing or abnormal cone pigments. Here, if the red cones are missing, the person will be unable to distinguish colours in the yellow-orange-red range of colours. Practically, this also causes difficulty with some shades of green, and is referred to as red-green anomaly. Slide 27 The Ishihara tests are used to detect colour vision abnormalities. The Ishihara test is constructed by first making one number in blue-yellow contrasting colours (1). Blueyellow colour blindness is very rare so almost everyone will perceive this. Then a different number is made from red-green contrasting colours (2). Combining the two gives a stimulus that will be seen as the number 6 (in this example) by a normal subject, because the redgreen contrast is dominant, but as a 5 by a red-green colour blind subject who can only perceive the blue-yellow contrast. 11
Slide 28 The two optic nerves meet at the optic chiasm (named for its X shape, after the Greek letter chi, χ). Some fibres cross over those from the nasal side of the retina (i.e. the side nearer the nose). This means that the information from the left side of each retina (where the image is formed from objects to our right) is processed on the left side of the brain, and vice versa. Slide 29 The primary visual cortex is at the occipital lobe of the brain (at the back). It detects edges and very simple features of what we see. It is Brodmann s area 17. Other areas nearby form the secondary visual cortex; areas in the secondary visual cortex deal with more complex features of images, like colour, shape and 3- dimensional position. The total area of the brain that deals with vision is very large (see shaded area on slide). Slide 30 In the primary visual cortex, a lot of cells (requiring a lot of the surface area of the cortex) are concerned with processing information from the foveal area, in the centre of the retina, which is the only area we can see clearly (for this reason, and also that shown in slide 22). 12
Slide 31 The main job of the primary visual cortex is to detect very simple features of the visual image, such as edges. The importance of edge detection in identifying objects is nicely demonstrated by this line drawing. Slides 32-34 As well as identifying objects, detecting edges is essential to distinguishing shading and also colour of objects. If we can t see an edge between two areas of a different shade, we can t see that they are different. Follow the animated slides of these illusions in the PowerPoint file on the website. Slide 35 Depth perception (judging how far away something is) can be done to a certain extent with one eye ( monocular depth cues ). If something is in front of or behind something else, or larger/smaller than a similar object, or moving relative to another object, we judge it as nearer or further away. 13
Slide 36 A well-known illusion using monocular depth cues. Slide 37 Because we have two forwardfacing eyes, we can judge distance based on the relative positions on the retina of objects that are nearer or further away. This is called the binocular disparity mechanism; binocular because it uses both eyes, and disparity because the positions of objects on each retina differ with distance. Slide 38 In the real world, binocular disparity would always be combined with other (monocular) depth cues, but it s possible to make a stimulus that uses only binocular disparity using random dots. The dots in the centre of these images are shifted, but those in the surrounding area are in identical positions. This makes the central square look further away or closer if you fuse the two images. 14