S & P Ch 2

Front 1

Describe the two ways used to conceptualize light.

Back 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.

Front 2

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

Back 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.

Front 3

3. What is the purpose of the cornea?

Back 3

Answer: 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.

Front 4

4. What is the purpose of the retina?

Back 4

Answer: 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.

Front 5

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

Back 5

Answer: 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.

Front 6

6. What is the difference between myopia and hyperopia?

Back 6

Answer: 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.

Front 7

7. What is astigmatism and how can it be fixed?

Back 7

Answer: 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).

Front 8

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

Back 8

Answer: 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.

Front 9

9. What are rods and cones?

Back 9

Answer: 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.

Front 10

10. Explain what happens in the process of hyperpolarization.

Back 10

Answer: 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.

Front 11

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

Back 11

Answer: 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.

Front 12

12. What is the role of horizontal cells?

Back 12

Answer: 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.

Front 13

13. What is visual acuity?

Back 13

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

Front 14

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

Back 14

Answer: 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.

Front 15

15. What is a receptive field?

Back 15

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

Front 16

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

Back 16

Answer: The center–surround organization of retinal ganglion cells 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.

Front 17

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

Back 17

Answer: 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.

Front 18

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

Back 18

Answer: 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.

Front 19

19. Explain how the pupil adapts to dark and light conditions.

Back 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.

Front 20

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

Back 20

Answer: We are generally not bothered by variations in overall light levels 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.

Front 21

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

Back 21

Answer: 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.

Front 22

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

Back 22

absorb

Front 23

The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects).

Back 23

accommodation

Front 24

A disease associated with aging that affects the macula. AMD gradually destroys sharp central vision, making it difficult to read, drive, and recognize faces. There are two forms of AMD: wet and dry.

Back 24

age-related macular degeneration (AMD)

Front 25

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

Back 25

amacrine cell

Front 26

The watery fluid in the anterior chamber of the eye.

Back 26

aqueous humor

Front 27

A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea.

Back 27

astigmatism

Front 28

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.

Back 28

bipolar cell

Front 29

An opacity of the crystalline lens.

Back 29

cataract

Front 30

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

Back 30

chromophore

Front 31

A photoreceptor specialized for daylight vision, fine visual acuity, and color.

Back 31

cone

Front 32

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

Back 32

contrast

Front 33

The transparent “window” into the eyeball.

Back 33

cornea

Front 34

A bipolar retinal cell whose processes are spread out to receive input from multiple cones.

Back 34

diffuse bipolar cell

Front 35

A unit of measurement of the optic power of a lens. It is equal to the reciprocal of the focal length, in meters. A 2-diopter lens will bring parallel rays of light into focus at ½ meter (50 cm).

Back 35

diopter (D)

Front 36

In reference to the retina, consisting of two parts: the rods and cones, which operate under different conditions.

Back 36

duplex

Front 37

The distance between the retinal image and the fovea.

Back 37

eccentricity

Front 38

The condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball.

Back 38

emmetropia

Front 39

An acoustic, electrical, electronic, or optic device, instrument, computer program, or neuron that allows the passage of some range of parameters (e.g., orientations, frequencies) and blocks the passage of others.

Back 39

filter

Front 40

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.

Back 40

fovea

Front 41

The back layer of the retina—what the eye doctor sees through an ophthalmoscope.

Back 41

fundus

Front 42

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.

Back 42

ganglion cell

Front 43

An electrical potential that can vary continuously in amplitude.

Back 43

graded potential

Front 44

A specialized retinal cell that contacts both photoreceptor and bipolar cells.

Back 44

horizontal cell

Front 45

Farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see near objects clearly.

Back 45

hyperopia

Front 46

An increase in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface.

Back 46

hyperpolarization

Front 47

A picture or likeness.

Back 47

image

Front 48

The part of a photoreceptor that lies between the outer segment and the cell nucleus.

Back 48

inner segment

Front 49

The colored part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil.

Back 49

iris

Front 50

A neuron located between the magnocellular and parvocellular layers of the lateral geniculate nucleus. This layer is known as the koniocellular layer.

Back 50

koniocellular cell

Front 51

Antagonistic neural interaction between adjacent regions of the retina.

Back 51

lateral inhibition

Front 52

The lens inside the eye that enables the changing of focus.

Back 52

lens

Front 53

A ganglion cell resembling a little umbrella that receives excitatory input from diffuse bipolar cells and feeds the magnocellular layer of the lateral geniculate nucleus.

Back 53

M ganglion cell

Front 54

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

Back 54

melanopsin

Front 55

A small bipolar cell in the central retina that receives input from a single cone.

Back 55

midget bipolar cell

Front 56

Nearsightedness, a common condition in which light entering the eye is focused in front of the retina and distant objects cannot be seen sharply.

Back 56

myopia

Front 57

A bipolar cell that responds to a decrease in light captured by the cones.

Back 57

OFF bipolar cell

Front 58

A cell that depolarizes in response to a decrease in light intensity in its receptive-field center.

Back 58

OFF-center cell

Front 59

A bipolar cell that responds to an increase in light captured by the cones.

Back 59

ON bipolar cell

Front 60

A cell that depolarizes in response to an increase in light intensity in its receptive-field center.

Back 60

ON-center cell

Front 61

The part of a photoreceptor that contains photopigment molecules.

Back 61

outer segment

Front 62

A small ganglion cell that receives excitatory input from single midget bipolar cells in the central retina and feeds the parvocellular layer of the lateral geniculate nucleus.

Back 62

P ganglion cell

Front 63

Activation by light.

Back 63

photoactivation

Front 64

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

Back 64

photon

Front 65

A light-sensitive receptor in the retina.

Back 65

photoreceptor

Front 66

Literally “old sight.” The age-related loss of accommodation, which makes it difficult to focus on near objects.

Back 66

presbyopia

Front 67

The dark, circular opening at the center of the iris in the eye, where light enters the eye.

Back 67

pupil

Front 68

The region on the retina in which visual stimuli influence a neuron’s firing rate.

Back 68

receptive field

Front 69

To redirect something that strikes a surface—especially light, sound, or heat—usually back toward its point of origin.

Back 69

reflect

Front 70

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.

Back 70

refract

Front 71

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.

Back 71

retina

Front 72

A progressive degeneration of the retina that affects night vision and peripheral vision. RP commonly runs in families and can be caused by defects in a number of different genes that have recently been identified.

Back 72

retinitis pigmentosa (RP)

Front 73

The visual pigment found in rods.

Back 73

rhodopsin

Front 74

A photoreceptor specialized for night vision.

Back 74

rod

Front 75

To disperse something—such as light—in an irregular fashion.

Back 75

scatter

Front 76

1. The ability to respond to transmitted signals. 2. In signal detection theory, a value that defines the ease with which an observer can tell the difference between the presence and absence of a stimulus or the difference between stimulus 1 and stimulus 2.

Back 76

sensitivity

Front 77

The location where axons terminate at the synapse for transmission of information by the release of a chemical transmitter.

Back 77

synaptic terminal

Front 78

To convert from one form of energy to another (e.g., from light to neural electrical energy, or from mechanical movement to neural electrical energy).

Back 78

transduce

Front 79

To convey something (e.g., light) from one place or thing to another.

Back 79

transmit

Front 80

Allowing light to pass through with no interruption, so that objects on the other side can be clearly seen.

Back 80

transparent

Front 81

A measure of the finest detail that can be resolved by the eyes.

Back 81

visual acuity

Front 82

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

Back 82

vitreous humor

Front 83

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.

Back 83

wave