Chapter 2 (M. Wolfe)

Question 1

Describe the two ways used to conceptualize light.

Answer 1

One way is to think of it as a wave that travels through a medium. Another is to think of it as a stream of photons, tiny particles, each consisting of one quantum of energy.

Question 2

2. Describe the difference between light that is reflected and light that is transmitted.

Answer 2

Reflected light occurs when a ray of light strikes a light-colored surface and then bounces back towards its point of origin. Transmitted light occurs when light is neither reflected nor absorbed by a surface. An example is a transparent window; light passes through the surface and is transmitted to the other side.

Question 3

What is the purpose of the cornea?

Answer 3

The cornea is a transparent surface on the exterior of the eye. It protects the eye from the outside world. Being transparent, it allows light to be transmitted through it and into the eye.

Question 4

4. What is the purpose of the retina?

Answer 4

The retina is a light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.

Question 5

How does the process of accommodation take place in the eye?

Answer 5

Accommodation takes place in the lens of the eye. The lens changes its refractive power by changing its shape. This causes the eye to be able to focus on a given object, whether it is near or far.

Question 6

6. What is the difference between myopia and hyperopia?

Answer 6

Myopia (i.e., “nearsightedness”) is a condition in which light entering the eye is focused in front of the retina, whereas hyperopia (i.e., “farsightedness”) is a condition in which light entering the eye is focused behind the retina. In both cases the retinal image is blurry without some sort of correction.

Question 7

What is astigmatism and how can it be fixed?

Answer 7

Astigmatism is a visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. It can be fixed by wearing lenses that have two focal points (that provide different amounts of focusing power in the horizontal and vertical planes).

Question 8

8. Why are photoreceptors important in the process of seeing?

Answer 8

Photoreceptors are the cells that make up the backmost layer of the retina. They are sensitive to light, and as soon as they sense it, they can cause neurons in the intermediate layers to fire action potentials. Photoreceptors are important in the process of seeing because they transduce the physical energy of light into neural energy that our brains can analyze.

Question 9

What are rods and cones?

Answer 9

Rods and cones are photoreceptors present in the retina. Rods are specialized for night vision; cones are specialized for daylight vision, fine visual acuity, and color.

Question 10

Explain what happens in the process of hyperpolarization.

Answer 10

Hyperpolarization is an increase in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface. This process is one in a sequence of events that occur once light is sensed by the photoreceptors

Question 11

11. Why can’t rods signal differences in color?

Answer 11

Rods cannot signal differences in color because they only have one type of photopigment. Cones, on the other hand, have three types of photopigments, which help them differentiate between colors.

Question 12

What is the role of horizontal cells?

Answer 12

Horizontal cells are specialized retinal cells that contact both photoreceptors and bipolar cells. They produce lateral inhibition, which allows the signals that reach retinal ganglion cells to be based on differences in activations between nearby photoreceptors rather than absolute levels of activation.

Question 13

What is visual acuity?

Answer 13

Visual acuity is a measure of the finest detail that one can resolve.

Question 14

What is the difference between an “ON” bipolar cell and an “OFF” bipolar cell?

Answer 14

An “ON” bipolar cell is a cone bipolar cell that depolarizes in response to an increase in light intensity. An “OFF” bipolar cell is a cone bipolar cell that depolarizes in response to a decrease in light intensity. These two cells have opposite reactions to light.

Question 15

What is a receptive field?

Answer 15

A receptive field is the region on the retina in which stimuli will activate a neuron. Receptive fields vary in size, shape, and complexity.

Question 16

Why is the center–surround organization of retinal ganglion cells so important?

Answer 16

It is important because it allows for sensitivity to contrast rather than absolute illumination levels. Ganglion cells are most sensitive to differences in the intensity of light in the center and in the surround, and they are relatively unaffected by the average intensity of light. This is useful because the average intensity of light falling on the retina will be quite variable, depending on whether the observer is indoors, outdoors, etc., but contrasts of light are relatively constant.

Question 17

What is a filter and how is it important in vision?

Answer 17

A filter is an acoustic, electrical, electronic, biological, or optical device, instrument, or computer program that allows the passage of some frequencies or digital elements and blocks others. Filters are important in vision because they allow the transformation of raw images into representations in the brain. Filters highlight certain important visual information while eliminating other unimportant information. The center–surround receptive fields of retinal ganglion cells are filters.

Question 18

What are some consequences of the differing sizes of M ganglion cell and P ganglion cell receptive fields?

Answer 18

P ganglion cells have smaller receptive fields than M ganglion cells at all eccentricities. This allows the M ganglion cells to respond to a larger portion of the visual field. In addition, they are much more sensitive to visual stimuli under low lighting conditions than P ganglion cells. P ganglion cells, on the other hand, provide finer resolution (greater acuity) than M ganglion cells, as long as there is enough light for them to operate.

Question 19

Explain how the pupil adapts to dark and light conditions.

Answer 19

Answer: The pupil has the ability to dilate and constrict, depending on amount of light. For example, under well-lit conditions, the pupil tends to constrict to let less light into the eye. Under dark conditions, the pupil dilates to allow more light into the eye.

Question 20

Explain why it is that we are generally not bothered by variations in overall light levels.

Answer 20

Because we have several mechanisms for regulating how much light enters the eye. One mechanism is pupil size. Another is the regeneration rates of pigments in our photoreceptors. Yet another is the rod/cone dichotomy—cones operate at moderate and high light levels while rods take over for low light levels. Finally, the neural circuitry of the retina itself helps stabilize external light variations by emphasizing contrasts in luminance rather than absolute light levels.

Question 21

What is the difference between age-related macular degeneration (AMD) and retinitis pigmentosa (RP)?

Answer 21

AMD is a disease associated with aging that affects the macula, or central visual field. RP is a progressive degenerative disease of the retina that affects peripheral vision.

Question 22

To take up something—such as light, noise, or energy—and not transmit it at all

Answer 22

Adsorb

Question 23

A retinal cell found in the inner synaptic layer that makes synaptic contacts with bipolar cells, ganglion cells, and other amacrine cells.

Answer 23

amacrine cell

Question 24

The watery fluid in the anterior chamber of the eye.

Answer 24

aqueous humor

Question 25

Describe a bipolar cell

Answer 25

A retinal cell that synapses with either rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells.

Question 26

An opacity of the crystalline lens

Answer 26

cataract

Question 27

The light-catching part of the visual pigments of the retina.

Answer 27

chromophore

Question 28

Describe contrast

Answer 28

The difference in luminance between an object and the background, or between lighter and darker parts of the same object.

Question 29

Describe eccentricity

Answer 29

The distance between the retinal image and the fovea

Question 30

Describe the fovea

Answer 30

A small pit, near the center of the macula, that contains the highest concentration of cones, and no rods. It is the portion of the retina that produces the highest visual acuity and serves as the point of fixation.

Question 31

A retinal cell that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells) and transmits information to the brain and midbrain.

Answer 31

ganglion cell

Question 32

what is lateral inhibition?

Answer 32

it is the antagonistic neural interaction between adjacent regions of the retina.

Question 33

A photopigment, found in a class of photoreceptive retinal ganglion cells, that is sensitive to ambient light.

Answer 33

melanopsin

Question 34

A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties.

Answer 34

photon

Question 35

refract

Answer 35

1. To alter the course of a wave of energy that passes into something from another medium, as water does to light entering it from the air. 2. To measure the degree of refraction in a lens or eye.

Question 36

The visual pigment found in rods.

Answer 36

rhodopsin

Question 37

vitreous humor

Answer 37

the transparent fluid that fills the vitreous chamber in the posterior part of the eye.

Question 38

Wave

Answer 38

An oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium.