Part 1-  What are the neurological structures of taste, touch, and smell? How do taste, touch, and smell affect behavior? Provide an example in your response.

Part 2-  The case of Mrs. R. and Dr. L. is presented in Chapter 6. What is a potential diagnosis for Mrs. R. given her presenting symptoms? Explain this diagnosis including common symptoms and areas of the brain associated with the diagnosis. Why is Mrs. R. having problems with “things” and “faces”? In your explanation describe how a medical or physical problem can actually appear as a mental health problem.


Review this week’s course materials and learning activities, and reflect on your learning so far this week. Respond to one or more of the following prompts in one to two paragraphs:

  1. Provide citation and reference to the material(s) you discuss. Describe what you found interesting regarding this topic, and why.
  2. Describe how you will apply that learning in your daily life, including your work life.
  3. Describe what may be unclear to you, and what you would like to learn.

chapter 6 Vision



· ■  The Stimulus

· ■  Anatomy of the Visual System

The Eyes


Connections Between Eye and Brain

Section Summary

· ■  Coding of Visual Information in the Retina

Coding of Light and Dark

Coding of Color

· Section Summary

· ■  Analysis of Visual Information: Role of the Striate Cortex

Anatomy of the Striate Cortex

Orientation and Movement

Spatial Frequency

Retinal Disparity


Modular Organization of the Striate Cortex

· Section Summary

· ■  Analysis of Visual Information: Role of the Visual Association Cortex

Two Streams of Visual Analysis

Perception of Color

Perception of Form

Perception of Movement

Perception of Spatial Location

Section Summary

Dr. L., a young neuropsychologist, was presenting the case of Mrs. R. to a group of medical students doing a rotation in the neurology department at the medical center. The chief of the department had shown them Mrs. R.’s CT scans, and now Dr. L. was addressing the students. He told them that Mrs. R.’s stroke had not impaired her ability to talk or to move about, but it had affected her vision.

A nurse ushered Mrs. R. into the room and helped her find a seat at the end of the table.

“How are you, Mrs. R.?” asked Dr. L.

“I’m fine. I’ve been home for a month now, and I can do just about everything that I did before I had my stroke.”

“Good. How is your vision?”

“Well, I’m afraid that’s still a problem.”

“What seems to give you the most trouble?”

“I just don’t seem to be able to recognize things. When I’m working in my kitchen, I know what everything is as long as no one moves anything. A few times my husband tried to help me by putting things away, and I couldn’t see them any more.” She laughed. “Well, I could see them, but I just couldn’t say what they were.”

Dr. L. took some objects out of a paper bag and placed them on the table in front of her.

“Can you tell me what these are?” he asked. “No,” he said, “please don’t touch them.”

Mrs. R. stared intently at the objects. “No, I can’t rightly say what they are.”

Dr. L. pointed to one of them, a wristwatch. “Tell me what you see here,” he said.

Mrs. R. looked thoughtful, turning her head one way and then the other. “Well, I see something round, and it has two things attached to it, one on the top and one on the bottom.” She continued to stare at it. “There are some things inside the circle, I think, but I can’t make out what they are.”

“Pick it up.”

She did so, made a wry face, and said, “Oh. It’s a wristwatch.” At Dr. L.’s request, she picked up the rest of the objects, one by one, and identified each of them correctly.

“Do you have trouble recognizing people, too?” asked Dr. L.

“Oh, yes!” she sighed. “While I was still in the hospital, my husband and my son both came in to see me, and I couldn’t tell who was who until my husband said something—then I could tell which direction his voice was coming from. Now I’ve trained myself to recognize my husband. I can usually see his glasses and his bald head, but I have to work at it. And I’ve been fooled a few times.” She laughed. “One of our neighbors is bald and wears glasses, too, and one day when he and his wife were visiting us, I thought he was my husband, so I called him ‘honey.’ It was a little embarrassing at first, but everyone understood.”

“What does a face look like to you?” asked Dr. L.

“Well, I know that it’s a face, because I can usually see the eyes, and it’s on top of a body. I can see a body pretty well, by how it moves.” She paused a moment. “Oh, yes, I forgot, sometimes I can recognize a person by how he moves. You know, you can often recognize friends by the way they walk, even when they’re far away. I can still do that. That’s funny, isn’t it? I can’t see people’s faces very well, but I can recognize the way they walk.”

Dr. L. made some movements with his hands. “Can you tell what I’m pretending to do?” he asked.

“Yes, you’re mixing something—like some cake batter.”

He mimed the gestures of turning a key, writing, and dealing out playing cards, and Mrs. R. recognized them without any difficulty.

“Do you have any trouble reading?” he asked.

“Well, a little, but I don’t do too badly.”

Dr. L. handed her a magazine, and she began to read the article aloud—somewhat hesitantly but accurately. “Why is it,” she asked, “that I can see the words all right but have so much trouble with thingsand with people’s faces?”

As we saw in  Chapter 3 , the brain performs two major functions: It controls the movements of the muscles, producing useful behaviors, and it regulates the body’s internal environment. To perform both these tasks, the brain must be informed about what is happening both in the external environment and within the body. Such information is received by the sensory systems. This chapter and the next are devoted to a discussion of the ways in which sensory organs detect changes in the environment and the ways in which the brain interprets neural signals from these organs.

We receive information about the environment from  sensory receptors —specialized neurons that detect a variety of physical events. (Do not confuse sensory receptors with receptors for neurotransmitters, neuromodulators, and hormones. Sensory receptors are specialized neurons, and the other types of receptors are specialized proteins that bind with certain molecules.) Stimuli impinge on the receptors and, through various processes, alter their membrane potentials. This process is known as  sensory transduction  because sensory events are transduced (“transferred”) into changes in the cells’ membrane potential. These electrical changes are called  receptor potentials . Most receptors lack axons; a portion of their somatic membrane forms synapses with the dendrites of other neurons. Receptor potentials affect the release of neurotransmitters and hence modify the pattern of firing in neurons with which these cells form synapses. Ultimately, the information reaches the brain.

image2 sensory receptor A specialized neuron that detects a particular category of physical events.

image3 sensory transduction The process by which sensory stimuli are transduced into slow, graded receptor potentials.

image4 receptor potential A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.


FIGURE 6.1 The Electromagnetic Spectrum

People often say that we have five senses: sight, hearing, smell, taste, and touch. Actually, we have more than five, but even experts disagree about how the lines between the various categories should be drawn. Certainly, we should add the vestibular senses; as well as providing us with auditory information, the inner ear supplies information about head orientation and movement. The sense of touch (or, more accurately, somatosensation) detects changes in pressure, warmth, cold, vibration, limb position, and several different kinds of events that damage tissue (that is, produce pain). Everyone agrees that we can detect all of these stimuli; the issue is whether we should say that they are detected by separate senses.

This chapter considers vision, the sensory modality that receives the most attention from psychologists, anatomists, and physiologists. One reason for this attention derives from the fascinating complexity of the sensory organs of vision and the relatively large proportion of the brain that is devoted to the analysis of visual information. Approximately 20 percent of the cerebral cortex plays a direct role in the analysis of visual information (Wandell, Dumoulin, and Brewer,  2007 ). Another reason, I am sure, is that vision is so important to us as individuals. A natural fascination with such a rich source of information about the world leads to curiosity about how this sensory modality works.  Chapter 7  deals with the other sensory modalities: audition, the vestibular senses, the somatosenses, gustation, and olfaction.

The Stimulus

As we all know, our eyes detect the presence of light. For humans, light is a narrow band of the spectrum of electromagnetic radiation. Electromagnetic radiation with a wavelength of between 380 and 760 nm (a nanometer, nm, is one-billionth of a meter) is visible to us. (See  Figure 6.1 . ) Other animals can detect different ranges of electromagnetic radiation. For example, honeybees can detect differences in ultraviolet radiation reflected by flowers that appear white to us. The range of wavelengths we call light is not qualitatively different from the rest of the electromagnetic spectrum; it is simply the part of the continuum that we humans can see.

The perceived color of light is determined by three dimensions: hue, saturation, and brightness. Light travels at a constant speed of approximately 300,000 kilometers (186,000 miles) per second. Thus, if the frequency of oscillation of the wave varies, the distance between the peaks of the waves will vary similarly but in inverse fashion. Slower oscillations lead to longer wavelengths, and faster ones lead to shorter wavelengths. Wavelength determines the first of the three perceptual dimensions of light:  hue . The visible spectrum displays the range of hues that our eyes can detect.

image6 hue One of the perceptual dimensions of color; the dominant wavelength.

Light can also vary in intensity, which corresponds to the second perceptual dimension of light:  brightness . If the intensity of the electromagnetic radiation is increased, the apparent brightness increases, too. The third dimension,  saturation , refers to the relative purity of the light that is being perceived. If all the radiation is of one wavelength, the perceived color is pure, or fully saturated. Conversely, if the radiation contains all visible wavelengths, it produces no sensation of hue—it appears white. Colors with intermediate amounts of saturation consist of different mixtures of wavelengths.  Figure 6.2  shows some color samples, all with the same hue but with different levels of brightness and saturation. (See  Figure 6.2 . )

image7 brightness One of the perceptual dimensions of color; intensity.

image8 saturation One of the perceptual dimensions of color; purity.


FIGURE 6.2 Saturation and Brightness

This figure shows examples of colors with the same dominant wavelength (hue) but different levels of saturations or brightness.

Anatomy of the Visual System

For an individual to see, an image must be focused on the retina, the inner lining of the eye. This image causes changes in the electrical activity of millions of neurons in the retina, which results in messages being sent through the optic nerves to the rest of the brain. (I said “the rest” because the retina is actually part of the brain; it and the optic nerve are in the central—not peripheral—nervous system.) This section describes the anatomy of the eyes, the photoreceptors in the retina that detect the presence of light, and the connections between the retina and the brain.

The Eyes

The eyes are suspended in the orbits, bony pockets in the front of the skull. They are held in place and moved by six extraocular muscles attached to the tough, white outer coat of the eye called the sclera. (See  Figure 6.3 . ) Normally, we cannot look behind our eyeballs and see these muscles because their attachments to the eyes are hidden by the conjunctiva. These mucous membranes line the eyelid and fold back to attach to the eye (thus preventing a contact lens that has slipped off the cornea from “falling behind the eye”).  Figure 6.4  illustrates the anatomy of the eye. (See  Figure 6.4 . )


FIGURE 6.3 The Extraocular Muscles, Which Move the Eyes

The eyes make three types of movements: vergence movements, saccadic movements, and pursuit movements.  Vergence movements  are cooperative movements that keep both eyes fixed on the same target—or, more precisely, that keep the image of the target object on corresponding parts of the two retinas. If you hold up a finger in front of your face, look at it, and then bring your finger closer to your face, your eyes will make vergence movements toward your nose. If you then look at an object on the other side of the room, your eyes will rotate outward, and you will see two separate blurry images of your finger.

image11 vergence movement The cooperative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas.

When you scan the scene in front of you, your gaze does not roam slowly and steadily across its features. Instead, your eyes make jerky  saccadic movements —you shift your gaze abruptly from one point to another. (Saccade comes from the French word for “jerk.”) When you read a line in this book, your eyes stop several times, moving very quickly between each stop. You cannot consciously control the speed of movement between stops; during each saccade the eyes move as fast as they can. Only by performing a  pursuit movement —say, by looking at your finger while you move it around—can you make your eyes move more slowly.

image12 saccadic movement ( suh  kad  ik ) The rapid, jerky movement of the eyes used in scanning a visual scene.

image13 pursuit movement The movement that the eyes make to maintain an image of a moving object on the fovea.


FIGURE 6.4 The Human Eye

The white outer layer of most of the eye, the sclera, is opaque and does not permit entry of light. However, the cornea, the outer layer at the front of the eye, is transparent. The amount of light that enters is regulated by the size of the pupil, which is an opening in the iris, the pigmented ring of muscles situated behind the cornea. The lens, situated immediately behind the iris, consists of a series of transparent, onionlike layers. Its shape can be altered by contraction of the ciliary muscles. These changes in shape permit the eye to focus images of near or distant objects on the retina—a process called  accommodation .

image15 accommodation Changes in the thickness of the lens of the eye, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina.

After passing through the lens, light traverses the main part of the eye, which is filled with vitreous humor(“glassy liquid”), a clear, gelatinous substance. After passing through the vitreous humor, light falls on the  retina , the interior lining of the back of the eye. In the retina are located the receptor cells, the  rods  and  cones  (named for their shapes), collectively known as  photoreceptors .

image16 retina The neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye.

image17 rod One of the receptor cells of the retina; sensitive to light of low intensity.

image18 cone One of the receptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision.

image19 photoreceptor One of the receptor cells of the retina; transduces photic energy into electrical potentials.

The human retina contains approximately 120 million rods and 6 million cones. Although they are greatly outnumbered by rods, cones provide us with most of the visual information about our environment. In particular, they are responsible for our daytime vision. They provide us with information about small features in the environment and thus are the source of vision of the highest sharpness, or acuity (from the Latin acus, “needle”). The  fovea , or central region of the retina, which mediates our most acute vision, contains only cones. Cones are also responsible for color vision—our ability to discriminate light of different wavelengths. Although rods do not detect different colors and provide vision of poor acuity, they are more sensitive to light. In a very dimly lighted environment we use our rod vision; therefore, in very dim light we are color-blind and lack foveal vision. (See  Table 6.1 . )

image20 fovea (foe  vee a ) The region of the retina that mediates the most acute vision of birds and higher mammals. Color-sensitive cones constitute the only type of photoreceptor found in the fovea.

Another feature of the retina is the  optic disk , where the axons conveying visual information gather together and leave the eye through the optic nerve. The optic disk produces a blind spot because no receptors are located there. We do not normally perceive our blind spots, but their presence can be demonstrated. If you have not found yours, you may want to try the exercise described in  Figure 6.5 .

image21 optic disk The location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot.

TABLE 6.1 Locations and Response Characteristics of Photoreceptors

Cones Rods
Most prevalent in the central retina; found in the fovea Most prevalent in the peripheral retina; not found in the fovea
Sensitive to moderate to high levels of light Sensitive to low levels of light
Provide information about hue Provide only monochromatic information
Provide excellent acuity Provide poor acuity

Close examination of the retina shows that it consists of several layers of neuron cell bodies, their axons and dendrites, and the photoreceptors.  Figure 6.6  illustrates a cross section through the primate retina, which is divided into three main layers: the photoreceptive layer, the bipolar cell layer, and the ganglion cell layer. Note that the photoreceptors are at the back of the retina; light must pass through the overlying layers to get to them. Fortunately, these layers are transparent. (See  Figure 6.6 . )

The photoreceptors form synapses with  bipolar cells , neurons whose two arms connect the shallowest and deepest layers of the retina. In turn, bipolar cells form synapses with the  ganglion cells , neurons whose axons travel through the optic nerves (the second cranial nerves) and carry visual information into the rest of the brain. In addition, the retina contains  horizontal cells  and  amacrine cells , both of which transmit information in a direction parallel to the surface of the retina and thus combine messages from adjacent photoreceptors. (Look again at  Figure 6.6 . )

image22 bipolar cell A bipolar neuron located in the middle layer of the retina, conveying information from the photoreceptors to the ganglion cells.

image23 ganglion cell A neuron located in the retina that receives visual information from bipolar cells; its axons give rise to the optic nerve.

image24 horizontal cell A neuron in the retina that interconnects adjacent photoreceptors and the outer processes of the bipolar cells.

image25 amacrine cell (amm  a krine ) A neuron in the retina that interconnects adjacent ganglion cells and the inner processes of the bipolar cells.

The primate retina contains approximately 55 different types of neurons: one type of rod, three types of cones, two types of horizontal cells, ten types of bipolar cells, 24–29 types of amacrine cells, and 10–15 types of ganglion cells (Masland,  2001 ).


Figure 6.7  shows a drawing of two rods and a cone. Note that each photoreceptor consists of an outer segment connected by a cilium to the inner segment, which contains the nucleus. (See  Figure 6.7 . ) The outer segment contains several hundred  lamellae , or thin plates of membrane. (Lamella is the diminutive form of lamina, “thin layer.”)

image26 lamella A layer of membrane containing photopigments; found in rods and cones of the retina.


FIGURE 6.5 A Test for the Blind Spot

With your left eye closed, look at the plus sign with your right eye and move the page nearer to and farther from you. When the page is about 20 cm from your face, the green circle disappears because its image falls on the blind spot of your right eye.


FIGURE 6.6 Details of Retinal Circuitry

(Adapted from Dowling, J. E., and Boycott, B. B. Proceedings of the Royal Society of London, B, 1966, 166, 80–111.)

Let’s consider the nature of transduction of visual information. The first step in the chain of events that leads to visual perception involves a special chemical called a photopigment.  Photopigments  are special molecules embedded in the membrane of the lamellae; a single human rod contains approximately 10 million of them. The molecules consist of two parts: an  opsin  (a protein) and  retinal  (a lipid). There are several forms of opsin; for example, the photopigment of human rods,  rhodopsin , consists of rod opsinplus retinal. (Rhod- refers to the Greek rhodon, “rose,” not to rod. Before it is bleached by the action of light, rhodopsin has a pinkish hue.) Retinal is synthesized from vitamin A, which explains why carrots, which are rich in this vitamin, are said to be good for your eyesight.

image29 photopigment A protein dye bonded to retinal, a substance derived from vitamin A; responsible for transduction of visual information.

image30 opsin (opp  sin ) A class of protein that, together with retinal, constitutes the photopigments.

image31 retinal (rett  i nahl ) A chemical synthesized from vitamin A; joins with an opsin to form a photopigment.

image32 rhodopsin ( roh  dopp  sin ) A particular opsin found in rods.


FIGURE 6.7 Photoreceptors

When a molecule of rhodopsin is exposed to light, it breaks into its two constituents: rod opsin and retinal. When that happens, the rod opsin changes from its rosy color to a pale yellow; hence, we say that the light bleaches the photopigment. The splitting of the photopigment produces the receptor potential: hyperpolarization of the membrane of the photoreceptor.

In the vertebrate retina, photoreceptors provide input to both bipolar cells and horizontal cells.  Figure 6.8 shows the neural circuitry from a photoreceptor to a ganglion cell. The circuitry is much simplified and omits the horizontal cells and amacrine cells. The first two types of cells in the circuit—photoreceptors and bipolar cells—do not produce action potentials. Instead, their release of the neurotransmitter (glutamate) is regulated by the value of their membrane potential; depolarizations increase the release, and hyperpolarizations decrease it. The contents of the circles indicate what would be seen on an oscilloscope screen recording changes in the cells’ membrane potentials in response to a spot of light shining on the photoreceptor.

The hyperpolarizing effect of light on the membrane of a photoreceptor is shown in the left circle. In the dark, photoreceptors constantly release their neurotransmitter. When light strikes molecules of the photopigment, the hyperpolarization that ensues reduces the amount of neurotransmitter released by the photoreceptor. Because the neurotransmitter normally hyperpolarizes the dendrites of the bipolar cell, a reduction in its release causes the membrane of the bipolar cell to depolarize. Thus, light hyperpolarizes the photoreceptor and depolarizes the bipolar cell. (See  Figure 6.8 . ) The depolarization of the bipolar cell causes it to release more neurotransmitter, which depolarizes the membrane of the ganglion cell and raises this cell’s rate of firing. Thus, light shining on the photoreceptor excites the ganglion cell and increases the rate of firing of its axon.

The circuit shown in  Figure 6.8  illustrates a ganglion cell whose firing rate increases in response to light. As we will see, other ganglion cells decrease their firing rate in response to light. These neurons are connected to bipolar cells that form different types of synapses with the photoreceptors. The functions of these two types of circuits are discussed in the next section, “Coding of Visual Information in the Retina.”


FIGURE 6.8 Neural Circuitry in the Retina

Light striking a photoreceptor produces a hyperpolarization, so the photoreceptor releases less neurotransmitter. Because the neurotransmitter normally hyperpolarizes the membrane of the bipolar cell, the reduction causes a depolarization. This depolarization causes the bipolar cell to release more neurotransmitter, which excites the ganglion cell.

(Adapted from Dowling, J. E., in The Neurosciences: Fourth Study Program, edited by F. O. Schmitt and F. G. Worden. Cambridge, Mass.: MIT Press, 1979.)

Connections Between Eye and Brain

The axons of the retinal ganglion cells bring information to the rest of the brain. They ascend through the optic nerves and reach the  dorsal lateral geniculate nucleus (LGN)  of the thalamus. This nucleus receives its name from its resemblance to a bent knee (genu is Latin for “knee”). It contains six layers of neurons, each of which receives input from only one eye. The neurons in the two inner layers contain cell bodies that are larger than those in the outer four layers. For this reason the inner two layers are called the  magnocellular layers , and the outer four layers are called the  parvocellular layers  (parvo- refers to the small size of the cells). A third set of neurons in the  koniocellular sublayers  are found ventral to each of the magnocellular and parvocellular layers. (Konis is Greek word for “dust.”) As we will see later, these three sets of layers belong to different systems, which are responsible for the analysis of different types of visual information. They receive input from different types of retinal ganglion cells. (See  Figure 6.9 . )

image35 dorsal lateral geniculate nucleus (LGN) A group of cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex.

image36 magnocellular layer One of the inner two layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex.

image37 parvocellular layer One of the four outer layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for perception of color and fine details to the primary visual cortex.

image38 koniocellular sublayer ( koh nee oh  sell  yew lur ) One of the sublayers of neurons in the dorsal lateral geniculate nucleus found ventral to each of the magnocellular and parvocellular layers; transmits information from short-wavelength (“blue”) cones to the primary visual cortex.

The neurons in the LGN send their axons through a pathway known as the optic radiations to the primary visual cortex—the region surrounding the  calcarine fissure  (calcarine means “spur-shaped”), a horizontal fissure located in the medial and posterior occipital lobe. The primary visual cortex is often called the  striate cortex  because it contains a dark-staining layer (striation) of cells. (See  Figure 6.10 . )

image39 calcarine fissure (kal  ka rine ) A horizontal fissure on the inner surface of the posterior cerebral cortex; the location of the primary visual cortex.

image40 striate cortex (stry  ate ) The primary visual cortex.


FIGURE 6.9 Lateral Geniculate Nucleus

This photomicrograph shows a section through the right lateral geniculate nucleus of a rhesus monkey (cresyl violet stain). Layers 1, 4, and 6 receive input from the contralateral (left) eye, and layers 2, 3, and 5 receive input from the ipsilateral (right) eye. Layers 1 and 2 are the magnocellular layers; layers 3–6 are the parvocellular layers. The koniocellular sublayers are found ventral to each of the parvocellular and magnocellular layers. The receptive fields of all six principal layers are in almost perfect registration; cells located along the line of the unlabeled arrow have receptive fields centered on the same point.

(Photomicrograph from Hubel, D. H., Wiesel, T. N., and Le Vay, S. Philosophical Transactions of the Royal Society of London, B, 1977, 278, 131–163. Reprinted with permission.)

Figure 6.11  shows a diagrammatical view of a horizontal section of the human brain. The optic nerves join together at the base of the brain to form the X-shaped  optic chiasm  (khiasma is the Greek for “cross”). There, axons from ganglion cells serving the inner halves of the retina (the nasal sides) cross through the chiasm and ascend to the LGN on the opposite side of the brain. The axons from the outer halves of the retina (the temporal sides) remain on the same side of the brain. (See  Figure 6.11 . ) The lens inverts the image of the world projected on the retina (and similarly reverses left and right). Therefore, because the axons from the nasal halves of the retinas cross to the other side of the brain, each hemisphere receives information from the contralateral half (opposite side) of the visual scene. That is, if a person looks straight ahead, the right hemisphere receives information from the left half of the visual field, and the left hemisphere receives information from the right. It is not correct to say that each hemisphere receives visual information solely from the contralateral eye. (Look again at  Figure 6.11 . )

image42 optic chiasm (ky  az’ m ) A cross-shaped connection between the optic nerves, located below the base of the brain, just anterior to the pituitary gland.

Besides the primary retino-geniculo-cortical pathway, fibers from the retina take several other pathways. For example, one pathway to the hypothalamus synchronizes an animal’s activity cycles to the 24-hour rhythms of day and night. (We will study this system in  Chapter 9 .) Other pathways, e