Vision: Lecture Objectives

This component of the Lecture will have these learning outcomes: • Introduce the concept of vision ~5 mins. • Disseminate a few (ir)relevant facts about the eye ~5 mins. • Understand the working of the eye ~10 mins. • Introduction to Illusions ~15 mins. • Optical Illusions ~20 mins. • Depth Perception and Movement Detection~15 mins. Vision: Lecture Objectives Damian Schofield

Lecture Objectives Damian Schofield

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996 … nothing could be easier than seeing. We just point our eyes where we want them to go, and gather in whatever there is to see. Nothing could be less in need of explanation. The world is flooded with light, and everything is available to be seen. We see people, pictures, landscapes, and whatever else we need to see, and with the help of science we can see galaxies and viruses and the insides of our own bodies. Seeing does not interfere with the world or take anything from it, and it does not hurt or damage anything. Seeing is detached and efficient and rational. Unlike the stomach or the heart, eyes are our own to command: they obey every desire and thought. SO ARE THESE STATEMENTS TRUE ? Seeing is Believing ? Damian Schofield

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996 Each one of those ideas is completely wrong. The truth is more difficult: seeing is irrational, inconsistent, and undependable. It is immensely troubled, cousin to blindness and sexuality, and caught up in the threads of the unconscious. Our eyes are not ours to command: they roam where they will and tell us they have only been where we have sent them. No matter how hard we look, we see very little of what we look at…. Seeing is Believing ? Damian Schofield

Seeing is Believing - Example Damian Schofield

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996 … Even when I am not thinking of the use of objects, they remind me of use. And there is a curious thing here that easily passes unnoticed: I do not focus on anything that is not connected in some way with my own desires and actions. I fail to see the stretches of wall between the lamp and the coffee cup, or the manila paper of the file folders, or the black plastic calendar holder. My eyes can understand only desire and possession. Anything else is meaningless and therefore invisible. Seeing is Believing - Example Damian Schofield

Biological vision is the process of using light reflected from the surrounding world as a way of modifying behavior. Generally, with humans, we say that the surrounding environment is interpreted by visual input. This usually implies some form of conscious understanding of the three-dimensional (3D) world from the two-dimensional (2D) projection that it forms on the retina of the eye. Biological Vision Damian Schofield

Solving the problem of converting light into ideas, of visually understanding features and objects in the world, is a complex task far beyond the abilities of the world's most powerful computers. Much of our visual computation is carried out unconsciously and often our interpretations can be fallacious. Vision requires distilling foreground from background, recognising objects presented in a wide range of orientations, and accurately interpreting spatial cues. Biological Vision Damian Schofield

In this part of the course we will briefly overview the human visual system and try to understand the ways in which this system interprets its input. Although not strictly correct, the analogy between machine vision and biological vision is currently the best model available. Biological vs. Machine Vision Damian Schofield

We use the human visual system as an existence proof that visual interpretation is even possible in the first place, and its response to optical illusions as a way to guide our development of algorithms that replicate the human system; We use our understanding of machine vision and our ability to generate ever more complex computer images as a way of modifying, or evolving, our visual system in its efforts to interpret the visual world. Biological vs. Machine Vision Damian Schofield

The eye is considered by most neuroscientists as part of the brain. It consists of a small spherical globe of approximately 2cm in diameter, which is free to rotate under the control of six extrinsic (or extraocular) muscles. The tough, outermost layer of the eye is called the sclera. It maintains the shape of the eye. The front sixth of this layer is clear and is called the cornea. All light must first pass through the cornea when it enters the eye. Eye Facts (1) Damian Schofield

The choroid is the second layer of the eye. It contains two structures: • The ciliary body - The ciliary body is a muscular area that is attached to the lens. It contracts and relaxes to control the size of the lens for focusing. • The iris - The iris is the colored part of the eye. The iris is an adjustable diaphragm around an opening called the pupil. • Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material called vitreous humor. The smaller, front section contains a clear, watery material called aqueous humor. Eye Facts (2) Damian Schofield

The lens is a clear, bi-convex structure about 10 mm (0.4 inches) in diameter. The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision. The eye is unique in that it is able to move in many directions to maximise the field of vision, yet is protected from injury by a bony cavity called the orbital cavity. The eye is embedded in fat, which provides some cushioning. Eye Facts (3) Damian Schofield

The eyelids protect the eye by blinking. This also keeps the surface of the eye moist by spreading tears over the eyes. Eyelashes and eyebrows protect the eye from particles that may injure it. Tears are produced in the lacrimal glands, which are located above the outer segment of each eye. The tears eventually drain into the inner corner of the eye, into the lacrimal sac, then through the nasal duct and into the nose. That is why your nose runs when you cry. Eye Facts (4) Damian Schofield

Pupil size can change from 2 millimeters to 8 millimeters. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times. Pupil size between 6 and 8 mm may indicate the use of marijuana, cocaine, crack, meth, hallucinogens, crystal, ecstasy or other stimulants. Pupil size between 1 and 2 mm may indicate the use of heroin, opiates or other depressants. Lastly, Pupils ….. Damian Schofield

We subconsciously pick up clues from others’ pupil sizes and use them to help us form opinions about people. Eckhard Hess, a Chicago biopsychologist, performed a study in which men were shown retouched photographs of women. In half the photographs, the pupils were made to appear larger than normal, and in the other half they were smaller. The men in the study invariably perceived the women with larger pupils as being more attractive and friendlier than the very same women when shown with smaller pupils. Lastly, Pupils ….. Damian Schofield

We subconsciously pick up clues from others’ pupil sizes and use them to help us form opinions about people. Eckhard Hess, a Chicago biopsychologist, performed a study in which men were shown retouched photographs of women. In half the photographs, the pupils were made to appear larger than normal, and in the other half they were smaller. The men in the study invariably perceived the women with larger pupils as being more attractive and friendlier than the very same women when shown with smaller pupils. So, following logically from this, does the use of drugs make you more attractive to potential sexual partners ? Lastly, Pupils ….. Damian Schofield

The Human Eye Damian Schofield

The retina is complex in itself. This thin membrane at the back of the eye is a vital part of our ability to see. • Its main function is to receive and transmit images to the brain, by converting light into neural signals that can be relayed to the cortex through the optic nerve. • The retina consists of a team of five types of cells whose role it is to collect light, extract basic information about color, form, and motion, and pass the pre-processed image on to centers in the brain. • These cell types are: • photoreceptors, • bipolarcells, • horizontalcells, • amacrinecells, • ganglioncells. The Retina Damian Schofield

The Layers of the Retina Light passes all the way through the retina before reaching the photoreceptor cells at the back. Photoreceptors convert light signals into neural impulses that are relayed to a variety of other cells types in the retina for processing. The ganglion cells at the front of the retina are the final relay station in the eye, and they pass signals into the brain via the optic nerve. The Retina Damian Schofield

The location of the optic nerve on the retina obviously prohibits the existence of photoreceptors at this point. This point is known as the blind spot and any light that falls upon it is not perceived by the viewer. Most people are unaware of their blind spot, although it is easy to demonstrate that it exists. The Retina Damian Schofield

Why not try decapitating your friends? Where the optic nerve exits the eyeball is a gap in the light-sensitive retinal cells, creating a "blind spot." You can locate your own spot by marking two X's side by side on a sheet of paper about three inches apart, then closing your right eye and viewing the X on the right with your left eye. Start with the paper about a foot and a half from your face, then bring it slowly toward you. At some critical distance (which you may need to "hunt" for), the left X will fall entirely on your blind spot and disappear. What good is all this? Now you're ready to play the decapitation game: From about 10 feet away, shut your right eye and stare at your friend’s head with your left eye, then slowly move your open eye horizontally toward the right until the head falls onto your blind spot and disappears. "When King Charles II heard about the blind spot, he took great delight in walking around his court decapitating his ladies in waiting or beheading criminals with his blind spot before they were actually guillotined.” Vision Worksheet 01 Damian Schofield

Why not try decapitating your friends? X X Vision Worksheet 01 Damian Schofield

The Fovea The fovea is the region of the retina that allows us to see detail. At the fovea, the top two layers of the retina thin out, allowing light to fall directly onto the photoreceptor cells. Foveal photoreceptors are mostly cone cells, meaning that the fovea is particularly good at seeing color in daylight. At night, the activity of color-insensitive rod cells in the periphery of the retina dominates vision. The Retina Damian Schofield

An image shining upon the retina traverses these layers to reach the photoreceptor cells, which absorb the incoming light and transform it into electrochemical signals. Photoreceptors are divided into two subtypes, rods and cones. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more cone shaped. Rods and Cones Damian Schofield

Rod cells are very sensitive to changes in contrast even at low light levels (they can detect a single photon) and can create black and white images without much light, but consequently are imprecise in detecting position (due to light scatter) and insensitive to color. Rods are generally located in the periphery of the retina and used for night vision.. Rods and Cones Damian Schofield

Once enough light is available (for example, daylight or artificial light in a room), cones give us the ability to see the colour and detail of objects. These are the high-precision cells that are specialised to detect red, green, or blue light. They are generally located in the center of the retina in a region of high spatial acuity (the fovea). Cones are responsible high acuity tasks like reading (they allow you to read this slide) they allow us to see at a high resolution. Rods and Cones Damian Schofield

Rods and Cones Damian Schofield

The photoreceptors are in a continuous turnover, and the outer segment ‘discs’ have a turnover of around 9 to 13 days. The rod disc shedding is maximal in the morning, and cone shedding maximal at dusk. These are probably mediated through melatonin. The information received by the rods and cones are then transmitted to the nearly 1 million ganglion cells in the retina. These ganglion cells interpret the messages from the rods and cones and send the information on to the brain by way of the optic nerve. Rods and Cones Damian Schofield

Signals from the photoreceptors pass forward into the next layer of the retina containing horizontal, bipolar, and amacrine cells. The Visual Pathway Damian Schofield

Signals from the photoreceptors pass forward into the next layer of the retina containing horizontal, bipolar, and amacrine cells. These cells form small networks that are able to extract information about form and motion from an image. The neurons in these two retinal layers exhibit complex receptive fields that enable them to detect contrast changes within an image; these changes might indicate edges or shadows. That information continues to the front of the retina where it is received by a layer of ganglion cells. The Visual Pathway Damian Schofield

The optic nerves within each eye meet in the front part of the head at a point called the optic chiasm, which functions like a cloverleaf on a highway. All the fibers from the left half of each retina turn towards the right side of the brain, and the fibers from the right half of each retina turn towards the left side of the brain. The end result of this is that the left half of the brain looks at the right visual world, and the right half of the brain looks at the left visual world. The Visual Pathway Damian Schofield

First stages of the visual pathway. A series of splits and crossing in the first parts of the visual system may appear complicated, but the end result is simple: the left half of the brain receives information about the right half of the world (in this case the spoon handle), and the right half of the brain receives information about the left half of the world (here the spoon tip) The Visual Pathway Damian Schofield

A small group of fibers in the optic nerve splits off and travels down to brainstem nuclei, which are groups of cells governing reflex actions. Those fibers mediate automatic responses, such as adjusting the size of the pupil, blinking, and coordinating the movement of the eyes. The majority of fibers in the optic nerve take another path that leads to the very back of the brain, to a part of the occipital lobe called primary visual cortex, or V1. The Visual Cortex Damian Schofield

The primary visual cortex (V1) is part of the cerebral cortex. The V1, also called striate cortex because of the distinctive stripe it bears, is responsible for creating the basis of a three-dimensional map of visual space, and extracting features about the form and orientation of objects. Once basic processing has occurred in V1, the visual signal enters the secondary visual cortex, V2, which surrounds V1. The Visual Cortex Damian Schofield

The cerebral cortex is the extensive outer layer of gray matter of the cerebral hemispheres, and is involved in higher brain functions, including sensation, voluntary muscle movement, thought, reasoning, and memory. The grooves on the brain's surface results in much of the cortex being buried; over 60% of the cerebral cortex in primates is buried and not visible from the surface. Cerebral cortex is typically 2-4mm thick and folded, but if unfolded it has the same area as a 21” television. The Cerebral Cortex Damian Schofield

Illusions That Won’t Go Away. Stare at some sights long enough, and you'll continue to see an afterimage even after you've looked away. Now vision scientists report that the brain can see afterimages of things that weren't even there in the first place, but were created by optical illusions. The cerebral cortex is responsible for the trick. Afterimages come in two basic types: those formed on the eye's retina and those conjured up in the brain. For example, a bright light can bleach pigments in the retina and cause the retina's neurons to adapt; these effects linger after the light disappears, creating an afterimage. But most other aftereffects, such as continuing to hear a sound when it has ceased or seeing color-based patterns after looking away, are formed in the brain's cerebral cortex. Now cognitive scientist Shinsuke Shimojo and his colleagues at the California Institute of Technology in Pasadena have discovered that the cerebral cortex can create afterimages even of illusory surfaces. They worked with a well-known effect called perceptual color spreading, or filling in. If you stare at the illustration long enough, you will start to see a box between the four circles with wedges cut out of them (researchers call them pacmen for their resemblance to the video game heroes). Shimojo and his colleagues noticed that if you shift your gaze to the blank area at right, you continue to see the box. Illusions, they found, can create afterimages. Then they fiddled with the pacmen to find out whether the newfound perception relies more on the retina-based afterimages created by the pacmen or on a cortex-based process of filling in. The cortex won, they report in the 31 August issue of Science. The demonstration that illusions can create afterimages is "cute," says vision scientist Mary Hayhoe of the University of Rochester in New York state. "No matter how much we study perception we still find it puzzling when we see something that isn't there," Hayhoe says. Visual Worksheet 02 Damian Schofield

Illusions That Won’t Go Away. Visual Worksheet 02 Damian Schofield

Visual acuity is a measure of the spatial resolving power of the visual system; it indicates the angular size of the smallest detail that can be resolved. Visual acuity is measured for various purposes. When determining the appropriate eyeglasses, the corrective lens power that permits the best visual acuity is prescribed. Visual acuity measurements are also used by some licensing authorities and employers as eligibility criteria for some occupations (e.g., airline pilot, police officer) and activities (e.g. driving). Visual acuity has traditionally been used as the primary indicator of the magnitude of functional impairment due to vision loss. Visual Acuity Damian Schofield

Visual Acuity Damian Schofield

The notation of visual acuity is written as a fraction, with normal vision being 20/20 (twenty twenty vision). At a 20 foot distance, (the top number in the fraction, or testing distance), a person with normal vision should be able to read the small 20/20 line on an eye chart. The smallest line that you can read on the chart is your visual acuity. If larger lines than the 20/20 line are all that can be read, the visual acuity may be 20/30, 20/60, etc. The larger the second number is, the worse is the vision. A person with 20/200 vision would have to come up to 20 feet to see a letter that a person with normal vision could see at 200 feet! If the vision is 20/10, it means that the vision is better than normal. A person with 20/10 vision can read a letter at 20 feet that a person with normal vision would have to come up to 10 feet to read. Certain visual acuities have special significance. Some of these are • 20/20 vision is considered normal vision • 20/40 vision in at least one eye is the vision required to pass the driving test • 20/200 vision or worse is the legal definition of blindness Visual Acuity Damian Schofield

Vision Testing Instructions The blue "standardization bar" must be measured in centimeters on your computer screen or printout. You should then stand in a position back from your computer screen so that your eye is this same number of feet from the screen. i.e. if the blue bar measures 9 1/2 centimeters in length, you should stand back so that your eye is 9 1/2 feet from the screen. Vision testing should be performed on each eye separately, wearing distance eyeglasses if required. There are six lines on the screen, you should note the last line number in which you can read most (50-75%) of the letters. Visual Worksheet 03 Damian Schofield

Vision Testing Instructions The middle letter of the top line is equivalent to 20/200. A person unable to read this letter with best eyeglass correction is considered legally blind. The "T" and the "B" on the top line are equivalent to 20/100. The second line is 20/50. Best corrected acuity in the range between 20/50 and 20/100 is considered disabling in occupations which require work with numbers or extensive reading. The third line is equivalent to 20/40, it is the "driver's test" line. You must be able to read most of the letters on this line in order to obtain an unrestricted drivers license in most US states. The bottom three lines represent 20/30, 20/20 and 20/10. These are considered to be "normal vision" with the 20/20 line being the traditional normal vision line. Visual Worksheet 03 Damian Schofield

Our eyes have to dark-adapt before we can see in the dark or dim light. Usually, this requires fifteen to twenty minutes (or more) in an environment as dark as the environment you will be viewing. First of all, the pupil needs to dilate to its maximum aperture in order to collect the most light. Another component of night vision is contained in the biochemistry of the eye. The rhodopsin contained in our rod cells is sensitive to very low light levels. We need to use our rods, as opposed to the color-vision cone cells, in order to see faint objects in the dark. Any source of bright light will saturate the rod cells, destroy the sensitivity of the rhodopsin, and require twenty minutes of further dark adaptation. Rhodopsin is less sensitive to red light than to other wavelengths. This is why astronomers read star charts and make log entries with the help of dim, red lights. Our red-absorbing cone cells allow us to read, while we maintain most of our dim-light sensitivity. Dark Adaptation Damian Schofield

Dark adaptation is how the visual system adapts when going from a bright environment to a dark one. Light adaptation happens when we go from a dark environment to a bright one. For example, you spend the afternoon in the movies and when you leave the sun is still shining, your eyes may hurt when you get outdoors. Or perhaps, on a snowy winters day you go from indoors where the lighting is moderate and you find that it is difficult to see for some seconds, perhaps as long as a minute. One of the major differences between dark adaptation and light adaptation is their time course. Whereas dark adaptation takes about 30 minutes to be complete, light adaptation happens very quickly, usually in less than a minute. Dark Adaptation Damian Schofield

Another difference between these to type of adaptation is that when you are light adapted and then go into a very dark room for a while you may not see anything at all. As you dark adapt more and more things become visible. When you go from a darker area to a very bright one you usually are not temporarily blind. It is just that your vision, temporarily, is not very good. In technical jargon, your contrast sensitivity is poor until you become light adapted. By that we mean that you will have difficulty in perceiving areas of low contrast. It is like every thing is all washed out. But as you quickly light adapt the darker areas become darker and the lighter areas become easier to see. Dark Adaptation Damian Schofield

Vision: Lecture Objectives