Human Perception and Memory Semester 2, 2009 1 Vision Human Visual Perception • Humans are visual creatures. • While your eye is like a camera, there’s no little guy inside your head watching T.V. • Rather, your subjective perception of the visual world is created by your brain: John Ross: “Vision is an hallucination triggered by sense inputs”. Notes For most people, vision is the dominant sense, at least in terms of gath- ering information and interacting with the world. This is particularly true of conventional user interfaces, although in part that’s due to limitations of technology. We have pretty good widely available technology, at least for cre- ating flat, 2D, imagery (LCD display screens, etc.), and for stereo sound. But at the moment we have only limited, and mostly experimental, technology for smell and touch. Human vision, like any other mode of perception, is far from simple. While the creation of your subjective perception is fundamentally a myste- rious process, we know a lot about it, enough to provide practical guidance for designing GUIs. The Human Eye See Figure 1. Notes The fovea (the term I’ll use), is also called the macula (as in Figure 1).
Figure 1: Cut-away view of the human eye. From http://en.wikipedia. org/wiki/Image:Human_eye_cross-sectional_view_grayscale.png . The Human Eye Functioning • focussing (cornea, lens) • aperture control (iris) • image capture (retina) – variable resolution (fovea) – photoreceptors (rods, cones) – integration time (1/15 sec.) – preprocessing • pointing (saccades) Focussing • Cornea and lens form an image on retina. • The lens is made out of special transparent crystalline protein. • Lens and ciliary muscles adjust focus for different distances— accommodation . • Focussing problems 2
• Age effects: presbyopia, yellowing, stiffening, “floaters”, . . . Notes • Just as a camera’s lens forms an image of the outside scene on its film or CCD array, so the cornea and lens of the human eye working together form an image on the retina. • The lens is made out of special transparent crystalline protein. • Since the difference in refractive index between the lens and its sur- rounding fluids is fairly small, it can’t bend light very much. • Most of the bending of light to form the image is actually done by the cornea (the curved, transparent front surface of the eyeball), because of the substantial difference in refractive index between the cornea and the air. This is why you can’t see clearly under water: The cornea loses almost all of its focussing ability, because its refractive index is not much different from water’s. Wearing a diving mask restores this cornea-air interface, so your eye can focus. • The lens functions mainly to adjust the focussing already done by the cornea. This is why people can still see after they’ve had the lens surgically removed because of cataracts (a clouding of the lens), though they’ll need spectacles or a contact lens to compensate. • And just as with a camera, for a particular setting, only objects at a certain distance will be perfectly in focus, therefore the focus setting generally needs to be changed depending on whether you’re looking at nearby or far away objects. Commonly, because of the shape of your cornea, lens or eyeball, you may not be able to focus properly. Usually this can be corrected by wearing eye glasses or contact lenses. • Cameras adjust focus by changing the distance between the lens and the image; the eye instead does it by changing the shape of the lens. The lens is attached by the zonular ligaments. In the normal, resting state, the ciliary muscle is relaxed, the ligaments are taut and pull the lens into a thinner shape that bends light less. This is for focussing on far-away objects. 3
When the ciliary muscle contracts, it loosens the ligaments, allowing the lens to spring back into a fatter shape that bends light more. This is for focussing on close-up objects. This process of focus adjustment is called accommodation . • Age effects: The protein of your lenses has to last your entire life. It never gets replaced, and deteriorates with time. It loses its trans- parency and elasticity and becomes yellowish with age. Think about what effects this will have. In particular, the lens loses its ability to relax into the fat shape needed to focus on close-up objects. This phe- nomenon is called presbyopia , from Greek words meaning “elder see- ing”. (The name of the Christian denomination, Presbyterian, comes from one of the same Greek words, since their churches are governed by “elders”.) This is why most people need reading glasses as they get older. Another thing that happens any time, but increasingly with age, is that little bits of stuff become detached and drift around inside the eyeball (“floaters”), casting a faint shadow on the retina. You can sometimes see these when you look up at a clear blue sky. Aperture Control • The iris is like the aperture control of a camera. • Part of adjustment for varying light levels. • Bigger aperture: – Harder to focus. – More effect of lens defects. • So vision much harder under low-light conditions. 4
Notes • The iris is the band of muscles in front of the lens. The opening in the iris is the pupil . It looks black because you’re looking into the relatively dark interior of the eyeball. The iris is pigmented and patterned: the “color” of somebody’s eyes is the color of his’r’her iris. Some security systems recognize people by the pattern in their iris, like a fingerprint. • The iris controls the size of the pupil and hence the amount of light entering the eye. It’s one of ways the eye adjusts to varying light levels: In bright light, the pupil contracts to reduce the amount of light getting into the eye. In low-light conditions, the pupil expands (to about 7mm across) to gather as much light as possible. The eye also adjusts to different light levels by chemically altering the reponsiveness of the photoreceptors. • Pupil adjustment is like the aperture control (f-ratio) of a camera. • As with a camera, the aperture also affects depth of focus (aka depth of field). To see an object clearly, your eye has to accommodate to focus on that object. Objects nearer or further away will be out of focus, blurry. This focus adjustment is much more critical for a large aperture than a small one. (This is how you can get cheap “focus-free” cameras: They have a small enough aperture that everything over some reasonable range of distances is acceptably in focus.) • Another issue is that with a large aperture (big pupil) the light is passing through more of your lens, and will be affected more by any optical defects in your lens. • The practical effect of these two effects is that vision is much more difficult under low-light conditions. In bright light, your pupil shuts down to a small aperture: this makes focussing easier and reduces the impact of lens defects. In low light, though, your pupil opens up to a large aperture: focussing becomes more difficult and lens defects show up more. Retina • Retina covers inside surface of back of eyeball. • Like CCD sensor of a camera. 5
• Specialized light-sensitive nerve cells, photoreceptors . • Rods and cones • Rods—“night vision”, scotopic vision, dark adaptation, blue sensitive. • Cones—normal light, photopic vision, color perception. • Retina: neural image-processing front-end computer. • Blind spot Notes • The photoreceptors contain special pigment molecules, which react to light, setting up a chain of chemical reactions in the cell causing it to send off nerve signals. • The photoreceptors come in two varieties, rods and cones . (The names come from the shapes of the cells.) • The rods are very sensitive to light, and operate only under low-light conditions. They are responsible for our “night vision” (known techni- cally as scotopic vision, from Greek for “dark seeing”). Rod-based night vision is of importance only in some applications, like for visual astronomers, police, emergency workers, military, and the like, also for ordinary citizens driving cars at night, particularly in the country with no street lights. In some of these situations, it can be of life-and-death importance. So it is useful to be aware of the following characteristics of rod-based vision: – Rods are active only in low-light conditions and are inactived by bright light. Worse than that, it takes some time in the dark for rods to become active, about 10 minutes—even longer, half an hour or more, for them to reach peak sensitivity. This is why, when you step out into a dark night from a well-lit room, you at first can’t see anything. Only as your eyes “get used to the dark” will you start to see anything. The reverse process, however, is not gradual. Even a brief expo- sure to bright light is enough to switch off your rods and disable your night vision. – Rod-based night vision is adversely affected by such common drugs as alcohol, caffeine, nicotine, even at quite low doses. 6
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