Chapter 3 Flashcards
1
Q
What are the 4 basic steps to vision?
A
1) distal stimulus
- ex; the tree
2) Light is reflected and transformed to create an image of the tree on the retina
3) Receptor processes: receptors transform light into electricity
4) Neural processing: signals travel in a network of neurons
2
Q
What is vision based on?
A
Visible light, which is a small part of the electromagnetic spectrum.
3
Q
What is the electromagnetic spectrum?
A
Continuum of electromagnetic energy that is produced by electric charges and is radiated as waves.
4
Q
What is a wavelength?
A
The distance between the peaks of the electromagnetic waves.
5
Q
What wavelengths are part of the visible light?
A
Between 400 to 700 nanometers (mm)
6
Q
What wavelengths are associated with blue, green, yellow orange and red?
A
Blue —> short wavelengths
Green —> middle wavelengths
Red (yellow, orange) —> long wavelengths
7
Q
What does light pass through before hitting the retina at the back of the eye?
A
1) pupil
2) focused by the cornea
3) focused by lens
4) hits retina
8
Q
What is the retina?
A
The network of neurons that covers the back of the eye and that contains the receptors for vision
9
Q
What are the receptors for vision called?
A
Photoreceptors
10
Q
What are the two types of photoreceptors?
A
Rods and cones
11
Q
What are the outer-segments to the rods and the cones?
A
They are the part of the receptors that contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals.
12
Q
How do the neural signals transduced from light exit the eye?
A
Signals from the receptors flow through the network of neurons that make up the retina and emerge from the back of the eye in the optic nerve.
13
Q
Which part of the retina contains only cones?
A
Fovea
14
Q
What part of the retina contains both rods and cones, but rods in an important amount?
A
Peripheral retina
15
Q
What is the name of the condition which is most common in older people that destroys the cone-rich fovea and a small area that surrounds it?
A
Macular degeneration
16
Q
What are the consequences of macular degeneration for vision?
A
This condition creates a blind region in central vision, so when a person looks directly at something, they lose sight of it.
17
Q
Name the condition that is a degeneration of the retina that is passed from one generation to the next and that attacks the peripheral rod receptors?
A
Retinitis pigmentosa
18
Q
What are the consequences of the condition called retinitis pigmentosa on vision?
A
It results in poor vision in the peripheral visual field.
Eventually, in severe cases, the foveal cone receptors are also attacked, resulting in complete blindness.
19
Q
What is the blind spot of the eye?
A
An area in the retina where there are no photoreceptors, which is where the nerve fibres that make up the optic nerve leave the eye.
20
Q
Why aren’t we actually aware of the blind spot?
A
1) the blind spot is located off to the side of our visual field, where objects are not in sharp focus.
2) a mechanism in the brain “fills in” the place where the image disappear.
21
Q
What is the cornea?
A
The transparent covering of the front of the eye.
It accounts for 80% of the eye’s focusing power.
It is fixed in place, so it cannot adjust its focus.
22
Q
What is the lens of the eye?
A
It supplies the remaining 20% of the eye’s focusing power.
It can change its shape to adjust the eye’s focus for objects located at different distances.
23
Q
How is the change in shape of the lens in the eye achieved?
A
By the action of ciliary muscles, which increase the focusing power of the lens by increasing its curvature.
24
Q
What is the process controlled by the adjustable lens?
A
Accommodation
25
What is accommodation?
The change in the lens’s shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker
26
What are called the errors that can affect the ability of the cornea and/or lens to focus the visual input onto the retina?
Refractive errors
27
What is the refractive error that often occurs in normal aging called presbyopia?
As people get older, their ability to accommodate decreases due to the hardening of the lens and weakening of the ciliary muscles.
28
How can presbyopia be dealt with?
By wearing reading glasses that bring near objects into focus by replacing the focusing power that can no longer be provided by the lens.
29
What is myopia?
Inability to see distant objects clearly
30
What causes myopia?
It occurs when the optical system brings parallel rays of light into focus at a point in front of the retina, so that the image that reaches the retina is blurred.
31
What are the two causes of myopia?
1) refractive myopia
- in which the cornea and/or the lens bends the light too much
2) axial myopia
- in which the eyeball is too long
32
What is hyperopia?
People can see distant objects clearly but have trouble seeing nearby objects
33
What are the causes of hyperopia?
The focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short
34
What is transduction?
The transformation of one form of energy into another form of energy.
35
Where does visual transduction occur?
In photoreceptors
36
What are the two parts to visual pigments?
A long protein called *opsin*
A much smaller light-sensitive component called *retinal*
37
What happens to the retinal when light hits it?
The visual pigment molecule absorbs the light, which causes the retinal within to change its shape, from being bent, to being straight.
That is called **isomerization**
38
What is isomerization?
The process of the retinal changing shape from bent to straight when it is hit by light
It activates thousands of charged molecules to create electrical signals in receptors
39
How do the visual pigments influence the aspects of visual perception?
1) how we adjust to darkness
2) how well we see light in different parts of the spectrum
40
The process of increasing sensitivity in the dark.
Dark adaptation
41
How is dark adaptation measured?
By determining a dark adaptation curve
42
What is the dark adaptation curve?
The function relating sensitivity to light to time in the dark, beginning when the lights are extinguished
43
What is light-adapted sensitivity?
The sensitivity measured in the light
Because it is measured while the eyed are adapted to the light
44
What is dark adaptation?
The sensitivity at the end of dark adaptation
45
How much do cones and rods adapt to the dark?
Cones adapt up to a certain point but are not very sensitive to light so they see badly in the dark
Rods are much more light-sensitive and can therefore adapt to the dark much better
46
What is the rare genetic defect called where people have no cones and only rods?
Rod monochromats
47
What is the rod-cone break?
The place where the rods begin to determine the dark adaptation curve instead of the cones.
48
What is visual pigment bleaching?
The change in shape and separation of the retinal from the opsin that causes the molecule to become lighter in colour
49
Can the opsin molecules still be useful for vision when they are in their bleached state after the retinal has changed shape and separated?
No. They are no longer useful then. In order to do their job of changing light energy into electrical energy, the retinal needs to return to its bent shape and become reattached to the opsin. This process is called **visual pigment regeneration**
50
What is the visual pigment regeneration?
The process of the retinal reattaching to the opsin and returning to its original shape in order to be useful to vision again
51
What do rushton’s measurements show about the time needed for regeneration of rods and cones?
6 minutes for cones
30 minutes for rods
52
What are the two important connections between perception and physiology found by Rushton?
1) our sensitivity to light depends on the concentration of a chemical - the visual pigment
2) the speed at which our sensitivity increases in the dark depends on a chemical reaction - the regeneration of the visual pigment
53
What is the condition called detached retina?
When a person’s retina becomes detached from the pigment epithelium, a layer that contains enzymes necessary for pigment regeneration.
54
What is spectral sensitivity?
It is the eye’s sensitivity to light as a function of the light’s wavelength.
55
How is spectral sensitivity measured?
By determining the spectral sensitivity curve— the relationship between wavelength and sensitivity
56
Which wavelengths are rods more sensitive to?
Short-wavelength light
Light of 500 nm
57
Which wavelengths are cones most sensitive to?
Long-wavelength
Light of 560 nm
58
How does vision shift from cones to rods after the eye has adapted to the dark?
As vision shifts from the cones in the light-adapted eye to the rods after the eye has become relatively more sensitive to short-wavelength light- that is- light nearer to the blue and green end of the spectrum
59
What is the **Purkinje shift**?
The enhanced perception of short-wavelengths during dark adaptation
60
Which pigments regenerate faster? The cones or the rods?
The cone pigments regenerate faster.
61
What is a pigment’s **absorption spectrum**?
A plot of the amount of light absorbed versus the wavelength of the light.
62
At which wavelength does the rod pigment absorb best?
500 nm
63
Why are there three absorption spectra for the cones?
Because there are three different cone pigments
64
At which wavelength does the short-wavelength pigment absorb light best?
419 nm
65
At which wavelength does medium-wavelength pigment absorbs light best?
531 nm
66
At which wavelength does long-wavelength pigment absorb light best?
558 nm
67
At which wavelength does the psychophysical spectral sensitivity curve peak?
560 nm
68
Why is the cone spectral sensitivity curve determined mainly by the medium and long wavelength pigments?
Because there are fewer short wavelength receptors and therefore much less short-wavelength pigment
69
What are the increase in sensitivity that occurs in the dark and the sensitivity to different wavelengths across the spectrum determined by?
The properties of the rod and cone visual pigment
70
How many types of neurons make up the layers of neural circuits that are within the retina?
5 types
71
Through which cells does the signal travel to starting from the photoreceptors?
Goes from the receptors to the **bipolar cells** to the **ganglion cells**
72
Which cells in the eyes have long axons and which have short axons?
Receptors = short axons
Bipolar cells = short axons
Ganglion cells = long axons
73
Which axons make up the optic nerve and transmit signals out of the retina?
The long axons of ganglion cells
74
What are the five types of neurons making up the retina?
1) rod and cone receptors
2) horizontal cell
3) bipolar cells
4) amacrine cells
5) ganglion cells
75
How many cones and rods on average send their signals to one ganglion cell?
Rods —> 120 to one ganglion cell
Cones —> 6 to one ganglion cells
76
Into which two differences in perception does the greater convergence of the rods compared to the cones translate to?
1) the rods result in better sensitivity than the cones (to light)
2) the cones result in better detail vision than the rods
77
Why can we see in dim light?
We use our rods to detect faint stimuli. Because rod sensitivity is greater than the cone
78
What are the reasons for the greater sensitivity of rods?
1) it takes less light to generate a response from an individual rod receptor than from an individual cone receptor
2) the rods have greater convergence than the cones
79
What is acuity?
The ability to see details, every small letters etc.
80
Which one of the rods and cones have better visual acuity?
Cones
81
Where is the visual acuity best on the retina?
Fovea
82
What does the cone’s lack of convergence have to do with acuity?
Cones usually have their own ganglion cells. Therefore, there is this separation of all cones. Rods are multiple on the same ganglion cells, there is therefore some convergence when they all get to the same ganglion cells, which reduces acuity.
83
Why is convergence of the rods a double-edge sword?
1) high-convergence results in hight sensitivity (reacts and fires more easily, needs less light to fire, etc.)
2) low convergence results in low sensitivity but high acuity (cones)
84
Who first discovered the property of neurons called the neuron’s **receptive field**?
H. Keffer Hartline
85
What did Hartline do to discover the ganglions’ receptive fields?
1) isolated a dingle ganglion cell axon in opened eyecup of frog
2) teased apart the optic nerve where it leaves the eye to isolate it.
3) illuminated different areas of retina and found that the cell he was recording from responded only when a small area of the retina was eliminated
4) called this area the **receptive field**
86
What is Hartline’s definition of receptive fields?
The region of the retina that must receive illuminations in order to obtain a response in any given fiber.
87
Is the ganglion’s receptive field the same size as the photoreceptors’?
No. The receptive fields of ganglion cells are much larger than a single photoreceptor.
It covers hundreds or even thousands of receptors
88
Can ganglion cells receptive fields of different ganglion cells overlap?
Yes
89
Who measured ganglion cell receptive fields in the cat and reported a property called **center-surround receptive fields**?
Stephen Kuffler
90
What are center-surround receptive fields and what do they look like?
It’s a type of receptive field found only in mammals.
These receptive fields are made of center and surround. There are two types of these center-surround receptive fields.
One type has an excitatory center and an inhibitory surround. The other has a inhibitory center and an excitatory surround
91
What are center-surround fields and what do they look like?
Center-surround fields are receptive fields only present in mammals.
There are two types. One has an excitatory center that increases the firing of ganglion cells if the light hits it, and its surround is inhibitory, which decreases the firing of ganglion cells. This kind of receptive field is called a **excitatory-center, inhibitory-surround receptive field**.
The other type is the other way around: excitatory surround and inhibitory center. That one is called an **inhibitory-center, excitatory surround receptive field.
92
Where are located the center-surround ganglion cell receptive fields?
In the mammalian retina
93
What is the updated definition of receptive fields by Hubel and Wiesel?
“The retinal region over which a cell in the visual system can be influenced (excited or inhibited) by light”
94
What is the effect called **center-surround antagonism**?
A small spot of light presented to the excitatory center of the receptive field causes a small increase in the rate of nerve firing: increasing the light’s size so it covers the entire center will increase the cell’s response.
**center-surround antagonism** is when the spot of light becomes large enough that it begins to cover the inhibitory area as well as the center.
- Stimulation of the inhibitory surround counteracts the center’s excitatory response, causing a decrease in the neuron’s firing rate.
95
What is the inhibition involved in center-surround antagonism and what does it do?
**Lateral inhibition**
- inhibition that is transmitted across the retinal (laterally)
96
How was lateral inhibition discovered in regards to center-surround antagonism? With which animal? What was the procedure?
**Limulus**
- Horseshoe crab
- Chosen because of the structure of its eye —> possible to stimulate individual receptors. Very large compared to humans so it is possible to illuminate and record from a single receptor without illuminating its neighboring receptors.
97
How does lateral inhibition function?
If you are stimulating point A, the excitatory center, the ganglion cell increases in firing.
When stimulating more than point A, and a part of point B, the inhibitory surround, the inhibition is transmitted from B to A from the horizontal and amacrine cells. The inhibition is transmitted laterally across the human retina.
98
Which cells are involved in the transmition of lateral inhibition across the human retina?
Horizontal and amacrine cells
99
How are center-surround receptive fields, convergence and lateral inhibition connected?
Ex: 7 receptors in one receptive fields numbered 1 through 7. 1, 2, 6 and 7 are in the inhibitory surround and 3, 4 and 5 are in the excitatory center. All of these light receptors converge on to a point called B before continuing down the path to the ganglion cell.
The surround will inhibit the excitatory response of the center and point B will sum up all of the incoming signals to produce ONE response.
See image page 57 (figure 3.29)
100
What do center-surround receptive fields contribute to on top of determining optimal stimuli for ganglion cells?
Edge enhancement
101
What is the edge-enhancement done by the center-surround receptive fields?
An increase in perceived contrast at borders between regions of the visual field
102
What is the Chevreul illusion and what is its role in edge enhancement?
**Chevreuil illusion**
- Ex: two gray bands one beside another. The intensity of the light is the same for both bands of light, but the perception of lightness is not. At the border of both bands there is a lightening and a darkening.
- The perceived light and dark bands at the borders, which are not present in the actual physical stimuli.
- Placing colours side by side alter their appearance. The edge looks sharper and more distinct, which demonstrates edge enhancement.
103
What are Mach bands?
Ex: placing one sheet of paper in front of another piece of paper. The shadow is a fuzzy border between light and dark, rather than the sharp border in Chevreuil illusion.
- Light and dark bands created at fuzzy borders are called **mach bands**
104
How does edge-enhancement work in the example of 4 ganglion cells receptive fields?
Ex: go look at p.59 (figure 3.32)
- The key to understanding how those neurons could cause edge enhancement is to compare the amount of inhibition for the different cells.
Ex: four center-surround receptive fields
- In lighter band: Cell A is inhibitory in the whole cell so it’s very inhibitory because it’s in lighter area. Cell B is at the border, about half of its receptive field is inhibitory so it is less inhibitory than cell A. The cell B’s response is higher than cell A because it is less inhibitory. Therefore it appears lighter which creates a light bar (part of the border). The opposite is true for the darker band. Cell D is not inhibitory at all so it appears dark, but cell C, at the border, is a bit more inhibitory than cell D, so it appears darker, which creates the border.
HONESTLY SEE FIGURE IN THE BOOK CAUSE THIS IS HARD TO EXPLAIN
105
What is the method that has been used to measure infant visual acuity?
Preferential looking technique
106
What is the preferential looking technique used in measuring infant visual acuity?
1) asking the right questions
- Can you tell the difference between the one on the left and the one on the right?
2) observing
- Ex: the experimenter presents two stimuli to the child, but is unaware of which stimuli are being presented. He then observed the eyes of the infant, and can see if the infant is looking more towards one stimulus than the other. Then he can conclude that the infant can tell the difference between the two.
107
What is the visual evoked potential?
Method to measure electrical signals, recorded by disc electrodes placed on infant’s head.
- Alternate gray field with checkerboard pattern.
108
What are the findings of the visual evoked potential with children and infants?
Visual acuity is poorly developed at birth.
Acuity increases rapidly over the first 6 to 9 months. FOllowed by a leveling-off period.
Adult acuity is reached sometime after 1 year of age.
109
Why is acuity less in infants?
Because their cones are shorter and contain less visual pigment and therefore do not absorb light as effectively as adult cones.
+
Infant cones are much more spaced apart than adult cones.
