Chapter 3

Question 1

Retinitis pigmentosa

Answer 1

A genetic eye disease that causes the retina to break down, leading to a slow loss of peripheral vision, or even total blindness in some cases. Could be treated with bionic eye technology (an array of electrodes implanted in the back of the eye that, through a camera mounted on eyeglasses, sends signals to the visual system about what is “out there” in the world). While it doesn’t completely restore vision, it allows the person to see contrasting lightness versus darkness, such the edge between where one object ends and another begins

Question 2

Wavelength

Answer 2

For light energy, the distance between one peak of a light wave and the next peak. The electromagnetic spectrum is a continuum of electromagnetic energy that is produced by electric charges and is radiated as waves. The wavelengths in the electromagnetic spectrum range from extremely short-wavelength gamma rays to long-wavelength radio waves.

Question 3

Visible light

Answer 3

The band of electromagnetic energy that activates the visual system and that, therefore, can be perceived. For humans, visible light has wavelengths between 400 and 700 nanometers. For humans and some other animals, the wavelength of visible light is associated with the different colors of the spectrum, with short wavelengths appearing blue, middle wavelengths green, and long wavelengths yellow, orange, and red.

Question 4

Eye

Answer 4

The eyeball and its contents, which include focusing elements, the retina, and supporting structures. The first eyes, which appeared back in the Cambrian period (570–500 million years ago), were eyespots on primitive animals such as flatworms that could distinguish light from dark but couldn’t detect features of the environment.

Question 5

Cornea

Answer 5

The transparent focusing element of the eye that is the first structure through which light passes as it enters the eye. The cornea is the eye’s major focusing element. It accounts for about 80% of the eye’s focusing power, but like the lenses in eyeglasses, it is fixed in place so it can’t adjust its focus.

Question 6

Lens

Answer 6

The transparent focusing element of the eye through which light passes after passing through the cornea and the aqueous humor. The lens’s change in shape to focus at different distances is called accommodation. Supplies the remaining 20% of the eye’s focusing power.

Question 7

Pupil

Answer 7

The opening through which light reflected from objects in the environment enters the eye.

Question 8

Retina

Answer 8

A complex network of cells that covers the inside back of the eye. These cells include the receptors, which generate an electrical signal in response to light, as well as the horizontal, bipolar, amacrine, and ganglion cells.

Question 9

Photoreceptors

Answer 9

The receptors for vision. There are 2 types:
- Rods: A cylinder-shaped receptor in the retina that is responsible for vision at low levels of illumination.
- Cones: Cone-shaped receptors in the retina that are primarily responsible for vision in high levels of illumination and for color vision and detail vision.

Question 10

Outer segments

Answer 10

Part of the rod and cone visual receptors that contains the light-sensitive visual pigment molecules (the reaction of this molecule to light results in the generation of an electrical response in the receptors)

Question 11

Optic nerve

Answer 11

Bundle of nerve fibers that carry impulses from the retina to the lateral geniculate nucleus and other structures. Each optic nerve contains about 1 million ganglion cell fibers.

Question 12

Fovea

Answer 12

A small area in the human retina that contains only cone receptors. The fovea is located on the line of sight, so that when a person looks at an object, the center of its image falls on the fovea. It is very small, so it contains only 1% of all the cones in the retina (so, around 50’000)

Question 13

Peripheral retina

Answer 13

The area of retina outside the fovea. It contains both rods and cones (but more rods than cones - there are 20x more rods than cones in the retina, 120 million vs. 6 million).

Question 14

Macular degeneration

Answer 14

A clinical condition that causes degeneration of the macula, an area of the retina that includes the fovea and a small surrounding area. This creates a blind region in central vision, so when a person looks directly at something, they sight of it.

Question 15

Blind spot

Answer 15

The small area where the optic nerve leaves the back of the eye. There are no visual receptors in this area, so small images falling directly on the blind spot cannot be seen. We aren’t aware of it because it is located off to the side of our visual field, where objects are not in sharp focus. Also, a mechanism in the brain “fills in” the place where the image disappears (it creates a perception that matches the surrounding pattern).

Question 16

Accommodation

Answer 16

In vision, bringing objects located at different distances into focus by changing the shape of the lens. It occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker. This increased curvature increases the bending of the light rays passing through the lens so the focus point is pulled from point B back to A to create a sharp image on the retina.

Question 17

Refractive errors

Answer 17

Errors that can affect the ability of the cornea and/or lens to focus incoming light onto the retina.

Question 18

Presbyopia

Answer 18

The inability of the eye to accommodate due to a hardening of the lens and a weakening of the ciliary muscles. It occurs as people get older. It can be dealt with by wearing reading glasses, which brings near objects into focus by replacing the focusing power that can no longer be provided by the lens.

Question 19

Myopia

Answer 19

An inability to see distant objects clearly. Also called nearsightedness. Myopia occurs when the optical system brings parallel rays of light into focus at a point in front of the retina, so the image that reaches the retina is blurred.

Question 20

Refractive myopia

Answer 20

Myopia (nearsightedness) in which the cornea and/or the lens bends the light too much.

Question 21

Axial myopia

Answer 21

Myopia (nearsightedness) in which the eyeball is too long.

Question 22

Hyperopia

Answer 22

A condition causing poor vision in which people can see objects that are far away but do not see near objects clearly. Also called farsightedness. Young people can bring the image forward onto the retina by accommodating. However, older people, who have difficulty accommodating, often use corrective lenses that bring the focus point forward onto the retina.

Question 23

Transduction

Answer 23

The transformation of one form of energy into another form of energy. Visual transduction occurs in photoreceptors (the rods and cones) and transforms light into electricity.

Question 24

Visual pigments composition

Answer 24

Visual pigments have two parts: a long protein called opsin and a much smaller light-sensitive component called retinal. Despite its small size compared to the opsin, retinal is the crucial part of the visual pigment molecule, because when the retinal and opsin are combined, the resulting molecule absorbs visible light.

Question 25

Isomerization

Answer 25

Change in shape of the retinal part of the visual pigment molecule that occurs when the molecule absorbs a quantum of light. Isomerization triggers the enzyme cascade that results in transduction from light energy to electrical energy in the retinal receptors.

Question 26

Dark adaptation

Answer 26

Visual adaptation that occurs in the dark, during which the sensitivity to light increases. This increase in sensitivity is associated with regeneration of the rod and cone visual pigments.

Question 27

Dark adaptation curve

Answer 27

The function that traces the time course of the increase in visual sensitivity that occurs during dark adaptation.

Question 28

Light-adapted sensitivity

Answer 28

The sensitivity of the eye when in the light-adapted state. Usually taken as the starting point for the dark adaptation curve because it is the sensitivity of the eye just before the lights are turned off.

Question 29

Dark-adapted sensitivity

Answer 29

The sensitivity of the eye after it has completely adapted to the dark.

Question 30

Pirates and eyepatches

Answer 30

There is a myth that pirates wore eye patches to preserve night vision in one eye so that when they went from the bright light outside to the darkness below decks, moving the patch to the light-adapted eye would enable them to see with the dark-adapted eye. Indeed, keeping an eye in the dark triggers the process of dark adaptation, which causes the eye to increase its sensitivity in the dark. However, whether pirates actually used patches to help them see below decks remains an unproven hypothesis.

Question 31

Measuring Cone Adaptation

Answer 31

To measure dark adaptation of the cones alone, we have to ensure that the image of the test light falls only on cones. We achieve this by having the participant look directly at the test light so its image falls on the all-cone fovea, and by making the test light small enough so that its entire image falls within the fovea. This curve, which measures only the activity of the cones, matches the initial phase of our original dark adaptation curve but does not include the second phase.

Question 32

Measuring rod adaptation

Answer 32

Because the cones are more sensitive to light at the beginning of dark adaptation, they control our vision during the early stages of adaptation, so we can’t determine what the rods are doing. In order to reveal how the sensitivity of the rods is changing at the very beginning of dark adaptation, we need to measure dark adaptation in a person who has no cones (rod monochromats). Tests show that rods are much less sensitive than the cone light-adapted sensitivity. Once dark adaptation begins, the rods increase their sensitivity, to reach their final dark-adapted levels in 25 minutes.

Question 33

Rod monochromats

Answer 33

A person who has a retina in which the only functioning receptors are rods. Their all-rod retinas provide a way for us to study rod dark adaptation without interference from the cones.

Question 34

Dark adaptation process

Answer 34

As soon as it’s dark, the sensitivity of both the cones and the rods begins increasing. However, because the cones are much more sensitive than the rods at the beginning of dark adaptation, we see with our cones right after the lights are turned out. However, after about 3 to 5 minutes in the dark, the cones have reached their maximum sensitivity. By about 7 minutes in the dark, the rods’ sensitivity catches up to the cones’, and gradually becomes higher until 25 minutes after the start.

Question 35

Rod-cone break

Answer 35

The point on the dark adaptation curve at which vision shifts from cone vision to rod vision.

Question 36

Visual Pigment Bleaching

Answer 36

The change in the color of a visual pigment that occurs when visual pigment molecules are isomerized by exposure to light. As the light remains on, more and more of the pigment’s retinal is isomerized and breaks away from the opsin, so the retina’s color changes.

Question 37

Visual pigment regeneration

Answer 37

Occurs after the visual pigment’s two components—opsin and retinal—have become separated due to the action of light. Regeneration, which occurs in the dark, involves a rejoining of these two components to reform the visual pigment molecule. This process depends on enzymes located in the pigment epithelium.

Question 38

Detached retina

Answer 38

A condition in which the retina is detached from the back of the eye (the pigment epithelium, a layer that contains enzymes necessary for pigment regeneration). It can occur as a result of traumatic injury to the eye/head. When this occurs, the bleached pigment’s separated retinal and opsin can no longer be recombined, and the person becomes blind in the area of the visual field served by the separated area of the retina.

Question 39

Cone vs. rods in regeneration of visual pigment

Answer 39

Cone pigment takes 6 minutes to regenerate completely, whereas rod pigment takes more than 30 minutes. The rate of cone dark adaptation matched the rate of cone pigment regeneration and the rate of rod dark adaptation matched the rate of rod pigment regeneration. These results demonstrated two important connections between perception and physiology:
- Our sensitivity to light depends on the concentration of a chemical—the visual pigment.
- The speed at which our sensitivity increases in the dark depends on a chemical reaction—the regeneration of the visual pigment.

Question 40

Spectral sensitivity

Answer 40

The sensitivity of visual receptors to different parts of the visible spectrum.

Question 41

Spectral sensitivity curve

Answer 41

The function relating a subject’s sensitivity to light to the wavelength of the light. The spectral sensitivity curves for rod and cone vision indicate that the rods and cones are maximally sensitive at 500 nm and 560 nm, respectively.

Question 42

Monochromatic light

Answer 42

Light that contains only a single wavelength. It can be created by using special filters or a device called a spectrometer. To determine a person’s spectral sensitivity, we determine the person’s threshold for seeing monochromatic lights across the spectrum using one of the psychophysical methods for measuring threshold.

Question 43

Cone spectral sensitivity

Answer 43

A plot of visual sensitivity versus wavelength for cone vision. Often measured by presenting a small spot of light to the fovea, which contains only cones. Can also be measured when the eye is light adapted, so cones are the most sensitive receptors.

Question 44

Rod spectral sensitivity

Answer 44

The curve plotting visual sensitivity versus wavelength for rod vision. This function is typically measured when the eye is dark adapted by a test light presented to the peripheral retina.

Question 45

Rods vs. cones spectral sensitivity

Answer 45

The rods are more sensitive to short-wavelength light than are the cones, with the rods being most sensitive to light of 500 nm and the cones being most sensitive to light of 560 nm. This difference in the sensitivity of cones and rods to different wavelengths means that as vision shifts from the cones in the light-adapted eye to the rods after the eye has become dark adapted, our vision shifts to become relatively more sensitive to short-wavelength light.

Question 46

Purkinje shift

Answer 46

The shift from cone spectral sensitivity to rod spectral sensitivity that takes place during dark adaptation. Named after Johann Purkinje, who described this effect in 1825. For example, green foliage seems to stand out more near dusk.

Question 47

Absorption spectrum and the 3 cone pigments

Answer 47

A plot of the amount of light absorbed by a visual pigment versus the wavelength of light. The short-wavelength pigment (S) absorbs light best at about 419 nm; the medium-wavelength pigment (M) absorbs light best at about 531 nm; and the long-wavelength pigment (L) absorbs light best at about 558 nm

Question 48

Cone spectral sensitivity curve

Answer 48

The absorption of the rod visual pigment closely matches the rod spectral sensitivity curve, and the short-, medium-, and long-wavelength cone pigments add together to result in a psychophysical spectral sensitivity curve that peaks at 560 nm. Because there are fewer short-wavelength receptors and therefore much less of the short-wavelength pigment, the cone spectral sensitivity curve is determined mainly by the medium- and long-wavelength pigments

Question 49

Bipolar cells

Answer 49

A retinal neuron that receives inputs from the visual receptors and sends signals to the retinal ganglion cells.

Question 50

Ganglion cells

Answer 50

A neuron in the retina that receives inputs from bipolar and amacrine cells. The axons of the ganglion cells are the nerve fibers that travel out of the eye in the optic nerve.

Question 51

Horizontal cells

Answer 51

A neuron that transmits signals laterally across the retina. Horizontal cells synapse with receptors and bipolar cells.

Question 52

Amacrine cells

Answer 52

A neuron that transmits signals laterally in the retina. Amacrine cells synapse with bipolar cells and ganglion cells.

Question 53

Neural convergence

Answer 53

Synapsing of a number of neurons onto one neuron. A great deal of convergence occurs in the retina because each eye has 126 million photoreceptors but only 1 million ganglion cells. Thus, on the average, each ganglion cell receives signals from 126 photoreceptors.

Question 54

Rod vs. cone convergence

Answer 54

An important difference between rods and cones is that the signals from the rods converge more than do the signals from the cones. On average, about 120 rods send their signals to one ganglion cell, but only about 6 cones send signals to a single ganglion cell. This difference between rod and cone convergence becomes even greater when we consider the cones in the fovea (each ganglion cell only receives input from 1 cone: no convergence). The greater convergence of the rods compared to the cones translates into two differences in perception: the rods result in better sensitivity than the cones, and the cones result in better detail vision than the rods.

Question 55

Why are rods more sensitive?

Answer 55

One reason for this greater sensitivity of rods, compared to cones, is that it takes less light to generate a response from an individual rod receptor than from an individual cone receptor. Another possible reason: The rods have greater convergence than the cones. For example, experiments show that it takes less incoming light to stimulate a ganglion cell that is receiving input from rods, since many rods converge onto that one ganglion cell.

Question 56

Why do cones have better visual acuity?

Answer 56

The cones have better visual acuity because they have less convergence. Acuity refers to the ability to see details. This implies that visual acuity is highest in the fovea.

Question 57

Rods vs cone acuity

Answer 57

When we present two spots of light next to each other, the rods’ signals cause the ganglion cell to fire. When we separate the two spots, the two separated rods still feed into the same ganglion cell and cause it to fire. In both cases, the ganglion cell fires. Thus, firing of the ganglion cell
provides no information about whether there are two spots close together or two separated spots.
When we present a light that stimulates two neighboring cones, as on the left, two adjacent ganglion cells fire. But when we separate the spots, as on the right, two separate ganglion cells fire. This separation provides information that there are two separate spots of light. The cones’ lack of convergence causes cone vision to have higher acuity than rod vision.

Question 58

Receptive field

Answer 58

The area on the receptor surface (the retina for vision; the skin for touch) that, when stimulated, affects the firing of that neuron. A ganglion cell’s receptive field covers a much greater area than a single photoreceptor. This proves that the cell is receiving converging signals from all of these photoreceptors. Also, the receptive fields of many different ganglion cells overlap (which means that g light on a particular point on the retina activates many ganglion cells).

Question 59

Center-surround receptive fields

Answer 59

A receptive field that has a center-surround organization. In these receptive fields, the area in the “center” of the receptive field responds differently to light than the area in the “surround” of the receptive field.

Question 60

Excitatory area

Answer 60

Area of a receptive field that is associated with excitation. Stimulation of this area causes an increase in the rate of nerve firing.

Question 61

Inhibitory area

Answer 61

Area of a receptive field that is associated with inhibition. Stimulation of this area causes a decrease in the rate of nerve firing.

Question 62

Excitatory-center, inhibitory-surround receptive field

Answer 62

A center-surround receptive field in which stimulation of the center area causes an excitatory response and stimulation of the surround causes an inhibitory response.

Question 63

Inhibitory-center, excitatory-surround receptive field

Answer 63

A center-surround receptive field in which stimulation of the center causes an inhibitory response and stimulation of the surround causes an excitatory response.

Question 64

Center-surround antagonism

Answer 64

The competition between the center and surround regions of a center-surround receptive field, caused by the fact that one is excitatory and the other is inhibitory. Stimulating center and surround areas simultaneously decreases responding of the neuron, compared to stimulating the excitatory area alone.

Question 65

Lateral inhibition

Answer 65

Inhibition that is transmitted laterally across a nerve circuit. In the retina, lateral inhibition is transmitted by the horizontal and amacrine cells. It is what underlies center-surround antagonism in center-surround ganglion cell receptive fields.

Question 66

Ommatidia

Answer 66

A structure in the eye of the Limulus that contains a small lens, located directly over a visual receptor. The Limulus eye is made up of hundreds of these ommatidia. The Limulus eye has been used for research on lateral inhibition because its receptors are large enough so that stimulation can be applied to individual receptors.

Question 67

Edge enhancement

Answer 67

An increase in perceived contrast at borders between regions of the visual field. In addition to determining optimal stimuli for ganglion cells, center-surround receptive fields contribute to edge enhancement

Question 68

Chevreul illusion

Answer 68

Occurs when areas of different lightness are positioned adjacent to one another to create a border. The illusion is the perception of a light band on the light side of the border and a dark band on the dark side of the border, even though these bands do not exist in the intensity distribution. By appearing lighter on one side of the border and darker on the other, the edge itself looks sharper and more distinct, which demonstrates edge enhancement.

Question 69

Mach bands

Answer 69

Light and dark bands perceived at light–dark borders. The same mechanism is thought to be responsible for the Mach and Chevreul effects.

Question 70

Preferential Looking (PL) technique

Answer 70

A technique used to measure perception in infants. Two stimuli are presented, and the infant’s looking behavior is monitored for the amount of time the infant spends viewing each stimulus. The reason preferential looking works is that infants have spontaneous looking preferences (they prefer to look at certain types of stimuli).

Question 71

Visual evoked potential (VEP)

Answer 71

An electrical response to visual stimulation recorded by the placement of disk electrodes on the back of the head. This potential reflects the activity of a large population of neurons in the visual cortex. The VEP usually indicates better acuity than does preferential looking, but both techniques indicate that visual acuity is poorly developed at birth.

Question 72

Why do infants have low visual acuity?

Answer 72

Although the rod-dominated peripheral retina appears adultlike in the newborn, the all-cone fovea contains widely spaced and very poorly developed cone receptors. The newborn’s cones have fat inner segments and very small outer segments, whereas the adult’s inner and outer segments are larger and are about the same diameter. The small size of the outer segment means that the newborn’s cones contain less visual pigment and therefore do not absorb light as effectively as adult cones.