SP – Chapter 2 and Lecture 3 PDF

Chapter 2 Alicia Hunsicker, Thought Forms, 2011  The First S te p s in Vision: From Light to Neural Signals Questions to Contemplate Think about the following questions as you read this chapter. B...

Chapter 2 Alicia Hunsicker, Thought Forms, 2011  The First S te p s in Vision: From Light to Neural Signals Questions to Contemplate Think about the following questions as you read this chapter. B y the chapter's end, you should be able to answer and discuss them. How are images of the world formed on the retina? How is energy from light converted into the electrical neural signals that lead to "seeing"? When the eye doctor says you have 20/20 vision, what does she mean? How is it that you are able to see over a huge range of brightness levels? magine looking into the night sky at your favorite star. The light coming from that star reaches your eye after traveling as far as 2000 light-years (almost 12 quadrillion miles in round numbers). Remarkably, in a dark winter sky far from city lights, the neighboring galaxy, Andromeda, is visible at over 2 million light- years away! This chapter describes the first steps in seeing. To understand how we see, we must first consider a little physics and optics, and then we'll look at how the eye is built to capture light and how specialized cells in the retina act to change physical light energy into electrical neural energy. nI Chapters 3-8, we'll see how light information gleaned by the eyes travels back through the head to the brain, as well as how the brain transforms this information into a meaningful interpretation of the outside world. 2.1 A Little Light Physics Light si a form of electromagnetic radiation- energy produced by vibrations of electrically charged material. There are two ways ot conceptualize light: as a wave or as a stream of photons, tiny particles that each consist of one quantum of ener- gy. This dual nature of light can be confusing to physics and psychology students alike. In this discussion, we'll try to avoid confusion as much as possible by treating light as being made up of waves when it moves around the world and being made up of photons when it is absorbed. The electromagnetic spectrum si made up of energy that varies over a very wide w a v e An oscillation that travels range of wavelengths (that is, the distances between successive points ni the wave), through a medium by transferring energy from one particle or point to and light makes up only a tiny portion of this spectrum. FIGURE 2.1A illustrates the another without causing any permanent electromagnetic spectrum, from gamma rays (which have very short wavelengths) displacement of the medium. to radio and television waves (which have very long wavelengths). Visible light waves photon Aquantum of visible light or have wavelengths between 400 and 700 nanometers (nm; 1 nm = 10-9 meter), as other form of electromagnetic radiation illustrated on the bottom of Figure 2.1A. Note that as the wavelength varies ni the demonstrating both particle and wave properties. visible spectrum, the hue we observe changes, from violet at about 400 nm, through hue The perceptual attribute of colors the whole spectrum of the rainbow, up to red at about 650 nm. (As we'll discuss ni that enables them to be classed as sim- Chapter 5, however, the light waves themselves are not colored; it si only after our ilar to red, green, or blue, or something visual system interprets an incoming wave that we perceive the light as a specific color.) in between.  34 Chapter 2 The First Steps ni Vision: From Light ot Neural Signals FIGURE 2.1 The ele ctro ma gne tic (A) Wavelength (nm) ene rgy spectr um (A) The spectrum of electromagnetic energy (specified ni nanometers, nm), with the visible spectrum 10-3 10-2 10-1 10 10₴ 103 10* 105 10% 107 108 10' 1010 10" 102 Gamma rays H (400-700 m) expanded. Note that 1nm = 10-9 meter. Heat Mic rowa ves Infrared FM radio Ultr avio let X-rays Television (B) Rayleigh scattering. Scattering of light causes the sky to look blue when the sun si high and ot look red when the sun is low. Wavelength (nm) (B) Some blue light is scattered Sun Observer A lot of blue Mostly reddish- light gets scattered. orange light remains. Atmosphere Earth Note that the electromagnetic wavelength spectrum in Figure 2. logarithmic 1A is on a scale. T o quote Tom Cornsweet, "If, instead, we consider or distances, not their logarithms, and we represent the range betweactual en A lengths M radio and X-ray wavelengths as the distance from New York to Los Angeles, then wavelengths we are able ot see would be represented on that scale as adistance hte than an eighth of an inch. Science had ot invent instruments to detect wavel of less represe nted by the rest of that distance" (Co engths rnsweet, 2017, p. 2). Let's consider what happens ot light on its way from space, the electromagnetic radiation from a star travelsa instara straig to an eye. nI empty speed of light (about 186,000 miles per second). Once ti reac ht line at the hes the atmosphere, some of hte starlights' photons are absorbed yb encounters whti dust, vaporized water, and os on; and some of hte light si scattered (sometimes called diffracted) yb these particles. Scatering fo sunlight yb smal particles (Rayleigh scat a b s o r b To take up something-such named after Lord Rayleigh) gives hte sky tering, its color: blue when the sun is high, because as light, noise, or energy—and not short-wavelength (blue) light si scatered more strongly than other wavelengths; erd transmit it at all. at sunset, when the sunsi near hte horizon, because hte sunlight must pas through scatter To disperse something-such as light—in an irregular fashion. more atmosphere near hte Earths' surface, scattering more of hte short-wavelength (bule) light F (G IURE 21.B3), alowing hte longer-wavelength light (red nad yeolw)  2.2 Eyes That Capture Light 35 to reach your eyes. Most of the photons, however, make ti through the atmosphere reflect To redirect something that and eventually hit the surface of an object. strikes a surface-especially light, If a ray of starlight were to strike alight-colored surface, most of the light would sound, or heat—usually back toward sti point of origin. be reflected. Indeed, the fact that most of the light bounces off the surface accounts for that surface's "light" appearance. However, most of the light striking a dark transmit To convey something (e.g., light) from one place or thing to another. surface is absorbed. Light that is neither reflected nor absorbed by the surface is refract 1. To alter the course of a wave transmitted through the surface. fI we are gazing at our star through awindow as of energy that passes into something the light travels from ari into theglass, some of the rays wil eb bent, or refracted, from another medium, as water does to as light is transmitted. light entering ti from the air. 2. To mea- Refraction also occurs when sure the degree of refraction in a lens light passes from air into water or into the eye- or eye. ball. In fact, the part of na eye exam ni whcih the eye doctor checks the patient's prescription is often called a refraction because the doctor determines how much image Apicture or likeness. cornea The transparent "window" into the eyeball. the next section, we'll es how the optic system of our eyes performs this same transparent Referring to the charac- kind of focusing. teristic of a material that allows light to pass through ti with no interruption such that o b j e c t s on the other side can b e 2.2 Eyes That Capture thgL i clearly seen. T o see stars or anything else, we need some type of physiological mechanism for sensing light. Even single-celled organisms such as amoebas respond to light, changing their direction of motion to avoid bright light when it is detected. But eyes go well beyond mere light detection. An eye can form an image of the outside world, enabling animals that possess eyes to use light to recognize objects, not just to determine whether light si present and what direction it's coming from. Before explaining how eyes form images, let's take a tour through the human eye to become familiar with its important parts. FIGURE 2.2 shows a front-to-back slice through a human eye, with the most important structures labeled. The first tissue that light from the star will encounter is the cornea. The cor- nea provides a window to the world because it is transparent (that is, most light (A) Sclera (B) H o r i z o n t a l cell Ciliary muscle A m a c r i n e cell Z o n u l e s of Z i n n. Choroid Cornea Retina Fovea - Rod Pupil Optic disc Light - Cone Iris Aqueous humor Lens Optic nerve Vitreous humor Blood vessels Inner Ganglion Bipolar limiting cells membrane cells Pigment epithelium FIGURE 2.2 The human right eye in cross section (viewed from above) that the Ron the retina si reversed right to left, and it is upside down. The "hole" in the ret- ina where the optic nerve leaves the eyeball si the optic disc (where the absence of photo- receptors results in a blind spot). (B) This enlargement of a cross section through the retina shows the main cell types. These will be discussed ni Section 2.3.  36 Chapter 2 The First Steps ni Vision: From Light to Neur al Signals photons are transmitted through it, rather than being reflected or absorbed). tI si transparent because ti si made of ahighly ordered arrangement of fibers and because it contains no blood vessels or blood, which would absorb light. The cornea does, however, have arich supply of transparent sensory nerve endings, which are there to force the eyes to close and produce tears fi the cornea si scratched, to preserve its transparency. If you have ever scratched your cornea or worn contact lenses too long, you know exactly how painful this can be! Fortunately, hte external layers of the cornea regenerate very quickly, so even when acornea si scratched, ti usually heals within 24 hours. If you wear contact lenses, you will know that they sit on a thin film of tears ni front of the cornea. The tear film si important because it provides hte eyes with a smooth, clear surface, helps to protect and lubricate the eyes, and washes away dust and particles. Tears also help to reduce the risk of eye infections. Besides, without tears, crying just wouldn't be the same. You might be wondering how the cells of the cornea get their oxygen and nu- trients if it has no blood supply. The aqueous humor, a fluid derived from blood fills the space immediately behind the cornea and supplies oxygen and nutrients to, and removes waste from, both the cornea and the lens. Like the cornea, the lens has no blood supply, so it can be completely transparent. As we'li see later, the shape of the lens is controlled by the ciliary muscles. To get to the lens, the light from our star must pass through the pupil, which si simply a hole ni a muscular structure called the iris. The iris gives the eye its a q u e o u s humor The watery fluid ni distinctive color and controls the size of the pupil, and thus the amount of light that the anterior chamber of the eye. reaches the retina, via the pupillary light reflex. When the level of light increases lens The structure inside the eye that or decreases, the iris automatically expands or contracts to allow more or less light enables the changing of focus. into the eye, respectively. Interestingly, like the aperture of a camera, the pupil of pupil The dark, circular opening at the the iris plays an important role in the image quality. Under low illumination, when center of the iris in the eye, where light the pupil is large, the depth of focus (the range of distances over which the image enters the eye. is sharply focused) is reduced, resulting in poor image quality. iris The colored part of the eye, con- After passing through the lens, our starlight will enter the vitreous chamber (the sisting of a muscular diaphragm sur- rounding the pupil and regulating the space between the lens and the retina), where it will be refracted for the fourth and light entering the eye by expanding and final time by the vitreous humor. This is the longest part of the journey through contr actin g the pupil. the eyeball; this chamber comprises 80% of the internal volume of the eye. The vitreous humor The transparent fluid vitreous is gel-like and viscous (a bit like egg white), and it si generally transparent. that fills the vitreous chamber in the While staring up at the bright blue sky on a lazy sunny day, however, you may have. posterior part of the eye noticed "floaters"" small bits of debris (biodebris) that drift around in the vitreous. retina A light-sensitive membrane in Floaters are quite common, and they are usually not a cause for concern. the back of the eye that contains pho- toreceptors and other cell types that Finally, after traveling through the vitreous chamber, the light emitted by our transduce light into electrochemical favorite star will (hopefully) be brought into focus at the retina. T o be abit more signals and transmit them to the brain precise, only some of the light wil actually reach the retina. Much of the light through the optic nerve. energy will have been lost ni space or the atmosphere, because of absorption and accommodation The process by which scattering, as described already. In addition, a good deal of light will have become the eye changes its focus (in which lost in the eyeball, so only about half of the starlight that arrives at the cornea will the lens gets fatter as gaze si directed reach the retina. The role of the retina is to detect light and "tell the brain about t o w a r d nearer objects). The distan ce betwe en aspects of light that are related ot objects ni the world" (Oyster, 1999). In other focal distan ce the lens (or mirror) a n d the viewe d words, the retina si where seeing really begins, because ti si here that light energy object, in meters. is turned into electrical neural signals—a process known as transduction. diop ter A unit of meas urem ent of the Interestingly, our ability to detect abriefly presented dim light (and many other optical power of a lens. It is equal to the visual stimuli) si limited not only by the retinal transduction processes and the reciprocal of the focal length, in meters. neural signals that it generates, but also by the noise ni the visual pathways, and by A 2-diopter lens will bring parallel other factors too. For example, detection thresholds can be enhanced by providing rays of light into focus at 0.5 meter (50 a sound at precisely the same time sa the dim light (Spence and Ngo, 2012). centimeters ).  2.2 Eyes That Capture Light 37 This "cross modal facilitation" may be Accommodated caused by the sound reducing uncer- tainty about when precisely the visual stimulus will appear (see Chapter 1) or Ciliary muscle by the top-down focusing of attention contracted at precisely the time that the visual - Zonules of Zinn relaxed stimulus appears (see Chapter.)7 - Accommodated Focusing Light onto the lens Retina To focus a distant star onto the retina, - Unaccommodated lens the refractive power of the four optical components of the eye-cornea, aque- - Zonules of Zinn ous humor, lens, and vitreous humor — under tension must be perfectly matched to the length Ciliary muscle of the eyeball. Because the cornea is relaxed highly curved and has ahigher refrac- tive index than air (1.376 versus 1,) it forms the most powerful refractive surface Unaccommodated in the eye. The aqueous and vitreous humors also help refract light. However, the refractive power of each of these three structures is fixed, so they cannot be used FIGURE 2.3 Accommodation changes to bring close objects into focus. This job si performed by the lens, which can a-l the pow er of the lens The left side of ter the refractive power by changing its shape—a process called accommodation. this cross section shows the relaxed (unac- commodated) lens. The right side (top) Accommodation (change ni focus) is accomplished through contraction of shows the bulging accommodated lens, the ciliary muscle. The lens is attached to the ciliary muscle through tiny fibers resulting from decreased tension of the (suspensory ligaments known as the zonules of Zinn) (FIGURE 2.3). When the zonules when the ciliary muscle contracts. ciliary muscle si relaxed, the zonules are stretched and the lens is relatively flat. In this state, the eye will be focused on very distant objects (like our star). But to focus on something closer-say, a wristwatch or smartphone—the ciliary muscle must presbyopia Literally "old sight"; the contract. This contraction reduces the tension on the zonules and enables the lens age-related loss of accommodation, which makes it difficult to focus on near to bulge. The fatter the lens is, the more power it has, and the closer you can focus. objects. Accommodation enables the power of the lens to vary. Lens power (P) = 1/f, where fis the focal distance in meters. So, fi your unaccommodated eyes were perfectly corrected for distant vision, 15 diopters of accommodation would enable 20 Amplitude of accommodation (diopters) you to read your watch at a distance of about 0.067 meter (1/15) or 6.7 centimeters Dond ers (cm; to convert meters to centimeters, simply multiply by 100). If you can read your Duane 15 Jackson watch at 6.7 cm (while wearing your distance correction), you are either very lucky Sheard or very young. Our ability to accommodate declines with age, starting from about Tu r n e r 8 years old, and we lose about 1 diopter of accommodation every 5 years up to age 10 30 (and even more after age 30). By the time most people are between 40 and 50 years old, they find that their arms are too short because they can no longer easily accommodate the 2.5 diopters or so needed to see clearly at 40 cm (1/0.4 = 2.5). This 5 condition si called presbyopia (meaning "old sight"), and it is, like death and taxes, inevitable! FIGURE 2.4 illustrates the precipitous drop in accommodation with age. Why do we all have presbyopia to look forward to? The main reason si that the lens 0 becomes harder, and the capsule that encircles the lens, enabling ti ot change shape, loses 10 20 30 40| 50 60 70 its elasticity. Lucky for us, Benjamin Franklin (1706-1790) invented bifocals-lenses Age (years) that have one power at the top (permitting us to see distant objects) and a different FIGURE 2.4 The precipitous drop in power at the bottom (allowing us ot focus on objects at a comfortable reading distance). amplitu de of accomm odation with Like the other optical components of the eye, the lens si normally transparent. It age The dashed line indicates the ampli- tude of accommodation required to focus is transparent because the crystallins (a class of proteins that make up the lens) are at a distance of 40 cm. Each colored symbol packed together very densely and therefore are very regular. Anything that interferes represents data from a different classical with the regularity of the crystallins wil result ni loss of transparency (that is, result study.  38 Chapter 2 The First Steps ni Vision: From Light ot Neural Signals A () Emmetropia FIGURE 2.5 Optics of hte human eye Examples of an eye htw ino visual defects pia) (B), m (emmetropia) (A), nearsightedness (myoectio yopia whti corection C (,) farsightedness corr n (E). (hyperopia) (D), and hyperopia with ni areas that are opaque, ro opacities). Opacities of the lens are known sa cataracts. Cataracts can occur at different ages and take many different forms. Congenital B () Myopia cataracts (present ta birth) are relatively rare, but if they are dense (and therefore interfere with retinal m i age quality), they can have devastating effects on normal visual devel o pment f i not treated promptly. Most cataracts are discovered after age 50, and the prevalence of cataracts increases with age, os by 70 almost everyone has some loss of transparency. Cataracts can interfere with vision because they absorb and scatter more light than hte normal lens does. Fortunately, treatment of cataracts n(i C )( Myopia with correction which the opacified lens si extracted and replaced with a plastic or silicone implant) has become quite routine-often just a 30-minute procedure ni the eye doctor's office. There si much ongoing effort ot develop new types of lens implants that can change focus. Hopefully these wil be perfected before you need cataract surgery! When the refractive power of the four optical components of the eye (cornea, aqueous humor, lens, and vitreous humor) are perfectly matched to the length of () Hyperopia D the eyeball, this is known as emmetropia (FIGURE 2.5A). Aperson who is emme- tropic does not need corrective lenses to see distant objects. On average, the adult human eye si 42 millimeters (mm) long, about the diameter of a quarter. However, eyeballs can be quite a bit longer or shorter and still eb emmetropic because eyes generally grow to match the power of the optical components we're born with. (Most newborns are hyperopic because the optical components of their eyes are (E) Hyperopia with correction relatively wel developed at birth compared with the length of their eyeballs.) Refractive errors occur when the eyeball is too long or too short relative to the power of the four optical components. fI the eyeball si too long for the optics (FIGURE 2.5B), the image of our star will be focused in front of the retina, and the star will thus be seen as a blur rather than a spot of light. This condition is called myopia (or nearsightedness). Individuals with myopia cannot see distant objects clearly; luckily, myopia can be corrected with negative (minus) lenses, which diverge the rays of starlight before they enter the eye (FIGURE 2.5C). fI the eyeball is too cataract An opacity of the crystalline short for the optics (FIGURE 2.5D), the image of our star will be focused behind the lens. retina— a condition called hyperopia (or farsightedness). If the hyperopia si not too emmetropia The condition in which severe, ayoung hyperope can compensate and see clearly by accommodating, thereby there si no refractive error, because the increasing the power of the eye. fI accommodation fails to correct the hyperopia, the refractive power of the eye si perfectly star's image wil again be blurred. Hyperopia can be corrected with positive (plus) matched to the length of the eyeball. refractive error A very common dis- lenses, which converge the rays of starlight before they enter the eye (FIGURE 2.5E). order in which the image of the world A noted s earlier, the most powerful refracting surface in the eye is the cornea, is not clearly focused on the retina. The most common refractive errors are which contributes about two-thirds of the eyes' focusing power. nI an emmetrope, the cornea is spherical, like a basketball or soccer ball (FIGURE 2.6A). However, fi myopia, hyperopia, astigmatism, and the cornea si not spherical, but rather shaped like a football (that is, the presbyopia. is different in the horizontal and vertical merid curvature myopia Nearsightedness, a common ians; FIGURE 2.6B), the result si condition in which light entering the eye is focused in front of the retina and dis- tant objects cannot be seen sharply. (A) B () FIGURE 2.6 Two balls, two shapes (A) Basketballs and soccer balls are spherical. (B) Rugby and American football balls are elliptical. fI the cornea si shaped like afootball, with different curvatures in the horizontal and vertical meridians, it will result ni astigmatism.  39 2.2 Eyes That Capture Light astigmatism. With astigmatism, vertical lines may be focused slightly in front of the retina, while horizontal lines may be focused slightly behind ti (or vice versa). fI you have a reasonable degree of uncorrected astigmatism, one or more of the lines in FIGURE 2.7 may appear to be out of focus while other lines appear sharp. Lenses that have two focal points (that is, lenses that provide different amounts of focusing power in the horizontal and vertical planes) can correct astigmatism. The develop- ment of refractive surgery such as LASIK (laser-assisted ni situ keratomileusis) as an alternative to glasses or contact lenses is based on the cornea's refractive power. FIGURE 2.7 Fan chart for astigmatism If you are a college student (and fi you are reading this, the odds are that you Take of your glasses (if you wear glasses) and view this "fan". fI you have a significant are), there's a high probability that you are myopic. While the causes of myopia degree of astigmatism, one or mor e of the are complex, involving both genes and environment (see Harb and Wildsoet, 2019, lines wil appear to have lower contrast. the level of for a review), it has long been suggested that there si a link between education and the development of myopia. Moreover, the prevalence of myopia is increasing, and myopia si predicted to affect more than half of the world's popula- hyperopia Farsightedness, a common tion in the next 30 years. One potentially important player in this increase may be condition ni which light entering the related to the amount of time we now spend no our devices (smartphones, tablets, eye is focused behind the retina and accommodation si required to see near and computers). This is an area of active research. objects clearly. The Retina astigmatism A visual defect caused by the uneq ual curving of one or more of The preceding discussion covered how the human visual system delivers a focused the refractive surfaces of the eye, usually image of our favorite star onto the retina, which is spread across the back of the t h e cornea. eyeball. The optics involved (see Figure 2.2) include a mechanism for regulating transdu ce To convert from one form the amount of light (the iris) and a lens for adjusting focal length (see Figure 2.3) of energy to another (e.g., from light to neural electrical energy, or from so that both near and distant objects can be focused on the retina. However, unlike mechan ical movem ent to neural elec- a camera, the human visual system has hte job of interpreting this image. This is trical energy). Neurons use electrical the difference between taking a picture and seeing a picture. And the process of signals in their communication. transduced seeing begins with the retina, where the light energy from our star is fundus The back layer of the retina: into neural energy that can be interpreted by the brain. what the eye doctor sees through an ophthalmoscope. What the Doctor Saw photoreceptor A light-sensitive look at the back sur- receptor in the retina. Eye doctors use an instrument called an ophthalmoscope to face of their patients' eyes, which si called the fundus (plural fundi). (You proba- bly remember all too well having that bright light shining into your eye while the doctor examined your fundus.) FIGURE 2.8 shows a photograph of a normal fundus. The white circle is known as the optic disc. This is the point wher e the arter ies and veins that feed the retina enter the eye and where the axons of ganglion cells (which we will get to shortly) Optic disc leave the eye via the optic nerve. This portion of the retina contains no photoreceptors, and is consequently it is blind. For that reason, it Fovea - referred to as the blind spot or physiological blind spot. You can experience your own blind spot, corresponding to the optic disc, by closing your left eye, fixating on the Fin FIGURE 2.9A with your right eye, and adjusting the distance of the book from your eyes until the red cir- cle disappears. You don't normally notice this large blind spot in your visual field because FIGURE 2.8 Fundus of the right eye of a human The branching blood ves- you have two eyes, and objects whose images sels are called the vascular tree.  40 Chapter 2 The First Steps ni Vision: From Light ot Neural Signals (A) fal into hte blind spot of one eye can be seen (see Chapter 6). However, even with the otherbyeyetheclootsheedr, hvete visual syestm "fills in" hte blind spot whti information orfm het surrounding area (FIGURE 2.9B). Another prominent feature of hte fundus si the fovea (hte F 15.-mm brownish spot ni Figure 2.8), whcih si located near hte center of the macula. The central ~05. m m of hte fovea blood vessels, thus allowing al of hte light ot pass throhausghon. This anatomy si responsible for the depression ni the center fo the fovea (this depression can eb seen more clearly ni Figure 2.10). Note that hte fovea is in the approximate center ofhte retina; however, ti appears ot be of ot hte side ni Figure 28, because of hte limited view of the fundus photo. The fundus (B) si the only place ni the body where one can see the arteries and veins directly, so ti provides doctors with an important F window on the well-being of the body's vascular system. The vascular "tree" (that is, the branched blood vessels) spreads out across the retina ni a characteristic way, but stops short of the fovea. FIGURE 2.9 Your blind spot To experience your blind spot, close your left eye, fixating on the F ni (A) with your right eye. You can see your own vascular tree by using a simple trick Hold the book about 15 cm away from your eye to begin and that requires only a penlight. nI a dark room, close your eyes adjust the distance of the book from your eyes until the red and place the penlight against the outside corner of one eye. circle disappears. This is your blind spot. Ordinarily you are not aware of it, because the visual system "fills in" the blind spot Holding the penlight against the eye, gently move the light around (up and down, and back and forth). Within a few seconds you with information from the surrounding area. fI you fixate on the F should see the shadows cast by your blood vessels looking like in (B) with your right eye and again adjust the distance, when the gap in the line falls in your blind spot, it will be filled in and you the branches of a tree. We don't normally see them, because will s e e a continu ous red line. the blood vessels move with our eyes, so their shadows are stabilized retinal images and, as with the blind spot, the visual system fills in behind them. The motion of the penlight makes the shadows move, enabling us to see them. Even when viewed through an ophthalmoscope with a lot of magnification, the fundus does not provide a detailed view of the retina. The retina is the neural structure of the eye where transduction takes place. To get a good look at the structure of the retina, we need a cross section, which reveals that the retina is a layered sheet of clear neurons, about half the thickness of a credit card (Rodieck, 1998), with another layer of darker cells, the pigment epithelium, lying behind fovea A small pit located near the the final layer. While your eye doctor can't see this level center of the macula and containing the of detail, she may use optical coherence tomography to see each layer of your retina highest concentration of cones and no in cross section rods. It is the portion of the retina that (FIGURE 2.10). Optical coherence tomography si a noninvasive imaging technique produces the highest visual acuity and that uses low-coherence light to capture high-resolution images from within serves as the point of fixation. light-scattering media (like the retina). macula The pigmented region with a As we'll see in the next section, together these neurons constitute a minico d i a m e t e r of a b o u t 5.5 millimeters near that begins the process of interpreting the information contained mputer t h e center of the retina. It is sometimes in visual images. The transduction of light energy into neural energy begins ni the backmost referred to as the macula lutea (from the of the retina, which is layer Latin) because of its yellow appearance. made up of photoreceptors. When photorecep tors sense rod A photoreceptor specialized for light, they can stimulate neurons ni the intermediate layers, including bipolar n i g h t vision. cells, horizontal cells, and amacri ne cells. These neurons then connect with the cone A photorecep tor specialized for frontmost layer of hte retina, made up of ganglion cells, whose axons pass through daylight vision, fine visual acuity, and the optic nerve to the brain. color. Before w e describe the function of these layers, we should address an obvious eccentricity The distance between the retinal image and the fovea. bucknig ,i in ehtechiefohet m ite legure ehohp peeposerert  2.2 Eyes That Capture Light 41 Outer: O u t e r Inner Inner nuclear plexitorm n u c l e a r plexifo rm Ganglion Retinal Extemal layer layer lay er layer: cel flayer pigment nilunig sa epithelium zone r e membran FIGURE 2.10 Optical coherence tomography of the retina Ahigh-resolution cross section of the retina and pigment epithelium showing the different layers (as labeled). Note the foveal depression where the ganglion cell layer thins. ganglion, horizontal, and amacrine cells before making contact with the photorecep- tors. However, these neurons are mostly transparent, whereas cells in the pigment epithelium, which provide vital nutrients and recycling (or housekeeping) functions to the photoreceptors, are opaque. Once we see that the photoreceptors must be next to both the pigment epithelium, for nutrition and recycling, and the other neurons, duplex In reference to the retina, in order to pass along their signals, the layering order makes much more sense. consisting of two parts: the rods and cones, which operate under different conditions. Retinal Geography and Function Each retina contains roughly 100 million photoreceptors. These are the neurons visual angle The angle that an object subtends at the eye. that capture light and initiate the act of seeing by producing chemical signals. The human retina contains at least two types of photoreceptors: rods and cones. Rod Cone These two types not only have different shapes (which si how they earned their names; FIGURE 2.11), but also have different distributions across the retina and Outer serve different functions. segment Humans have many more rods (about 90 million in each eye) than cones (about 4-5 million in each eye), and the two types of cells have very different geographic Inner distributions on the retina (FIGURE 2.12). Rods are completely absent from the segment center of the fovea, and their density increases to a peak at about 20 degrees and then declines again. The cones are most concentrated in the center of the fovea, and their density drops off dramatically with retinal eccentricity (distance from the fovea). The fovea is the "pit" in the inner retina that is specialized for seeing fine detail. Because human retinas have both rods and cones, they are considered duplex retinas. Some animals, such as rats and owls, have mostly rod retinas; others (e.g., certain lizards) have mostly cone retinas. Synaptic As the photographs of photoreceptors at different eccentricities in Figure 2.12 terminal illustrate, in the foveal center (0.0 mm) the cones are smaller and more tightly packed than in other areas of the retina. This rod-free area (about 300 square micrometers (um] on the retina) is directly behind the center of the pupil and subtends a visual FIGURE 2.11 P h o t o r e c e p t o r s Rod angle (the angle at the eye) of about 1 degree. How big is 1degree? Here's a rule and cone.  43 2.2 Eyes That Capture Light ofriest file atiy het sure, undein snity shoestiney, deite hte absence findinera teriteral prophetie a compaerd wnietr persofhta out of the "corner of the eye"). The cones become larger and more sparse away from the foveal center, so the small cells that appear outside the fovea at 1.35 mm in Figure 2.12 are rods (they are about the same size as the cones are in the fovea, at 0.0 mm). In all of the mi- crographs except for the one at 0.0, the large cells are always the cones. Rods and cones operate best under different lighting conditions: Rods function relatively well under conditions of dim (scotopic) illumination (which is why animals e brighter such as opossums with all-rod retinas are nocturnal), but cones requir stargazers know that it is often easier to spot a dim star by looking out of the corner of one's eye than by looking directly at it. We will revisit photopic and scotopic vision again in Chapter 5. Rods and cones differ functionally in another important way. Because all rods have the same type of photopigment, they cannot signal differences in color. Each cone, however, has one of three different photopigments that differ in the wavelengths at which they absorb light most efficiently. Therefore, cones can signal information about wavelength, and thus they provide hte basis for our color vision. You may wonder why we have both rods and cones and why 95% of our pho- someepent aer mo,digviey ones thenswelerni htsi yad a-50 munil casi ago, a particular fishy ancestor of ours developed rods (they already had cones), and this provided an advantage in survival at very low light levels at the bottom of the ocean. That advantage has survived till now (Lamb, 2016). Rods and cones use a great deal of energy, which they get from the retinal pigment epithelium, which lies below the retina. Recent work suggests that the rods and cones burn glucose, converting the leftovers into lactate, which they TABLE 2.1 Properties of the fovea and periphery ni human vision Property Foveal Periphery Photoreceptor type Mostly cones Mostly rods Bipolar cell type Midget Diffuse Co nve rge nce Low High Receptive-field size Small Large Acuity (detail) High Low Light sensitivity Low High  44 Chapter 2 The First Steps ni Vision: From Light to Neural Signals feed backot hte retinal pgiment epithelium, whcih its energy (Kanow et al., 2017). ni utrn uses het lactate orf FURTHER DISCUSSION of cones and color detection can be foundni S e c t i o n 5.2. 2.3 Dark and Light Adaptation When you ente dark orom from bright sunlight, hte number entering your reyeam gihteb reduced yb afactor of several bilion of(mpohreotonhtasnfogilht units) FIURE 2.14) Ini ou wil have trouble seeing anything, bu 21olg. (G 30 minutes ni the dark, ytoiualywilybe able ot detect even just afew pho t after about purple curve ni FG IURE 21.5 illustrates hte change ni the threshold ltiogns. T eh solid ht intensity, soriontheofleast light thresho needed , ot det ect lds). Initially hte threaperipheral s p ot ( see C hapt er 1 for adsicus. shold si very high, indicating ow But over 20 minutes or so, hte thresho increased ld si greatly reduced (meaningl sens sensitivity. itivitysi ). And when you emerge from wil be able ot see almost instantly. How dthoees dhtaerk viansdualretur n ot hte sunlight, you system alter its sensitivity over such a large operating range? There are four primary ways ni which the visual system illumination: pupil size, photopigment regeneration, hte dupleadjusts ot changes ni x retina, and neural circuitry. Pupil Size When aflashlight si shone ni someone's eye ni constricts. The diameter of the pupil can vary byadimly lit room, the pupil quickly about a factor of ,4 from about 2 mm in bright illumination to about 8 mm in the dark (FIGURE 2.16). Because hte amount of light entering the eye is proportional to the area of the pupil, the 4-fold increase in diameter accounts for a 16-fold improvem ent in sensitivity. In other words, 16 times as many quanta can enter the eye when the pupil si completely di- lated, compared with when it si constricted. Although this adaptive ability helps, pupil dilation has a time course of a few seconds, while dark adaptat certainly ion takes many minutes. Thus, pupil size accounts for only a small part of the visual system's overall ability to adapt to light and dark conditions. Starlight Moonlight Indoor lighting Sunlight Luminance of white paper in: Nor erion Good color vision Best acuity Visual function - Scotopic - - Mesopic - Photopic. 50% bleach Absolute Cone Rod threshold threshold Best Indirect Damage saturati on acuity ophthalmo- possible begins scope 0 2 6 8 Luminance (log candelas/ m2) FIGURE 2.14 Luminance levels The visua nance levels, from just a few photons to l system operates over ahuge range foulm-i very bright sunshine.  2.3 Dark and Light Adaptation 45 Photopigment Regeneration Rods only Asecond mechanism for achieving a large sensitivity range is provided by the way photopigments are used up and replaced ni receptor cells. In dim lighting condi- Cones only tions, plenty of photopigment is available, and rods and cones absorb and respond Threshold to as many photons as they can. As already noted, rods provide better sensitivity in such situations than do cones. Indeed, the rod system is capable of detecting a single quantum of light! After a photopigment molecule si bleached (used to de- tect a photon), the molecule must be regenerated before it can be used again to absorb another photon. As the overall light level increases, the number of photons starts to overwhelm the system: photopigment molecules cannot be regenerated fast enough to detect all 0 5 10 15 20 25 30 the photons hitting the photoreceptors. This slow regeneration is a good thing for Time in the dark (minutes) increasing our sensitivity range. fI photons are scarce, we use them all to see; fi we FIGURE 2.15 Dark adaptation The have an overabundance, we simply throw some of them away and use the leftovers. solid purple curve shows the threshold Interestingly, at very low light levels, we are about a factor of 2 more sensitive to light intensity required to detect a periph- a decrease in light levels (as might occur fi there were a shadow) than to an increase eral spot following several minutes of adap- (Patel and Jones, 1968). This might be important for survival fi you are a mouse tation to a bright light. The dashed red curve illustrates the rapid adaptation of the foraging for food in dim light trying to avoid being eaten by a cat! Recent work cones. The dashed blue curve shows the suggests that near threshold, decreases in illumination in dim light are detected slower recovery of the rods to much lower threshold intensities (that is, greater sen- via OFF retinal ganglion cells (Westö et al., 2022; Fain, 2022). sitivity). The solid purple curve represents the more sensitive of the two at any The Duplex Retina The light compensation mechanism is enhanced by humans' duplex retinas. Rods provide exquisite sensitivity at low light levels, but they become overwhelmed when the background light becomes moderately bright, leading to a loss in information quality. Cones are much less sensitive than rods (they function poorly under very dim light), but their operating range is much larger, stretching from about ten photons per second (just enough light to see color) to hundreds of thousands of photons per second (e.g., a snowcapped mountain in bright sunlight). So we use rods to see when the light is low, and the cones take over when there is too much light for the rods to function well. After adapting to a bright light, cones recover sensitivity quickly (dashed red curve in Figure 2.15) and then saturate. They are not in very sensitive to very dim light. Rods recover more slowly (dashed blue curve Figure 2.15), but after 20 minutes or so they are very sensitive to dim light. When our eyes are fully dark adapted, lights that are close to the detection threshold appear colorless. (A) Darkness (B) Bright illumination 8-mm pupil 2-mm pupil FIGURE 2.16 Two pupils The black spots in the middle of these two irises show the pos- sible range of pupil sizes as we go from darkness (A) into bright illumination (B). 46 Chapter 2 The First Steps ni Vision: From Light ot Neural Signals receptive field The region on the Neural Circuitry Ahtlough pupilsize, photopigment regeneration rats impotentd/cone dchio retina in which visual stim uli influence a neuron's firing rate. tmy balothpaleyredaryoblevari ni adtiaorknsandni viloevraerallight lleevveelsls hhasasotot dood witant reasonewaer not ag e-r ela ted macular de ge ne rat ion A disease associated l ight affects the macula. with aging that hte retina. A s we wil se below, aganglion cel si mostwsensi ith hte neural circuitry fo hte intensity ofhte lightni the center nad ni hte surround oftiveits otrecedpifteivreencefiseldn.i It gradually destroy sha rp cen tral vision, s making it difficult to rea d, drive, an d rec og are tw o forms: we t niz e faces. There hte regionno hte retina (and the corresponding region ni visual space) ni wchi an d dry. visual stimuli influence hte neurons' firing rate. Ganglion cels aer less afectedy retinitis pig me nto sa A deg ene rat ion of the retinapro gressive b tha t affects het average intensity fohte light, Buthteyevfield, as el tana above-sponatneous nghus,sahtehte photoreceptors nig ht vision and per iph era l fraeetedinw ghen hte liggahntgliofal nlscneolsh teaer ennottirecorm ecpelpetetivlye satatuurat rateesd. olT vision. It commonly runs in families and caused by defects ni a number of dif- can be encode hte pattern of relatively light and relatively dark areas ni hte reliotinnalcem l wli gang And the patern fo ilumination, not hte overal light level, si hte primary coinceag ferent genes that have recent identified. ly been e. of the rest of the visual system. rn oT sum up, the answer ot the question of how hte visual sysetm deals whti such large variationsni overall light levels has wto parts. First, w e reduce hte scael fohet p robl em yb r e gul a t ing types of photoreceptors t he am o un t f o l ight ent e r ing the eyebal, yb using diferent ni different situations, and by effectively throwing w aya photons we don't need. Second, by responding ot the contras t between adjacent retinal regions, the ganglion cells do their best to ignore whatever variation ni overall light level is left over. Sensation & Perception in Everyday Life W h e n G o o d Reti na G o e s Bad Millions of people around the world suffer from auditory signals (J. Ward and Meijer, 2010). Lkie the blinding diseases ni which the rods and/or cones earlier attempts at sensory substitution, this appro degen erate. These include age-re lated macular ach has limitations. degen eratio n and retinitis pigmentosa. At present, Fortunately, there are several exciting technologi- ther e are no effective cures to prevent the progres- cal developments that provide hope for people who sive degeneration of the photoreceptors that occurs are blind. These are all based on the notion that while in these diseases. For people with age-related macu- the photoreceptors are dead or dying, postreceptoral lar degeneration, this may lead to an inability to read neurons and their con nec tion s are largely or recognize faces. For people with long-standing intact. One retinitis pigmentosa, this leads inevitably to irrevers- approach is o t substitute an electronic prosthesis ible blindn ess. (an artificial device to replace or augment a missing or impaired part of the body) into the retina. Typical- Early attempts to assist the blind were based on ly, the prosthesis uses a camera to convert light into "sensory substitution" —hte idea that the sense of touch could act as a substitu te for the loss of vision. energy; an array of electrodes implanted in the retina generates an electrical stimulation pattern based Bach-y-Rita and his colleagues (Bach-y-Rita et al., on the light pattern on the camera and delivers this V camera to an array of 400 1969) connected a T stimulation pattern to the intact postreceptoral neu- tactile stimulat ors that vibrated against the skin of the rons (FIGURE 2.17). Unfortunately, while these retinal backs of blind study participants. Following extensive prostheses can restore some sight, there are technical training, these individuals were able to discriminate challenges to implanting them, and they suffer low the orientation of lines and recognize some geometric shapes. Unfortunately, the skin has very poor resolu- spatial resolution (Weiland, Cho, and Humayun, 2011), allowing only perception of spots of light and very- tion compared with vision, and this approach was high-contrast edges. largely abandoned. A more recent attempt to substi- Another approach that has had some early success tute anothe r sense for vision uses auditory substitution—a device that converts visual signals to ni animal models si to use gene therapy to express light-activated channels ni surviving photoreceptors  2.4 Retinal Information Processing 47 Sensation &Perception in Everyday Life (continued) using adeno-associated viral vectors. This approach is their low light sensitivity and inability to adapt to has been successfully used in several clinical trials in changes ni ambient lighting. However, recent work patients. (Berry et al., 2019) suggests that medium-wavelength Athird strategy si to chemically modify endog- cone opsin overcomes these limitations, enabling enous channels in retinal ganglion cells to make blind mice with retinitis pigmentosa to function well ni them light-sensitive. This approach essentially adds low-light conditions and to adapt to changes ni ambi- a synthetic small molecule "photoswitch" to confer ent lighting light sensitivity onto retinal ganglion cells, and it has Each of these approaches provides promise for been shown to reinstate light sensitivity ni blind mice new treatments for people with blinding retinal (Tochitsky et al., 2014). One limitation of this approach disorders. Retina Video Laser camera o r RF Epiretinal A r e a of implant photo- receptors destroyed by disease Subretinal implant FIGURE 2.17 Retinal prostheses The insert shows sensors implanted in t h e retina. Ganglion cells Photoreceptors 2.4 Retinal Information Processing The retina contains five major classes of neurons: photoreceptors, horizontal cells, bipolar cells, amarine cells, and ganglion cells mentioned earlier in this chapter. Let's take a closer look at the functions of each of these cell types. Light Transduction by Rod and Cone Photoreceptors When photoreceptors capture light, they produce chemical changes that start a cascade of neural events ending ni a visual sensation. Photoreceptors send their signals by way of the synaptic terminals, specialized structures for contacting other outer segment The part of a photo- retinal neurons. Figure 2.11 shows examples of rod and cone synaptic terminals. receptor that contains photopigment molecules. The synaptic terminals contain connections from the neurons that photoreceptors "talk to": the horizontal and bipolar cells. inner segment The part of a photore- Both types of photoreceptors consist of an outer segment (which si adjacent ot ceptor that lies between the outer seg- ment and the cell nucleus. the pigment epithelium), an inner segment, and a synaptic terminal. Molecules called visual pigments are made in the inner segment (which is like a little factory, filled synaptic terminal The location where axons terminate at the synapse for trans- with mitochondria) and stored ni hte outer segment, where they are incorporated mission of information by the release of into the membrane. Each visual pigment molecule consists of a protein (an opsin), a chemical transmitter.  48 Chapter 2 The First Steps ni Vision: From Light ot Neural Signals chromophore The lig of the visual pigments ht-catching part het structure fowhchi determines whcih wavelengths folight hte pigment moelcuel of the retina. absorbs, and achromophore, whcih captures light photons. The chromophoresi rhodopsin The visual pigment in ro d s. found hte part fohte pgiment molecule that determines its coloryb selectively absorbing melanopsin specific wavelengths fo light. The chromophore, known as Retinal, si derived orfm A photo sensitive to ambien pigment that is vitamin A,hich si ni turn manufactured from beta-carotene, which si why t light. photoactivation Activ mother told you ot eat your carrots! The opsin and chromophore aer connecyteodur. ation by light. Each photoreceptor has onyl one of hte four types of visual pigments found ni het hyperpolarization A change ni mem- brane potential such that the inner human retina. The pigm ent rhodopsin si found ni the rods, concentrated maniyl m e m b r a n e sur fac e b e c o m e s mo re neg- ni the stack of membranous discs ni the outer segment. Each cone has one fohte ative than the outer membrane surface. other three pigments, each of whcih respondsot long, medium, roshort wavelengths graded potential An electrical ure 51. shows the absorption spectra of the four photoreceptor types. only.EviFdigence potential that can vary con amplitude. tinuously in suggests that there may be another type of photoreceptor-one that "lives" among hte ganglion cells and that is involved ni adjusting our biological rhythm phot s ot omrsatchaer htesenday and night fo hte external world (Baringa, 2002). These orecept sitive ot hte ambient light level and contain hte photopig- ment melanopsin, and they send their signals ot the supra home of the brain's circadian clock, which regulates 24-hochiasm atic nucleus, hte ur patterns of behavior and physiology. They are known sa melanopsin-containing retinal ganglion cels ro intrinsically photosensitive retinal ganglion cells (ipRCs). T may also influence pupil responses (Spitschan et al., 2014). eh melanopsin signals When a photon from our favorite star makes way into the outer of a rod and is absorbed by a molecule of rhodopsiits segment n, it transfers its energy ot the chromophore portion of the visual pigment molecule. This proce ss, known photoactivation (also referred ot as bleaching), initiates a biochemical cascad as events eventually resulting ni the closing of cell membrane e of that normally allow chann els ions to flow into the rod's outer segment. these channels alters the balance of elec Closing trical current between the inside and outside of the rod's outer segment, making the inside of the cell more negatively charged. This process si known larization. Hyperpolarization closes voltage-gated as hyperpo- calcium channels at the synaptic terminal, thereby reducing the concentration calcium inside the cells. The lowerin of free in turn, g of the calcium concentration, reduces the concentration of neu rotransmitter (glutamate) molecules released in the synapse, and this change signals to the bipolar cell that the rod has captured a ton. T of events takes only a matter of millisecopho he entire sequence nds. While we have focused this discussion on rhodopsin, cone visu al pigment molecules act ni a qualitatively similar fashion. The amount of glutamate present in the photoreceptor-bipolar cell synapse at any one time si inversely pro of photons being absorbed by the portional ot the number photoreceptor. Thus, unlike mo other types of neurons, photoreceptors st do not respond in an all-or- nothing fashion. They pass their inform graded potentials, which vary ni ation on ot bipolar cells via size, instead of all-or-none action potentials or spikes, which are found (see Chapter 1). throughout the nervous system FIGURE 2.18 Con es in a live hum an red represent the S-, M-, and L-cones, respective Blue, green, and ly, o The three cone photopigments are not distributed equally among living human being in a patch of retina at 1 degree f a hte cones (FIGURE 2.18). Short wavel constitute only about 5-10% of hte totaength-sensitive cones (S-cones) the fovea. This pseudocolor image was mad from e by the use of adaptive optics, to measure and bypass the aberration s of essentially missing from the center of thel cone population, and they are si dichromatic (it has only two color-senfove a. Thus, hte foveal center the eye, and of selective bleaching, to isola te the different photopigments. sitive cone types). W know that there are more e also long wavelength-sensitive cones (L-cones)  2.4 Retinal Information Processing 49 TABLE 2.2 Properties of human photopic and scotopic vision Property Photopic system Scotopic system Photoreceptor 4-5 million cones 9 0 million r o d s L o c a t i o n in r e t i n a Throughout retina, with highest Outside fovea concentration close to fovea Spatial acuity (detail) High Low Light sensitivity Low High Response speed Fast Slow Saturation No saturation Saturate at ~twilight levels Dark adaptation Recovery ni 5~ minutes Recovery in ~40 minutes Light adaptation Fast (Weber's law) Slow (square root law) C o l o r vision Trichromatic None than medium waveleng