Lecture 5

Question 1

The First Steps in Vision:

Answer 1

From Light to Neural Signals

Question 2

The Man Who Could Not Read

Answer 2

Imagine not being able to read, drive, or recognize faces. For some people, a specific type of retinal disease can rob them of their high acuity central vision

AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma

Question 3

Age-related Macular Degeneration (AMD)

Answer 3

A disease associated with aging that affects the macula.

AMD gradually destroys sharp central vision.

AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma

Question 4

Age-related Macular Degeneration (AMD)

Answer 4

A disease associated with aging that affects the macula.

AMD gradually destroys sharp central vision.

AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma.

There are 2 varieties of AMD:
Wet and Dry

Question 5

Macula

Answer 5

The central part of the retina containing the fovea

Question 6

AMD

Answer 6

Loss in central vision —> blindness

1.75 million Americans

Double prevalence by 2020

Caused by formation of new veins or thinning of tissue.

Question 7

AMD Details

Answer 7

Loss in central vision —> blindness

1.75 million Americans

Double prevalence by 2020

Caused by formation of new veins or thinning of tissue.

Question 8

Wet AMD

Answer 8

Abnormal blood vessels grow under the macula and can leak blood and fluid, raising the macula and impairing central vision.

As the macula is displaced, straight lines may look wavy

Question 9

Dry AMD

Answer 9

More common.

Occurs when macula cones degenerate.

Sometimes dry AMD turns into wet AMD.

Once dry AMD is advanced, no treatment can reverse the loss of vision.

Question 10

Age-related macular degeneration (AMD)

Two varieties:

Answer 10

Wet AMD:
Abnormal blood vessels grow under the macula and can leak blood and fluid, raising the macula and impairing central vision.
As the macula is displaced, straight lines may look wavy

Dry AMD: 
More common. 
Occurs when macula cones degenerate. 
Sometimes dry AMD turns into wet AMD. 
Once dry AMD is advanced, no treatment can reverse the loss of vision.

HOWEVER: medical treatment can slow or stop further loss of vision

Some treatments can delay and possibly prevent AMD from progressing.

Studies show taking high-dose anti-oxidants and zinc significantly reduces risk of advanced AMD and associated vision loss.

Question 11

Studies show taking high-doses of _______ significantly reduces risk of advanced AMD and associated vision loss.

Answer 11

anti-oxidants and zinc

Question 12

Convergence

Answer 12

There is greater convergence of rods than cones onto ganglion cells

Thus, there is greater summation of rod signals,
and less stimulation per rod is required to obtain a response.

Question 13

Cone vision can see finer details than rod vision.

Answer 13

Greater convergence of rods than cones onto ganglion cells limits their spatial sensitivity.

Question 14

horizontal pathway

Answer 14

Horizontal cells: Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells.
Responsible for lateral inhibition, which creates the center–surround receptive field structure of retinal ganglion cells

Amacrine cells: These synapse horizontally between bipolar cells and retinal ganglion cells.
Have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time).

Question 15

Horizontal cells

Answer 15

Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells.

Responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells

Question 16

vertical pathway

Answer 16

The retina’s vertical pathway: Photoreceptors, bipolar cells, and ganglion cells

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

Question 17

vertical pathway

Answer 17

The retina’s vertical pathway: Photoreceptors, bipolar cells, and ganglion cells

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

P ganglion cells: Connect to the parvocellular pathway

M ganglion cells: Connect to the magnocellular pathway

Question 18

Midget bipolar cell

Answer 18

A small bipolar cell that receives input from a single cone

Question 19

Bipolar cell

Answer 19

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

Diffuse bipolar cell:
A bipolar cell that receives input from multiple photoreceptors

Midget bipolar cell:
A small bipolar cell that receives input from a single cone

Question 20

P ganglion cells

Answer 20

Connect to the parvocellular pathway.

Receive input from midget bipolar cells.

Parvocellular (“small cell”) pathway is involved in fine visual acuity, color, and shape processing.

Poor temporal resolution but good spatial resolution

Question 21

M ganglion cells

Answer 21

Connect to the magnocellular pathway

Receive input from diffuse bipolar cells

Magnocellular (“large cell”) pathway is involved in motion processing.

Excellent temporal resolution but poor spatial resolution

Question 22

Receptive Field

Answer 22

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

Question 23

Lateral Inhibition

Answer 23

The response properties of retinal ganglion cells can be explained by the neural circuit depicted above.

Note that the neuron being recorded receives excitatory inputs from one group of receptors and inhibitory inputs from receptors in surrounding regions

Question 24

Lateral Inhibition

Answer 24

The response properties of retinal ganglion cells can be explained by the neural circuit depicted above.

Note that the neuron being recorded receives excitatory inputs from one group of receptors and inhibitory inputs from receptors in surrounding regions.

Question 25

Kuffler

Answer 25

Kuffler mapped out the receptive fields of individual retinal ganglion cells in the cat retina

ON-center ganglion cells:
Excited by light falling on center, inhibited by light falling surround.

OFF-center ganglion cells:
Inhibited by light falling on center, excited by light falling on surround

Question 26

ON-center ganglion cells

Answer 26

Excited by light falling on center, inhibited by light falling surround

Question 27

OFF-center ganglion cells

Answer 27

Inhibited by light falling on center, excited by light falling on surround

Question 28

Why center-surround receptive fields?

Answer 28

Each ganglion cell will respond best to spots of a particular size (and respond less to spots that are too big or too small).

Retinal ganglion cells act like a filter for information coming to the brain.

Retinal ganglion cells are most sensitive to differences in the intensity of light between center and surround and are relatively unaffected by the average intensity.

Luminance variations tend to be smooth within objects and sharp between objects.

Thus, center-surround receptive fields help to emphasize object boundaries.

Question 29

One consequence of Center-Surround architecture:

Mach bands

Answer 29

Illusory stripes that emphasize differences in luminance.

The stripes are a product of your visual system’s center-surround receptive fields

Question 30

P and M ganglion cells revisited

Answer 30

P ganglion cells: Small receptive fields, high acuity, work best in high light situations, sustained firing.
Provide information mainly about the contrast in the retinal image

M ganglion cells: Large receptive fields, low acuity, work best in low light situations, burst firing.
Provide information about how an image changes over time

Question 31

Hermann Grid

Answer 31

An Illusion:
Why do spots appear at the junctions, and why do they disappear when a junction is fixated?

Point of fixation

Spots do not appear at the fixated junctions because receptive fields in the fovea are smaller than in more peripheral regions.

Cells with receptive fields at the junction receive more inhibition (2 more minus signs), so that the junctions appear darker.

Question 32

Dark and Light Adaptation

Answer 32

One of the most remarkable things about the human visual system is the incredible range of light levels we can adjust to.

2 mechanisms for dark and light adaptation:
Pupil Dilation
Photoreceptors and their replacement

Question 33

Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels.

Answer 33

The amount of photopigment available in photoreceptors changes over time.

The more light entering the retina, the faster the photopigments are used up, and the fewer photopigments there are to process more light.

The less light entering the retina, the more slowly photopigments are used up, and the more photopigments there are to process what little light is there

Question 34

The visual system regulates the amount of light entering the eye, and ignores whatever variation in overall light level is left over

Answer 34

In bright light, the pupil constricts, letting in less light

Next, the number of photopigments in the photoreceptors decreases over a few minutes

Being light-adapted means that even though there are more photons coming into the eye, there are fewer photopigments available to process them, so much of the light is unused.

Question 35

Retinitis pigmentosa (RP)

Answer 35

Family of hereditary diseases involving the progressive death of photoreceptors and degeneration of the pigment epithelium.

Question 36

The Man Who Could Not See Stars

Answer 36

Many people may not notice the onset of retinitis pigmentosa at first because it primarily affects peripheral vision.