Chapter 6- Vision

Question 1

Statement that whatever excites a particular nerve always sends the same kind of information to the brain

Answer 1

Law of specific nerve energies

The brain somehow interprets the action potential’s from the auditory nerve as sounds, those from the olfactory nerve as odors, and so forth.
For example, if you rub your eyes, you may see spots or flashes of light even in a totally dark room. You applied mechanical pressure, which excited visual receptors in your eyes. Anything that excites those receptors is perceived as light.

Question 2

You perceive an object when it emits or reflects energy that stimulates receptors that transmit information to your brain. How does your brain make sense of that information?

Answer 2

Your brain encodes the information in a way that doesn’t resemble what you see. Your brain stores a representation of information in terms of altered activity in many neurons. Your brain codes the information in terms of which neurons respond, their amount of response, and the timing of their responses

Question 3

If someone electrically stimulated the auditory receptors in your ear, what would you perceive?

Answer 3

Because of the law of specific nerve energies, you would perceive it as sound, not as shock. Of course, a strong enough shock would spread far enough to excite pain receptors also

Question 4

If it were possible to flip your entire brain upside down, without breaking any of the connections to sense organs or muscles, what would happen to your perceptions of what you see, hear, and so forth?

Answer 4

Your perceptions would not change. The way visual or auditory information is coded in the brain does not depend on the physical location within the brain. Seeing something as “on top” or “to the left” depends on which neurons are active but does not depend on the physical location of those neurons

Question 5

An opening in the centre of the iris where light enters

Answer 5

Pupil

Light enters the eye through the pupil and is focussed by the lens, which is adjustable, and cornea, which is not adjustable, and projected onto the retina.

Question 6

The rear surface of the eye, which is lined with visual receptors

Answer 6

Retina

Light from the left side of the world strikes the right half of the retina, and vice versa. Light from above strikes the bottom half of the retina, and light from below strikes the top half. The inversion of the image poses no problem for the nervous system.

Question 7

Type of neuron in the retina that receives input directly from the receptors

Answer 7

Bipolar cells

Question 8

Type of neuron in the retina that receives input from the bipolar cells

Answer 8

Ganglion cells

Question 9

Ganglion cell axons that exit through the back of the eye and continue to the brain

Answer 9

Optic nerve

Question 10

Area at the back of the retina where the optic nerve exits; it is devoid of receptors

Answer 10

Blind spot

Images sometimes disappear because they strike the blind spot, and when the blind spot interrupts a straight line or other regular pattern, your brain fills in the gap

Question 11

Additional cells besides bipolar cells and ganglion cells called ______ cells get information from bipolar cells and send it to other bipolar, amacrine, and ganglion cells. Various types of these cells refine input to ganglion cells, enabling them to respond specifically to shapes, movements, or other visual features

Answer 11

Amacrine

Question 12

In every day life, you never notice your blind spot for two reasons:

Answer 12

1. Your brain feels in the gap for straight lines or other regular patterns
2. Anything in the blind spot of one eye is visible to the other eye

Question 13

What makes the blind spot of the retina blind?

Answer 13

The blind spot has no receptors because it is occupied by exiting axons and blood vessels

Question 14

Ganglion cells in the fovea of humans and other primates

Answer 14

Midget ganglion cells

It is called midget ganglion cells because each is small and responds to just a single cone. As a result, each cone in the fovea is connected to the brain with a direct route that registers the exact location of the input. Because the midget ganglion cells provide 70% of the input to the brain, our vision is dominated by what we see in the fovea

Question 15

A tiny area of the retina specialized for acute, detailed vision

Answer 15

Fovea

Means “pit”

Because blood vessels and ganglion cell axons are almost absent near the fovea, it has nearly unimpended vision. The tight packing of receptors also aids perception of detail

Question 16

More important for perceiving detail, each receptor in the fovea connects to a single _____ ____, which in turn connects to a single ______ ___, which has an axon to the brain

Answer 16

Bipolar cell; ganglion cell

Question 17

Describe visual perception for many bird species

Answer 17

Many birds eyes occupy most of the head, compared to only 5% of the head in humans.
Many bird species have two foveas per eye, one pointing ahead and one pointing to the side. The extra foveas enable perception of detail in the periphery.
Hawks and other predatory birds have a greater density of visual receptors on the top half of their retinas, looking down, than on the bottom half, looking up. That arrangement is adaptive because predatory birds spend most of their day soaring high in the air looking down. However, to look up, the bird must turn its head.

Question 18

Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells. As a result, the brain cannot detect the exact location or shape of a peripheral light source. However, the summation enables perception of ______ ______ in the periphery. In short, foveal vision has better ______, or sensitivity to detail, and peripheral vision has better sensitivity to ___ ____

Answer 18

Fainter lights; acuity; dim light

In the periphery, your ability to detect detail is limited by interference from other nearby objects

Question 19

Type of retinal receptor that detects brightness of light

Answer 19

Rods

Abundant in the periphery of the human retina

Respond to faint light but are not useful in daylight because bright light bleaches them

Question 20

Type of retinal receptor that contributes to colour perception

Answer 20

Cones

Abundant in and near the fovea

Less active in dim light, more useful in bright light, and essential for colour vision

Question 21

Chemicals contained in rods and cones that release energy when struck by light

Answer 21

Photopigments

Question 22

Because of the distribution of rods and cones, you have a good colour vision in the _____ but not in the _____

Answer 22

Fovea; periphery

Question 23

Although rods out

number cones by about 20 to 1 in the human retina, cones provide about ____% of the brain’s input

Answer 23

90%

Question 24

You sometimes find that you can see a faint star on a dark night better if you look slightly to the side of the star instead of straight at it. Why?

Answer 24

If you look slightly to the side, the light falls on an area of the retina with more rods and more convergence of input

Question 25

If you found a species with a high ratio of cones to rods in its retina, what would you predict about its way of life?

Answer 25

We should expect this species to be highly active during the day and seldom active at night

Question 26

Theory that colour is perceived through the relative rates of response by three kinds of cells, each one maximally sensitive to a different set of wavelengths

Answer 26

Trichromatic theory or young-Helmholtz theory Short-wavelength, medium-wavelength, and long-wavelength cone types. Each cone responds to a broad range of wavelengths but to some more than others. According to the trichromatic theory, we discriminate among wave links by the ratio of activity across the three types of cones. For example, light at 550 nm excites the medium-wavelength and long-wavelength receptors about equally and the short-wavelength receptor almost not at all. This ratio of responses among the three cones determines a perception of yellow-green. When all three types of cones are equally active, we see white or gray

Question 27

Area of the world that an individual can see at any time

Answer 27

Visual field

Question 28

Result of staring at a coloured object for a prolonged length of time and then looking at a white surface, the image is seen as a negative image, with a replacement of red with green, green with red, yellow and blue with each other, and a black and white with each other

Answer 28

Negative colour after image

Question 29

Idea that we perceive colour in terms of opposites

Answer 29

Opponent process theory That is, the brain has a mechanism that perceives colour on a continuum from red to green, and another from yellow to blue, and another from white to black. Because demonstrations show that you do not always see the correct colour of after image, after images depend on the whole context, not just the light on individual receptors. The cerebral cortex must be responsible, not the bipolar or ganglion cells

Question 30

The ability to recognize colours despite changes in lighting

Answer 30

Colour constancy

Question 31

Concept that the cortex compares information from various parts of the retina to determine the brightness and colour for each area

Answer 31

Retinex theory Proposed by Edwin Land to account for colour and brightness constancy.

Question 32

Suppose a bipolar cell receives excitatory input from medium-wavelength cones and inhibitory input from all three kinds of cones. When it is highly excited, what colour would one see? When it is inhibited, what colour perception would result?

Answer 32

Excitation of the cell should yield a perception of green under normal circumstances. Inhibition would produce the opposite sensation, red

Question 33

When a television set is off, it's screen appears gray. When you watch a program, parts of the screen appear black, even though more light is actually showing on the screen than when the set was off. What accounts for the black perception?

Answer 33

The black experience arises by contrast with the other brighter areas. The contrast occurs by comparison within the cerebral cortex, as in the retinex theory of colour vision

Question 34

Figure 6.9 shows 500 nm light as blue and 550 nm light as yellow. Why should we nevertheless not call them "blue light" and "yellow light"?

Answer 34

Colour perception depends not just on the wavelength of light from a given spot but also the light from surrounding areas. As in figure 6.14, the context can change the colour perception

Question 35

Inability to perceive colour differences

Answer 35

Colour vision deficiency For genetic reasons, some people lack one or two of the types of cones. Some have three kinds of cones, but one kind is abnormal. In red-green colour deficiency, the most common form of colour deficiency, people have trouble distinguishing red from green because their long and medium wavelength cones have the same photopigment instead of different ones.

Question 36

Most people can use varying amounts of three colours to match any other colour that they see. Who would be an exception to this rule, and how many colours would they need?

Answer 36

Red-green color-deficient people would need only two colors. Women with four kinds of cones might need four

Question 37

Type of cell that receives input from receptors and delivers inhibitory input to bipolar cells

Answer 37

Horizontal cell The rods and cones of the retina make synapses with horizontal cells and bipolar cells. The horizontal cells make inhibitory contact onto bipolar cells, which in turn makes synapses onto amacrine cells and ganglion cells. All these cells are within the eyeball

Question 38

Thalamic nucleus that receives incoming visual information

Answer 38

Lateral geniculate nucleus Most ganglion cell axons go to the lateral geniculate nucleus. A smaller number of axons go to the superior colliculus and other areas, including part of the hypothalamus that controls the waking-sleeping schedule. The lateral geniculate, in turn, send axons to other parts of the thalamus and occipital cortex. The cortex returns many axons to the thalamus, so the thalamus and cortex constantly feed information back and forth

Question 39

The axons of the ganglion cells form the ____ ____, which leaves the retina and travels along the lower surface of the brain.

Answer 39

Optic nerve

Question 40

The optic nerves from the two eyes meet at the ____ _____, where, in humans, half of the axons from each eye cross to the opposite side of the brain.

Answer 40

Optic chiasm Information from the nasal half of each eye, the side closer to the nose, crosses to the contralateral hemisphere. Information from the temporal half, the side toward the temporal cortex, goes to the ipsilateral hemisphere. The percentage of crossover varies from one species to another depending on the location of the eyes.

Question 41

Where does the optic nerve start and where does it end?

Answer 41

It starts with the ganglion cells in the retina. Most of its axons go to the lateral geniculate nucleus of the thalamus; some go to the hypothalamus and superior colliculus

Question 42

The reduction of activity in one neuron by activity in neighbouring neurons

Answer 42

Lateral inhibition Lateral inhibition is the retina's way of sharpening contrasts to emphasize the borders of objects When light falls on a surface, the bipolars just inside the border are most excited, and those outside the border respond the least

Question 43

When light strikes a receptor, does the receptor excite or inhibit the bipolar cells? What effect does it have on horizontal cells? What effect does the horizontal cell have on bipolar cells?

Answer 43

The receptor excites both the bipolar cells and the horizontal cell. The horizontal cell inhibits the same bipolar cell that was excited plus additional bipolar cells in the surrounding area.

Question 44

If light strikes only one receptor, what is the net effect (excitatory or inhibitory) on the nearest bipolar cell that is directly connected to that receptor? What is the effect on bipolar cells to the sides? What causes that effect?

Answer 44

It produces more excitation than inhibition for the nearest bipolar cell. For surrounding bipolar cells, it produces only inhibition. The reason is that the receptor excites a horizontal cell, which inhibits all bipolar cells in the area

Question 45

Examine figure 6.18. You should see greyish diamonds at the crossroads among the black squares. Explain why

Answer 45

In the parts of your retina that look at the long white arms, each neuron is maximally inhibited by input on two of its sides (either above and below or left and right). In the crossroads, each neuron is maximally inhibited by input on all four sides. Therefore, the response in the crossroads is decreased compared to that in the arms.

Question 46

Area of the cortex responsible for the first stage of visual processing

Answer 46

Primary visual cortex or area V1 or striate complex If you close your eyes and imagine seeing something, activity increases in area V1 in a pattern similar to what happens when you actually see that object. Although we do not know whether conscious visual perception occurs in area V1, it is apparently necessary for it. People with damage to area V1 report no conscious vision, no visual imagery, and no visual images in their dreams. In contrast, adults who lose vision because of eye damage continue to have visual imagery and visual dreams.

Question 47

The ability to respond in limited ways to visual information without perceiving it consciously

Answer 47

Blindsight Occurs in people with damage to area V1. Within the damaged part of the visual field, they have no awareness of visual input, not even to distinguish between bright sunshine and outer darkness. Nevertheless, they might be able to. Accurately do something in the area where they cannot see, or move their eyes toward it, while insisting that they are "just guessing".

Question 48

The area in visual space that excites or inhibits any neuron

Answer 48

Receptive field The receptive field of a receptor is simply the point in space from which light strikes the cell. Other visual cells derive their receptive fields from the connections they receive. A rod or cone has a tiny receptive field and space to which it is sensitive. A small group of rods or cones connects to a bipolar cell, with a receptive field that is the sum of those of the cells connected to it. Several bipolar cells report to a ganglion cell, which therefore has a still larger receptive field. The receptive fields of several ganglion cells converge to form the receptive field at the next level, and so on. Receptive fields in the visual system include both excitation and inhibition.

Question 49

The receptive field of a ganglion cell has a circular centre with an antagonistic donut-shaped surround. That is, light in the centre of the receptive field might be excitatory, with the surround inhibitory, or the opposite. Primate ganglion cells fall into three categories:

Answer 49

Parvocellular, magnocellular, and koniocellular

Question 50

Small cell bodies with small receptive fields in or near the fovea

Answer 50

Parvocellular neurons

Question 51

Large cell bodies with large receptive fields that are distributed evenly throughout the retina

Answer 51

Magnocellular neurons

Question 52

Small ganglion cells that occur throughout the retina

Answer 52

Koniocellular neurons

Question 53

Compare parvocellular, magnocellular, and koniocellular neurons

Answer 53

Parvocellular neurons: small cell bodies and small receptive fields. Mostly in or near the fovea. Well suited to detect visual details. Also respond to color, each neuron being excited by some wavelengths and inhibited by others. The high sensitivity to detail and colour relates to the fact that parvocellular cells are located mostly in and near the phobia, which has many cones. Magnocellular neurons: Larger cell bodies and receptive fields. Distributed evenly throughout the retina. Respond strongly to movement and large overall patterns, but they do not respond to colour or fine details. Found throughout the right now, including the periphery, where we are sensitive to movement but not colour or details. Koniocellular Neurons: Small cell bodies, and occur throughout the retina. Have several functions, and their axons terminate in several locations.

Question 54

As we progress from bipolar cells to ganglion cells to later cells in the visual system, are receptive fields ordinarily larger, smaller, or the same size? Why?

Answer 54

They become larger because each cells receptive field is made by inputs converging at an earlier level.

Question 55

What are the differences between the magnocellular and parvocellular systems?

Answer 55

Neurons of the parvocellular system have small cell bodies with small receptive fields, are located mostly in and near the fovea, and are specialized for detailed and colour vision. Neurons of the magnocellular system have large cell bodies with large receptive fields, are located in all parts of the retina, and are specialized for perception of large patterns and movement

Question 56

What are the three types of cells in the primary visual cortex discovered by David Hubel and Torston Wiesel?

Answer 56

Simple cells, complex cells, and end-stopped cells

Question 57

Type of visual cortex cell that has a receptive field with fixed excitatory and inhibitory zones

Answer 57

Simple cell The more light shines in the excitatory zone, the more the cell responds. The more light shines in the inhibitory zone, the less the cell responds. For example: if the receptor field is a vertical bar, tilting the bar slightly decreases the cells response because light then strikes inhibitory regions as well. Moving the bar left, right, up, or down also reduces the response. Most simple cells have bar-shaped or edge-shaped receptive fields. More respond to horizontal or vertical orientations than to diagonals.

Question 58

Type of visual cortex cell located in areas V1 and V2 that responds to a pattern of light in a particular orientation anywhere within its large receptive field

Answer 58

Complex cell For example: a complex cell responds to a pattern of light in a particular orientation, such as a vertical bar, anywhere within its large receptive field Responds most strongly to a moving stimulus. The best way to classify a cell as simple or complex is to present a stimulus in several locations. A cell that responds to a stimulus in only one location is a simple cell. One that responds equally throughout a large area is a complex cell

Question 59

Type of visual cortex cell that resembles complex cells; responds best to stimuli of a precisely limited type, anywhere in a large receptive field, with a strong inhibitory field at one end of its field

Answer 59

End-stopped cell or hypercomplex Has a strong inhibitory area at one end of its bar-shaped receptive field. The cell responds to a bar-shaped pattern of light anywhere in it's broad receptive field, provided the bar does not extend beyond a certain point.

Question 60

How could a researcher determine whether a given neuron in the visual cortex is simple or complex?

Answer 60

First identify a stimulus, such as a horizontal line, that stimulates the cell. Then present the stimulus in a different location. If the cell responds only in one location, it is a simple cell. If it responds in several locations, it is a complex cell

Question 61

What do cells within a column of the visual cortex have in common?

Answer 61

They respond best to lines in the same orientation. Also, they are similar in their preference for one eye or the other, or both equally.

Question 62

Neurons whose responses indicate the presence of a particular feature

Answer 62

Feature detectors The waterfall illusion supports the idea of feature detectors because it suggests that you have fatigued the neurons that detect downward motion, leaving unopposed the detectors for the opposite motion. (if you stare at a waterfall for a minute or more and then look away, the rocks and trees next to it appear to flow upward) However, later researchers found that a cortical cell that responds well to a single bar or line responds even more strongly to a sine wave grading of bars or lines. Many cortical neurons respond best to a particular spatial frequency and hardly at all to other frequencies. Most visual researchers therefore believe that neurons in area V1 detect spatial frequency's rather than bars or edges. A branch of mathematics called Fourier Analysis demonstrates that a combination of sine waves can produce an unlimited variety of other patterns. Therefore, a series of spatial frequency detectors, some sensitive to horizontal patterns and others to vertical patterns, could represent any possible display.

Question 63

What is a feature detector?

Answer 63

It is a neuron that detects the presence of a particular aspect of an object, such as a shape or a direction of movement

Question 64

Stimulation from both eyes

Answer 64

Binocular input

Question 65

Time early in development when experiences have a particularly strong and enduring influence

Answer 65

Sensitive period For each aspect of visual experience, researchers identify a sensitive period when experiences have a particularly strong and enduring influence. The sensitive period ends with the onset of certain chemicals that stabilize synapses and inhibit axonal sprouting. However, even long after the sensitive period, a prolonged experience – such as a full week without visual stimulation to one eye - produces a measurable effect on the visual cortex.

Question 66

What is the effect of closing one eye early in life? What is the effect of closing both eyes?

Answer 66

If one eye is closed during early development, the cortex becomes unresponsive to it. If both eyes are closed, cortical cells remain somewhat responsive for several weeks and then gradually become sluggish and unselective in their responses

Question 67

The discrepancy between what the left and right eyes see

Answer 67

Retinal disparity Experience fine-tunes binocular vision, and abnormal experience disrupts it. The behavioural result of abnormal experience is poor depth perception

Question 68

A condition in which the eyes do not point in the same direction

Answer 68

Strabismus or strabismic amblyopia or lazy eye

Question 69

A decreased responsiveness to one kind of line or another, caused by an asymmetric curvature of the eyes

Answer 69

Astigmatism

Question 70

What early experience is necessary to maintain binocular input to the neurons of the visual cortex?

Answer 70

To maintain binocular responsiveness, cortical cells must receive simultaneous activity from both eyes fixating on the same object at the same time

Question 71

What causes astigmatism?

Answer 71

Astigmatism results when the eyeball is not quite spherical. As a result, the person sees one direction of lines more clearly than the other

Question 72

Why is it important to correct astigmatism early?

Answer 72

If the visual cortex sees one kind of line better than another, it becomes more responsive to the kind it sees well and less sensitive to the other. That tendency could persist even after the astigmatism is corrected

Question 73

What happens when a kitten spends its entire early sensitive period wearing goggles with horizontal lines painted on them?

Answer 73

Nearly all it's visual cortex cells become responsive only to horizontal lines. Even after months of later experience, the cat does not respond to vertical lines.

Question 74

Area of the brain that processes information from the primary visual cortex and transmits it to additional areas

Answer 74

Secondary visual cortex or area V2 The connections in the visual cortex are reciprocal. For example, V1 sends information to V2, and V2 returns information to V1.

Question 75

Visual paths in the temporal cortex that are specialized for identifying and recognizing objects; the "what" path

Answer 75

Ventral stream

Question 76

Visual path in the parietal cortex that helps the motor system locate objects; the "where" path

Answer 76

Dorsal stream

Question 77

Describe what happens when people damage the dorsal stream, or the parietal cortex

Answer 77

They seem in most ways to have normal vision. They can read, recognize faces, and describe objects in detail. They would probably pass the vision test for a drivers license. But although they know what things are, they don't know where they are. They cannot accurately reach out to grasp an object. While walking, they can describe what they see, but they bump into objects, oblivious to their location. Although they can describe from memory what their furniture looks like, they cannot remember how it is arranged in rooms at their house.

Question 78

Describe what happens when a person damages the ventral stream

Answer 78

They see "where" but not "what". One gentleman could not read, recognize faces, or identify objects by sight. He could make no sense of a television program except from the sound. However, he could take a walk, accurately going around obstacles in his way and could reach out to grasp objects or shake hands. He could see where objects were even though he had trouble identifying what they were.

Question 79

Portion of the cortex where neurons are highly sensitive to complex aspects of the shape of visual stimuli within very large receptive fields

Answer 79

Inferior temporal cortex Cells in the temporal cortex respond according to what the viewer perceives, not what the stimulus is physically. Cells that respond to the sight of a particular object continue responding about the same way despite changes in its position, size, and angle. The cell somehow learn to recognize all the different views as being the same object.

Question 80

An inability to recognize objects despite otherwise satisfactory vision

Answer 80

Visual agnosia Usually results from damage in the temporal cortex. Someone might be able to point to visual objects and slowly describe them but failed to recognize what they are.

Question 81

Brain area of the inferior temporal cortex that recognizes faces

Answer 81

Fusiform gyrus

Question 82

The inability to recognize faces due to damage of several brain areas

Answer 82

Prosopagnosia

Question 83

Suppose someone can describe an object in detail but stumbles and fumbles when trying to walk toward it and pick it up. Which is probably damaged, the dorsal path or the ventral path?

Answer 83

The inability to guide movement based on vision implies damage to the dorsal path

Question 84

What is prosopagnosia, and what does its existence tell us about separate shape recognition systems in the visual cortex?

Answer 84

Prosopagnosia is the inability to recognize faces. It's existence implies that the cortical mechanism for identifying faces is different from the mechanism for identifying other complex stimuli

Question 85

Area V4 is important for colour constancy. What is colour constancy?

Answer 85

It is the ability to recognize the colour of an object despite changes in the lighting

Question 86

Area of the brain that detects moving objects

Answer 86

Middle temporal cortex or Area MT or V5 An adjacent region, area MST or medial superior temporal cortex is also activated by motion These areas receive input mostly from the magnocellular path, which the text overall patterns, including movement over large areas of the visual field. Given that the magnocellular path is color-insensitive, MT is also colour insensitive

Question 87

Describe the responses of area MT

Answer 87

Most cells in area MT respond to selectively when something moves at a particular speed in a particular direction. They detect acceleration or deceleration as well as the absolute speed, and they respond to motion in all three dimensions. Also responds to photographs that imply movement.

Question 88

Describe the responses of cells in the area MST

Answer 88

Cells in the dorsal part of area MST respond best to more complex stimuli, such as the expansion, contraction, or rotation of a large visual scene. That kind of experience occurs when you move forward or backward or tilt your head. Cells in the ventral part of area MST respond when an object moves relative to its background. They therefore react either when the object moves or when the object is steady and the background moves

Question 89

An impaired ability to perceive movement

Answer 89

Motion blindness

Question 90

Voluntary eye movements

Answer 90

Saccades If you focus on your left eye while looking in a mirror and then look at your right eye, you will not see your eyes move. You do not see your own eyes move because several of the visual areas of your brain decrease their activity during voluntary eye movements, known as saccades. In short, during a voluntary eye-movement, you become temporarily motion blind

Question 91

Under what circumstances does someone with an intact brain become motion blind, and what accounts for the motion blindness?

Answer 91

People become motion blind shortly before and during a saccade or voluntary eye movement, because of suppressed activity in several brain areas, including area MT

Question 92

What symptoms occur after damage limited to area MT? What may occur if MT is intact but area V1 is damaged?

Answer 92

Damage in area MT can produce motion blindness. If area MT is intact but area V1 is damaged, the person may be able to report motion direction despite no conscious identification of the moving object