{"id":4921,"date":"2015-10-28T15:56:01","date_gmt":"2015-10-28T15:56:01","guid":{"rendered":"https:\/\/opentextbc.ca\/biology\/chapter\/17-5-vision\/"},"modified":"2021-03-04T00:07:54","modified_gmt":"2021-03-04T00:07:54","slug":"17-5-vision","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/biology\/chapter\/17-5-vision\/","title":{"raw":"17.5 Vision","rendered":"17.5 Vision"},"content":{"raw":"<div class=\"section module\" title=\"36.5.\u00a0Vision\" xml:lang=\"en\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\nBy the end of this section, you will be able to:\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n \t<li class=\"listitem\">Explain how electromagnetic waves differs from sound waves<\/li>\n \t<li class=\"listitem\">Trace the path of light through the eye to the point of the optic nerve<\/li>\n \t<li class=\"listitem\">Explain tonic activity as it is manifested in photoreceptors in the retina<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp54466688\"> <\/span><span id=\"m44761-autoid-cnx2dbk-id1829916\"> <\/span><strong>Vision<\/strong> is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.\n<div class=\"section\" title=\"Light\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h3 id=\"m44761-fs-idp118381680\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Light<\/span><\/span><\/h3>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp48440384\"> <\/span>As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium\u2014a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium; light can travel in a vacuum (<a class=\"xref target-figure\" title=\"Figure\u00a036.16.\u00a0\" href=\"#attachment_1206\">Figure 17.16<\/a>). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light\u2014a packet of electromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.\n\n<\/div>\n<span id=\"m44761-fs-idp165729760\"> <\/span>Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).\n\n[caption id=\"attachment_1206\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_01.jpg\"><img class=\"wp-image-4912\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1.jpg\" alt=\"Figure_36_05_01\" width=\"600\" height=\"254\"><\/a> Figure 17.16.\u00a0 In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of work by NASA)[\/caption]\n\n<div id=\"m44761-fig-ch36_05_01\" class=\"figure\" title=\"Figure\u00a036.16.\u00a0\">\n<div class=\"title\"><\/div>\n<span id=\"m44761-fs-idm30533744\"> <\/span>Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the <em class=\"glossterm\"><span id=\"m44761-autoid-cnx2dbk-id1840022\"> <\/span>candela<\/em>, which is approximately the luminous intensity of a one common candle.\n\n<span id=\"m44761-fs-idp32400112\"> <\/span>Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed \u201cwhite light\u201d is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light.\n\n<\/div>\n<div class=\"section\" title=\"Anatomy of the Eye\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idm51896288\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Anatomy of the Eye<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp137612992\"> <\/span>The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the <em class=\"glossterm\"><span id=\"m44761-autoid-cnx2dbk-id1840053\"> <\/span>retina<\/em> (shown in <a class=\"xref target-figure\" title=\"Figure\u00a036.17.\u00a0\" href=\"#attachment_1207\">Figure 17.17<\/a>) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (<a class=\"xref target-figure\" title=\"Figure\u00a036.17.\u00a0\" href=\"#attachment_1207\">Figure 17.17<\/a><span class=\"bold\"><strong>b<\/strong><\/span>). The <span id=\"m44761-autoid-cnx2dbk-id1822745\"> <\/span><strong>cornea,<\/strong> the front transparent layer of the eye, and the crystalline <strong><span id=\"m44761-autoid-cnx2dbk-id1822749\"> <\/span>lens<\/strong>, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The <span id=\"m44761-autoid-cnx2dbk-id1822754\"> <\/span><strong>iris<\/strong>, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.\n<div id=\"m44761-fs-idp52568064\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44761-fig-ch36_05_02\" class=\"figure\" title=\"Figure\u00a036.17.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<div class=\"mediaobject\">\n\n[caption id=\"attachment_1207\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_02.png\"><img class=\"wp-image-4913\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1.png\" alt=\"Figure_36_05_02\" width=\"600\" height=\"332\"><\/a> Figure 17.17.\u00a0 (a) The human eye is shown in cross section. (b) A blowup shows the layers of the retina.[\/caption]\n\n<\/div>\n<span id=\"m44761-fs-idp56683904\"> <\/span>Which of the following statements about the human eye is false?\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n \t<li class=\"listitem\">Rods detect color, while cones detect only shades of gray.<\/li>\n \t<li class=\"listitem\">When light enters the retina, it passes the ganglion cells and bipolar cells before reaching photoreceptors at the rear of the eye.<\/li>\n \t<li class=\"listitem\">The iris adjusts the amount of light coming into the eye.<\/li>\n \t<li class=\"listitem\">The cornea is a protective layer on the front of the eye.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp218174192\"> <\/span>The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called <span id=\"m44761-autoid-cnx2dbk-id1823015\"> <\/span><strong>presbyopia<\/strong> results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called <span id=\"m44761-autoid-cnx2dbk-id1823021\"> <\/span><strong>hyperopia<\/strong> caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. <span id=\"m44761-autoid-cnx2dbk-id1823026\"> <\/span><strong>Myopia<\/strong> (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.\n\n<span id=\"m44761-fs-idp79092400\"> <\/span>There are two types of photoreceptors in the retina: <span id=\"m44761-autoid-cnx2dbk-id1823036\"> <\/span><strong>rods<\/strong> and <span id=\"m44761-autoid-cnx2dbk-id1823040\"> <\/span><strong>cones<\/strong>, named for their general appearance as illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a036.18.\u00a0\" href=\"#attachment_1208\">Figure 17.18<\/a>. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.\n\n[caption id=\"attachment_1208\" align=\"aligncenter\" width=\"450\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_03.jpg\"><img class=\"wp-image-4914\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03.jpg\" alt=\"Figure_36_05_03\" width=\"450\" height=\"398\"><\/a> Figure 17.18.\u00a0 Rods and cones are photoreceptors in the retina. Rods respond in low light and can detect only shades of gray. Cones respond in intense light and are responsible for color vision. (credit: modification of work by Piotr Sliwa)[\/caption]\n\n<div id=\"m44761-fig-ch36_05_03\" class=\"figure\" title=\"Figure\u00a036.18.\u00a0\">\n\n<span id=\"m44761-fs-idp275200720\"> <\/span>The <strong><span id=\"m44761-autoid-cnx2dbk-id1825968\"> <\/span>fovea<\/strong> is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object\u2019s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.\n<div id=\"m44761-fs-idp119887296\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44761-fs-idp16200784\"> <\/span><img src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/eye_diagram.png\" alt=\"QR Code representing a URL\" width=\"120\"><\/div>\n<span id=\"m44761-fs-idp14651376\"> <\/span>Review the <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/eye_diagram\" target=\"\" rel=\"noopener noreferrer\">anatomical structure<\/a> of the eye, clicking on each part to practice identification.\n\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Transduction of Light\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp57896800\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Transduction of Light<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idm30468672\"> <\/span>The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, <strong><span id=\"m44761-autoid-cnx2dbk-id1841681\"> <\/span>rhodopsin<\/strong>, has two main parts <a class=\"xref target-figure\" title=\"Figure\u00a036.19.\u00a0\" href=\"#attachment_1210\">Figure 17.19<\/a>): an opsin, which is a membrane protein (in the form of a cluster of \u03b1-helices that span the membrane), and retinal\u2014a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (<span class=\"emphasis\"><em>cis<\/em><\/span>) form of the molecule to its linear (<span class=\"emphasis\"><em>trans<\/em><\/span>) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na<sup>+<\/sup> channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (<a class=\"xref target-figure\" title=\"Figure\u00a036.20.\u00a0\" href=\"#attachment_1211\">Figure 17.20<\/a>).\n\n[caption id=\"attachment_1210\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_04.jpg\"><img class=\"wp-image-4916\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04.jpg\" alt=\"Figure_36_05_04\" width=\"500\" height=\"578\"><\/a> Figure 17.19.\u00a0 (a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and retinal. When light strikes retinal, it changes shape from (b) a cis to a trans form. The signal is passed to a G-protein called transducin, triggering a series of downstream events.[\/caption]\n\n<div id=\"m44761-fig-ch36_05_04\" class=\"figure\" title=\"Figure\u00a036.19.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<\/div>\n<div id=\"m44761-fig-ch36_05_05\" class=\"figure\" title=\"Figure\u00a036.20.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n\n[caption id=\"attachment_1211\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_05.jpg\"><img class=\"wp-image-4917\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1.jpg\" alt=\"Figure\u00a036.20.\u00a0 When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.\" width=\"500\" height=\"508\"><\/a> <br>Figure 17.20.\u00a0<br>When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.[\/caption]\n\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Trichromatic Coding\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp182930816\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Trichromatic Coding<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp95869440\"> <\/span>There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in <a class=\"xref target-figure\" title=\"Figure\u00a036.21.\u00a0\" href=\"#attachment_1212\">Figure 17.21<\/a>. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (\u201cS\u201d for \u201cshort\u201d); others respond maximally to waves of 530 nm (M cones, for \u201cmedium\u201d); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or \u201clong\u201d cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.\n\n<span id=\"m44761-fs-idp26402176\"> <\/span>The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.\n\n[caption id=\"attachment_1212\" align=\"aligncenter\" width=\"450\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_06.jpg\"><img class=\"wp-image-4918\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06.jpg\" alt=\"Figure\u00a036.21.\u00a0 Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.\" width=\"450\" height=\"282\"><\/a> Figure 17.21.\u00a0<br>Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.[\/caption]\n\n<div class=\"section\" title=\"Retinal Processing\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp201989520\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Retinal Processing<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp44044176\"> <\/span>Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.\n\n<span id=\"m44761-fs-idp51630656\"> <\/span>Photoreceptors in the retina continuously undergo <span id=\"m44761-autoid-cnx2dbk-id1816308\"> <\/span><strong>tonic activity<\/strong>. That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline; while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.\n\n<span id=\"m44761-fs-idm24639296\"> <\/span>You can demonstrate this using an easy demonstration to \u201ctrick\u201d your retina and brain about the colors you are observing in your visual field. Look fixedly at <a class=\"xref target-figure\" title=\"Figure\u00a036.22.\u00a0\" href=\"#attachment_1213\">Figure 17.22<\/a> for about 45 seconds. Then quickly shift your gaze to a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall; the afterimage of the flag should continue to appear as red, white, and blue. What causes this? According to an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the brain interpreted this abrupt downshift as if the ganglion cells were responding now to their \u201copponent\u201d colors: red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state, the false perception of color will disappear.\n\n[caption id=\"attachment_1213\" align=\"aligncenter\" width=\"450\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_07.jpg\"><img class=\"wp-image-4919\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07.jpg\" alt=\"Figure_36_05_07\" width=\"450\" height=\"325\"><\/a> Figure 17.22.\u00a0 View this flag to understand how retinal processing works. Stare at the center of the flag (indicated by the white dot) for 45 seconds, and then quickly look at a white background, noticing how colors appear.[\/caption]\n\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Higher Processing\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp47985552\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Higher Processing<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44761-fs-idp10571392\"> <\/span>The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for \u201ccrossing\u201d) found at the base of the brain and allows us to coordinate information from both eyes.\n\n<span id=\"m44761-fs-idp164837360\"> <\/span>Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus, the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (\u201cwhere\u201d) information. A second stream projects to the temporal lobe and carries both magnocellular (\u201cwhere\u201d) and parvocellular (\u201cwhat\u201d) information.\n\n<span id=\"m44761-fs-idp5649760\"> <\/span>Another important visual route is a pathway from the retina to the <span id=\"m44761-autoid-cnx2dbk-id1831512\"> <\/span><strong>superior colliculus<\/strong> in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to the <span id=\"m44761-autoid-cnx2dbk-id1831517\"> <\/span><strong>suprachiasmatic nucleus<\/strong> (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body\u2019s internal clock, which controls our <strong><span id=\"m44761-autoid-cnx2dbk-id1831522\"> <\/span>circadian<\/strong> (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep\/wake patterns and annual cycles.\n<div class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44761-fs-idp101323136\"> <\/span><img src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/sense_of_sight.png\" alt=\"QR Code representing a URL\" width=\"120\"><\/div>\n<span id=\"m44761-fs-idp29317200\"> <\/span>View this\n\n<\/div>\n<\/div>\n<a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/sense_of_sight\" target=\"\" rel=\"noopener noreferrer\">interactive presentation<\/a> to review what you have learned about how vision functions.\n<h2>Summary<\/h2>\nVision is the only photo responsive sense. Visible light travels in waves and is a very small slice of the electromagnetic radiation spectrum. Light waves differ based on their frequency (wavelength = hue) and amplitude (intensity = brightness).\n\n<span id=\"m44761-fs-idp30108000\"> <\/span>In the vertebrate retina, there are two types of light receptors (photoreceptors): cones and rods. Cones, which are the source of color vision, exist in three forms\u2014L, M, and S\u2014and they are differentially sensitive to different wavelengths. Cones are located in the retina, along with the dim-light, achromatic receptors (rods). Cones are found in the fovea, the central region of the retina, whereas rods are found in the peripheral regions of the retina.\n\n<span id=\"m44761-fs-idp69177296\"> <\/span>Visual signals travel from the eye over the axons of retinal ganglion cells, which make up the optic nerves. Ganglion cells come in several versions. Some ganglion cell axons carry information on form, movement, depth, and brightness, while other axons carry information on color and fine detail. Visual information is sent to the superior colliculi in the midbrain, where coordination of eye movements and integration of auditory information takes place. Visual information is also sent to the suprachiasmatic nucleus (SCN) of the hypothalamus, which plays a role in the circadian cycle.\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<ol>\n \t<li>Which of the following statements about mechanoreceptors is false?\n<ol>\n \t<li>Pacini corpuscles are found in both glabrous and hairy skin.<\/li>\n \t<li>Merkel\u2019s disks are abundant on the fingertips and lips.<\/li>\n \t<li>Ruffini endings are encapsulated mechanoreceptors.<\/li>\n \t<li>Meissner\u2019s corpuscles extend into the lower dermis.<\/li>\n<\/ol>\n<\/li>\n \t<li><span id=\"m44761-fs-idp4420208\">Why do people over 55 often need reading glasses?<\/span>\n<ol>\n \t<li>Their cornea no longer focuses correctly.<\/li>\n \t<li>Their lens no longer focuses correctly.<\/li>\n \t<li>Their eyeball has elongated with age, causing images to focus in front of their retina.<\/li>\n \t<li>Their retina has thinned with age, making vision more difficult.<\/li>\n<\/ol>\n<\/li>\n \t<li><span id=\"m44761-fs-idp47635056\">Why is it easier to see images at night using peripheral, rather than the central, vision?<\/span>\n<ol>\n \t<li>Cones are denser in the periphery of the retina.<\/li>\n \t<li>Bipolar cells are denser in the periphery of the retina.<\/li>\n \t<li>Rods are denser in the periphery of the retina.<\/li>\n \t<li>The optic nerve exits at the periphery of the retina.<\/li>\n<\/ol>\n<\/li>\n \t<li><span id=\"m44761-fs-idm100375248\"><span id=\"m44761-fs-idp25394608\"><\/span>A person catching a ball must coordinate her head and eyes. What part of the brain is helping to do this?<\/span><\/li>\n \t<li><span id=\"m44761-fs-idp51697088\">How could the pineal gland, the brain structure that plays a role in annual cycles, use visual information from the suprachiasmatic nucleus of the hypothalamus?<\/span>\n<div class=\"body\">\n<div id=\"m44761-fs-idp1347488\" class=\"solution labeled\">\n<div class=\"body\"><\/div>\n<\/div>\n<\/div><\/li>\n \t<li><span id=\"m44761-fs-idp186033728\">How is the relationship between photoreceptors and bipolar cells different from other sensory receptors and adjacent cells?<\/span><\/li>\n<\/ol>\n<strong>Answers<\/strong>\n<ol>\n \t<li>D<\/li>\n \t<li>B<\/li>\n \t<li>C<\/li>\n \t<li>D<\/li>\n \t<li>The pineal gland could use length-of-day information to determine the time of year, for example. Day length is shorter in the winter than it is in the summer. For many animals and plants, photoperiod cues them to reproduce at a certain time of year.<\/li>\n \t<li>The photoreceptors tonically inhibit the bipolar cells, and stimulation of the receptors turns this inhibition off, activating the bipolar cells.<\/li>\n<\/ol>\n<\/div>\n<div class=\"cnx-eoc summary\">\n<div class=\"cnx-eoc art-exercise\">\n<div class=\"section empty\">\n<div class=\"section empty\">\n<div class=\"section\">\n<div class=\"body\">\n<div class=\"exercise\">\n<div class=\"title\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"glossary\" title=\"Glossary\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<dl>\n \t<dt><strong>circadian<\/strong><\/dt>\n \t<dd>describes a time cycle about one day in length<\/dd>\n \t<dt><strong>cone<\/strong><\/dt>\n \t<dd>weakly photosensitive, chromatic, cone-shaped neuron in the fovea of the retina that detects bright light and is used in daytime color vision<\/dd>\n \t<dt><strong>cornea<\/strong><\/dt>\n \t<dd>transparent layer over the front of the eye that helps focus light waves<\/dd>\n \t<dt><strong>fovea<\/strong><\/dt>\n \t<dd>region in the center of the retina with a high density of photoreceptors and which is responsible for acute vision<\/dd>\n \t<dt><strong>hyperopia<\/strong><\/dt>\n \t<dd>(also, farsightedness) visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry<\/dd>\n \t<dt><strong>iris<\/strong><\/dt>\n \t<dd>pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye<\/dd>\n \t<dt><strong>lens<\/strong><\/dt>\n \t<dd>transparent, convex structure behind the cornea that helps focus light waves on the retina<\/dd>\n \t<dt><strong>myopia<\/strong><\/dt>\n \t<dd>(also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making images in the distance blurry, but close-up images clear<\/dd>\n \t<dd>in the basilar membrane, the site of the transduction of sound, a mechanical wave, to a neural signal<\/dd>\n \t<dt><strong>presbyopia<\/strong><\/dt>\n \t<dd>visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry; caused by age-based changes in the lens<\/dd>\n \t<dt><strong>rhodopsin<\/strong><\/dt>\n \t<dd>main photopigment in vertebrates<\/dd>\n \t<dt><strong>rod<\/strong><\/dt>\n \t<dd>strongly photosensitive, achromatic, cylindrical neuron in the outer edges of the retina that detects dim light and is used in<\/dd>\n \t<dt><strong>superior colliculus<\/strong><\/dt>\n \t<dd>paired structure in the top of the midbrain, which manages eye movements and auditory integration<\/dd>\n \t<dt><strong>suprachiasmatic nucleus<\/strong><\/dt>\n \t<dd>cluster of cells in the hypothalamus that plays a role in the circadian cycle<\/dd>\n \t<dt><strong>tonic activity<\/strong><\/dt>\n \t<dd>in a neuron, slight continuous activity while at rest<\/dd>\n \t<dt><strong>vision<\/strong><\/dt>\n \t<dd>sense of sight<\/dd>\n<\/dl>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>","rendered":"<div class=\"section module\" title=\"36.5.\u00a0Vision\" xml:lang=\"en\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n<li class=\"listitem\">Explain how electromagnetic waves differs from sound waves<\/li>\n<li class=\"listitem\">Trace the path of light through the eye to the point of the optic nerve<\/li>\n<li class=\"listitem\">Explain tonic activity as it is manifested in photoreceptors in the retina<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp54466688\"> <\/span><span id=\"m44761-autoid-cnx2dbk-id1829916\"> <\/span><strong>Vision<\/strong> is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.<\/p>\n<div class=\"section\" title=\"Light\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h3 id=\"m44761-fs-idp118381680\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Light<\/span><\/span><\/h3>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp48440384\"> <\/span>As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a medium\u2014a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium; light can travel in a vacuum (<a class=\"xref target-figure\" title=\"Figure\u00a036.16.\u00a0\" href=\"#attachment_1206\">Figure 17.16<\/a>). The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light\u2014a packet of electromagnetic radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light.<\/p>\n<\/div>\n<p><span id=\"m44761-fs-idp165729760\"> <\/span>Certain variables are important when discussing perception of light. Wavelength (which varies inversely with frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).<\/p>\n<figure id=\"attachment_1206\" aria-describedby=\"caption-attachment-1206\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_01.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4912\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1.jpg\" alt=\"Figure_36_05_01\" width=\"600\" height=\"254\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1-300x127.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1-768x325.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1-65x27.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1-225x95.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/10\/Figure_36_05_01-1024x433-1-350x148.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1206\" class=\"wp-caption-text\">Figure 17.16.\u00a0 In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of work by NASA)<\/figcaption><\/figure>\n<div id=\"m44761-fig-ch36_05_01\" class=\"figure\" title=\"Figure\u00a036.16.\u00a0\">\n<div class=\"title\"><\/div>\n<p><span id=\"m44761-fs-idm30533744\"> <\/span>Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the <em class=\"glossterm\"><span id=\"m44761-autoid-cnx2dbk-id1840022\"> <\/span>candela<\/em>, which is approximately the luminous intensity of a one common candle.<\/p>\n<p><span id=\"m44761-fs-idp32400112\"> <\/span>Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is termed \u201cwhite light\u201d is light that is perceived as white by the human eye. This effect is produced by light that stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors) that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other wavelengths of light.<\/p>\n<\/div>\n<div class=\"section\" title=\"Anatomy of the Eye\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idm51896288\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Anatomy of the Eye<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp137612992\"> <\/span>The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the <em class=\"glossterm\"><span id=\"m44761-autoid-cnx2dbk-id1840053\"> <\/span>retina<\/em> (shown in <a class=\"xref target-figure\" title=\"Figure\u00a036.17.\u00a0\" href=\"#attachment_1207\">Figure 17.17<\/a>) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (<a class=\"xref target-figure\" title=\"Figure\u00a036.17.\u00a0\" href=\"#attachment_1207\">Figure 17.17<\/a><span class=\"bold\"><strong>b<\/strong><\/span>). The <span id=\"m44761-autoid-cnx2dbk-id1822745\"> <\/span><strong>cornea,<\/strong> the front transparent layer of the eye, and the crystalline <strong><span id=\"m44761-autoid-cnx2dbk-id1822749\"> <\/span>lens<\/strong>, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The <span id=\"m44761-autoid-cnx2dbk-id1822754\"> <\/span><strong>iris<\/strong>, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.<\/p>\n<div id=\"m44761-fs-idp52568064\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"body\">\n<div id=\"m44761-fig-ch36_05_02\" class=\"figure\" title=\"Figure\u00a036.17.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<div class=\"mediaobject\">\n<figure id=\"attachment_1207\" aria-describedby=\"caption-attachment-1207\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_02.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4913\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1.png\" alt=\"Figure_36_05_02\" width=\"600\" height=\"332\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1.png 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1-300x166.png 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1-768x425.png 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1-65x36.png 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1-225x124.png 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_02-1024x566-1-350x193.png 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1207\" class=\"wp-caption-text\">Figure 17.17.\u00a0 (a) The human eye is shown in cross section. (b) A blowup shows the layers of the retina.<\/figcaption><\/figure>\n<\/div>\n<p><span id=\"m44761-fs-idp56683904\"> <\/span>Which of the following statements about the human eye is false?<\/p>\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n<li class=\"listitem\">Rods detect color, while cones detect only shades of gray.<\/li>\n<li class=\"listitem\">When light enters the retina, it passes the ganglion cells and bipolar cells before reaching photoreceptors at the rear of the eye.<\/li>\n<li class=\"listitem\">The iris adjusts the amount of light coming into the eye.<\/li>\n<li class=\"listitem\">The cornea is a protective layer on the front of the eye.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp218174192\"> <\/span>The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called <span id=\"m44761-autoid-cnx2dbk-id1823015\"> <\/span><strong>presbyopia<\/strong> results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called <span id=\"m44761-autoid-cnx2dbk-id1823021\"> <\/span><strong>hyperopia<\/strong> caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. <span id=\"m44761-autoid-cnx2dbk-id1823026\"> <\/span><strong>Myopia<\/strong> (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.<\/p>\n<p><span id=\"m44761-fs-idp79092400\"> <\/span>There are two types of photoreceptors in the retina: <span id=\"m44761-autoid-cnx2dbk-id1823036\"> <\/span><strong>rods<\/strong> and <span id=\"m44761-autoid-cnx2dbk-id1823040\"> <\/span><strong>cones<\/strong>, named for their general appearance as illustrated in <a class=\"xref target-figure\" title=\"Figure\u00a036.18.\u00a0\" href=\"#attachment_1208\">Figure 17.18<\/a>. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.<\/p>\n<figure id=\"attachment_1208\" aria-describedby=\"caption-attachment-1208\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_03.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4914\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03.jpg\" alt=\"Figure_36_05_03\" width=\"450\" height=\"398\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03.jpg 544w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03-300x265.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03-65x57.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03-225x199.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_03-350x309.jpg 350w\" sizes=\"auto, (max-width: 450px) 100vw, 450px\" \/><\/a><figcaption id=\"caption-attachment-1208\" class=\"wp-caption-text\">Figure 17.18.\u00a0 Rods and cones are photoreceptors in the retina. Rods respond in low light and can detect only shades of gray. Cones respond in intense light and are responsible for color vision. (credit: modification of work by Piotr Sliwa)<\/figcaption><\/figure>\n<div id=\"m44761-fig-ch36_05_03\" class=\"figure\" title=\"Figure\u00a036.18.\u00a0\">\n<p><span id=\"m44761-fs-idp275200720\"> <\/span>The <strong><span id=\"m44761-autoid-cnx2dbk-id1825968\"> <\/span>fovea<\/strong> is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object\u2019s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.<\/p>\n<div id=\"m44761-fs-idp119887296\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action<br \/>\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44761-fs-idp16200784\"> <\/span><img decoding=\"async\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/eye_diagram.png\" alt=\"QR Code representing a URL\" width=\"120\" \/><\/div>\n<p><span id=\"m44761-fs-idp14651376\"> <\/span>Review the <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/eye_diagram\" target=\"\" rel=\"noopener noreferrer\">anatomical structure<\/a> of the eye, clicking on each part to practice identification.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Transduction of Light\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp57896800\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Transduction of Light<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idm30468672\"> <\/span>The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, <strong><span id=\"m44761-autoid-cnx2dbk-id1841681\"> <\/span>rhodopsin<\/strong>, has two main parts <a class=\"xref target-figure\" title=\"Figure\u00a036.19.\u00a0\" href=\"#attachment_1210\">Figure 17.19<\/a>): an opsin, which is a membrane protein (in the form of a cluster of \u03b1-helices that span the membrane), and retinal\u2014a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (<span class=\"emphasis\"><em>cis<\/em><\/span>) form of the molecule to its linear (<span class=\"emphasis\"><em>trans<\/em><\/span>) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na<sup>+<\/sup> channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (<a class=\"xref target-figure\" title=\"Figure\u00a036.20.\u00a0\" href=\"#attachment_1211\">Figure 17.20<\/a>).<\/p>\n<figure id=\"attachment_1210\" aria-describedby=\"caption-attachment-1210\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_04.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4916\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04.jpg\" alt=\"Figure_36_05_04\" width=\"500\" height=\"578\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04.jpg 800w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04-259x300.jpg 259w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04-768x888.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04-65x75.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04-225x260.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_04-350x405.jpg 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1210\" class=\"wp-caption-text\">Figure 17.19.\u00a0 (a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and retinal. When light strikes retinal, it changes shape from (b) a cis to a trans form. The signal is passed to a G-protein called transducin, triggering a series of downstream events.<\/figcaption><\/figure>\n<div id=\"m44761-fig-ch36_05_04\" class=\"figure\" title=\"Figure\u00a036.19.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\"><\/div>\n<\/div>\n<div id=\"m44761-fig-ch36_05_05\" class=\"figure\" title=\"Figure\u00a036.20.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<figure id=\"attachment_1211\" aria-describedby=\"caption-attachment-1211\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_05.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4917\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1.jpg\" alt=\"Figure\u00a036.20.\u00a0 When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.\" width=\"500\" height=\"508\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1.jpg 1008w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1-295x300.jpg 295w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1-768x780.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1-65x66.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1-225x229.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_05-1008x1024-1-350x356.jpg 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1211\" class=\"wp-caption-text\">Figure 17.20.\u00a0<br \/>When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Trichromatic Coding\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp182930816\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Trichromatic Coding<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp95869440\"> <\/span>There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in <a class=\"xref target-figure\" title=\"Figure\u00a036.21.\u00a0\" href=\"#attachment_1212\">Figure 17.21<\/a>. Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (\u201cS\u201d for \u201cshort\u201d); others respond maximally to waves of 530 nm (M cones, for \u201cmedium\u201d); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or \u201clong\u201d cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.<\/p>\n<p><span id=\"m44761-fs-idp26402176\"> <\/span>The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.<\/p>\n<figure id=\"attachment_1212\" aria-describedby=\"caption-attachment-1212\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_06.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4918\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06.jpg\" alt=\"Figure\u00a036.21.\u00a0 Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.\" width=\"450\" height=\"282\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06.jpg 544w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06-300x188.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06-65x41.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06-225x141.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_06-350x219.jpg 350w\" sizes=\"auto, (max-width: 450px) 100vw, 450px\" \/><\/a><figcaption id=\"caption-attachment-1212\" class=\"wp-caption-text\">Figure 17.21.\u00a0<br \/>Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.<\/figcaption><\/figure>\n<div class=\"section\" title=\"Retinal Processing\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp201989520\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Retinal Processing<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp44044176\"> <\/span>Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.<\/p>\n<p><span id=\"m44761-fs-idp51630656\"> <\/span>Photoreceptors in the retina continuously undergo <span id=\"m44761-autoid-cnx2dbk-id1816308\"> <\/span><strong>tonic activity<\/strong>. That is, they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline; while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.<\/p>\n<p><span id=\"m44761-fs-idm24639296\"> <\/span>You can demonstrate this using an easy demonstration to \u201ctrick\u201d your retina and brain about the colors you are observing in your visual field. Look fixedly at <a class=\"xref target-figure\" title=\"Figure\u00a036.22.\u00a0\" href=\"#attachment_1213\">Figure 17.22<\/a> for about 45 seconds. Then quickly shift your gaze to a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall; the afterimage of the flag should continue to appear as red, white, and blue. What causes this? According to an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the brain interpreted this abrupt downshift as if the ganglion cells were responding now to their \u201copponent\u201d colors: red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state, the false perception of color will disappear.<\/p>\n<figure id=\"attachment_1213\" aria-describedby=\"caption-attachment-1213\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_36_05_07.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4919\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07.jpg\" alt=\"Figure_36_05_07\" width=\"450\" height=\"325\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07.jpg 544w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07-300x217.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07-65x47.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07-225x163.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_36_05_07-350x253.jpg 350w\" sizes=\"auto, (max-width: 450px) 100vw, 450px\" \/><\/a><figcaption id=\"caption-attachment-1213\" class=\"wp-caption-text\">Figure 17.22.\u00a0 View this flag to understand how retinal processing works. Stare at the center of the flag (indicated by the white dot) for 45 seconds, and then quickly look at a white background, noticing how colors appear.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Higher Processing\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44761-fs-idp47985552\" style=\"text-align: left;\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Higher Processing<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44761-fs-idp10571392\"> <\/span>The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for \u201ccrossing\u201d) found at the base of the brain and allows us to coordinate information from both eyes.<\/p>\n<p><span id=\"m44761-fs-idp164837360\"> <\/span>Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus, the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (\u201cwhere\u201d) information. A second stream projects to the temporal lobe and carries both magnocellular (\u201cwhere\u201d) and parvocellular (\u201cwhat\u201d) information.<\/p>\n<p><span id=\"m44761-fs-idp5649760\"> <\/span>Another important visual route is a pathway from the retina to the <span id=\"m44761-autoid-cnx2dbk-id1831512\"> <\/span><strong>superior colliculus<\/strong> in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to the <span id=\"m44761-autoid-cnx2dbk-id1831517\"> <\/span><strong>suprachiasmatic nucleus<\/strong> (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body\u2019s internal clock, which controls our <strong><span id=\"m44761-autoid-cnx2dbk-id1831522\"> <\/span>circadian<\/strong> (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep\/wake patterns and annual cycles.<\/p>\n<div class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action<br \/>\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44761-fs-idp101323136\"> <\/span><img decoding=\"async\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/sense_of_sight.png\" alt=\"QR Code representing a URL\" width=\"120\" \/><\/div>\n<p><span id=\"m44761-fs-idp29317200\"> <\/span>View this<\/p>\n<\/div>\n<\/div>\n<p><a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/sense_of_sight\" target=\"\" rel=\"noopener noreferrer\">interactive presentation<\/a> to review what you have learned about how vision functions.<\/p>\n<h2>Summary<\/h2>\n<p>Vision is the only photo responsive sense. Visible light travels in waves and is a very small slice of the electromagnetic radiation spectrum. Light waves differ based on their frequency (wavelength = hue) and amplitude (intensity = brightness).<\/p>\n<p><span id=\"m44761-fs-idp30108000\"> <\/span>In the vertebrate retina, there are two types of light receptors (photoreceptors): cones and rods. Cones, which are the source of color vision, exist in three forms\u2014L, M, and S\u2014and they are differentially sensitive to different wavelengths. Cones are located in the retina, along with the dim-light, achromatic receptors (rods). Cones are found in the fovea, the central region of the retina, whereas rods are found in the peripheral regions of the retina.<\/p>\n<p><span id=\"m44761-fs-idp69177296\"> <\/span>Visual signals travel from the eye over the axons of retinal ganglion cells, which make up the optic nerves. Ganglion cells come in several versions. Some ganglion cell axons carry information on form, movement, depth, and brightness, while other axons carry information on color and fine detail. Visual information is sent to the superior colliculi in the midbrain, where coordination of eye movements and integration of auditory information takes place. Visual information is also sent to the suprachiasmatic nucleus (SCN) of the hypothalamus, which plays a role in the circadian cycle.<\/p>\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<ol>\n<li>Which of the following statements about mechanoreceptors is false?\n<ol>\n<li>Pacini corpuscles are found in both glabrous and hairy skin.<\/li>\n<li>Merkel\u2019s disks are abundant on the fingertips and lips.<\/li>\n<li>Ruffini endings are encapsulated mechanoreceptors.<\/li>\n<li>Meissner\u2019s corpuscles extend into the lower dermis.<\/li>\n<\/ol>\n<\/li>\n<li><span id=\"m44761-fs-idp4420208\">Why do people over 55 often need reading glasses?<\/span>\n<ol>\n<li>Their cornea no longer focuses correctly.<\/li>\n<li>Their lens no longer focuses correctly.<\/li>\n<li>Their eyeball has elongated with age, causing images to focus in front of their retina.<\/li>\n<li>Their retina has thinned with age, making vision more difficult.<\/li>\n<\/ol>\n<\/li>\n<li><span id=\"m44761-fs-idp47635056\">Why is it easier to see images at night using peripheral, rather than the central, vision?<\/span>\n<ol>\n<li>Cones are denser in the periphery of the retina.<\/li>\n<li>Bipolar cells are denser in the periphery of the retina.<\/li>\n<li>Rods are denser in the periphery of the retina.<\/li>\n<li>The optic nerve exits at the periphery of the retina.<\/li>\n<\/ol>\n<\/li>\n<li><span id=\"m44761-fs-idm100375248\"><span id=\"m44761-fs-idp25394608\"><\/span>A person catching a ball must coordinate her head and eyes. What part of the brain is helping to do this?<\/span><\/li>\n<li><span id=\"m44761-fs-idp51697088\">How could the pineal gland, the brain structure that plays a role in annual cycles, use visual information from the suprachiasmatic nucleus of the hypothalamus?<\/span>\n<div class=\"body\">\n<div id=\"m44761-fs-idp1347488\" class=\"solution labeled\">\n<div class=\"body\"><\/div>\n<\/div>\n<\/div>\n<\/li>\n<li><span id=\"m44761-fs-idp186033728\">How is the relationship between photoreceptors and bipolar cells different from other sensory receptors and adjacent cells?<\/span><\/li>\n<\/ol>\n<p><strong>Answers<\/strong><\/p>\n<ol>\n<li>D<\/li>\n<li>B<\/li>\n<li>C<\/li>\n<li>D<\/li>\n<li>The pineal gland could use length-of-day information to determine the time of year, for example. Day length is shorter in the winter than it is in the summer. For many animals and plants, photoperiod cues them to reproduce at a certain time of year.<\/li>\n<li>The photoreceptors tonically inhibit the bipolar cells, and stimulation of the receptors turns this inhibition off, activating the bipolar cells.<\/li>\n<\/ol>\n<\/div>\n<div class=\"cnx-eoc summary\">\n<div class=\"cnx-eoc art-exercise\">\n<div class=\"section empty\">\n<div class=\"section empty\">\n<div class=\"section\">\n<div class=\"body\">\n<div class=\"exercise\">\n<div class=\"title\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"glossary\" title=\"Glossary\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<dl>\n<dt><strong>circadian<\/strong><\/dt>\n<dd>describes a time cycle about one day in length<\/dd>\n<dt><strong>cone<\/strong><\/dt>\n<dd>weakly photosensitive, chromatic, cone-shaped neuron in the fovea of the retina that detects bright light and is used in daytime color vision<\/dd>\n<dt><strong>cornea<\/strong><\/dt>\n<dd>transparent layer over the front of the eye that helps focus light waves<\/dd>\n<dt><strong>fovea<\/strong><\/dt>\n<dd>region in the center of the retina with a high density of photoreceptors and which is responsible for acute vision<\/dd>\n<dt><strong>hyperopia<\/strong><\/dt>\n<dd>(also, farsightedness) visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry<\/dd>\n<dt><strong>iris<\/strong><\/dt>\n<dd>pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye<\/dd>\n<dt><strong>lens<\/strong><\/dt>\n<dd>transparent, convex structure behind the cornea that helps focus light waves on the retina<\/dd>\n<dt><strong>myopia<\/strong><\/dt>\n<dd>(also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making images in the distance blurry, but close-up images clear<\/dd>\n<dd>in the basilar membrane, the site of the transduction of sound, a mechanical wave, to a neural signal<\/dd>\n<dt><strong>presbyopia<\/strong><\/dt>\n<dd>visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry; caused by age-based changes in the lens<\/dd>\n<dt><strong>rhodopsin<\/strong><\/dt>\n<dd>main photopigment in vertebrates<\/dd>\n<dt><strong>rod<\/strong><\/dt>\n<dd>strongly photosensitive, achromatic, cylindrical neuron in the outer edges of the retina that detects dim light and is used in<\/dd>\n<dt><strong>superior colliculus<\/strong><\/dt>\n<dd>paired structure in the top of the midbrain, which manages eye movements and auditory integration<\/dd>\n<dt><strong>suprachiasmatic nucleus<\/strong><\/dt>\n<dd>cluster of cells in the hypothalamus that plays a role in the circadian cycle<\/dd>\n<dt><strong>tonic activity<\/strong><\/dt>\n<dd>in a neuron, slight continuous activity while at rest<\/dd>\n<dt><strong>vision<\/strong><\/dt>\n<dd>sense of sight<\/dd>\n<\/dl>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":90,"menu_order":37,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[],"license":[57],"class_list":["post-4921","chapter","type-chapter","status-publish","hentry","license-cc-by"],"part":4884,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4921","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/users\/90"}],"version-history":[{"count":1,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4921\/revisions"}],"predecessor-version":[{"id":4922,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4921\/revisions\/4922"}],"part":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/parts\/4884"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4921\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/media?parent=4921"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapter-type?post=4921"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/contributor?post=4921"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/license?post=4921"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}