Visual system - 1

Question 1

What is meant with the psychology of Gestalt:

“What we see represents not just the properties of objects but, more importantly, the organization of sensations by the brain.”

Answer 1

The brain makes certain assumptions about what is to be seen in the world, expectations that seem to derive in part from experience and in part from the built-in neural wiring for vision.

Question 2

Where is vision based on?

Answer 2

On the interrelationship of images → how certain parts of images relate to each other.

This is just like a melody, we don’t recognize the sequences of notes, but we recognize the interrelationship between notes.

Question 3

What is figure-ground dichotomy and how does it relate to vision?

Answer 3

Figure-ground dichotomy defines our ability to visually separate elements based on contrasts such as light an dark, black and white.

This dichotomy proves that in general your brain cannot see both the fore- and background and thus makes a distinguishment between the two. It supports the fact that vision is based on the interrelationship between parts of an image.

Question 4

How is our brain able to see patterns in a picture like this?

Answer 4

Seeing patterns is based on experience, our brain will try to make connections based on the things we’ve seen before.

Question 5

What is the Muler-Lyer illusion?

Answer 5

When we start making assumptions about the size, based on the shape.

Question 6

What emphasizes the fact that vision is based on the interrelationship of objects?

Answer 6

That the perceived size of an object depends on other subjects in the visual field. In the picture is an illusion where the doors look equal to size, but in fact this isn’t the case.

Question 7

What can we conclude based on the questions discussed before about seeing patterns?

Answer 7

Our visual system is constantly making assumptions about the outside world based on experience and build-in neuronal wiring (the modulating influence of descending projections).

Question 8

Describe characteristics of the retina.

Answer 8

• The retina processes the first part of the incoming visual information.
• There’s a special spot in the retina that processes much of the visual information (the darker spot in the middle of the retina), the rest of the retina processes much less visual information.
• There’s also a blind spot in the retina, where the optical nerve leaves the retina (right spot on the retina).

Question 9

Explain the pathway that visual information takes from the retina.

Answer 9

1. Light entering the eye triggers photochemical reactions in rods and cones at the back of retina.
2. Chemical reaction in turn acitvates bipolar cells.
3. Information is sent to the visual cortex via the thalamus.

Question 10

What are rods and cones?

Answer 10

These are the two types of photoreceptors in the retina of the eye and are composed of a inner (cell body) and outer segment (synaptic terminal).

Question 11

What’s the difference between rods and cones in regard to the length of inner and outer segments?

Answer 11

Inner and outer segments → rods have longer outer segments that contain a lot of optic discs compared to cones that have shorter outer segments with fewer optic discs.

Question 12

What are optic discs?

Answer 12

Optic discs are part of the membrane of the pigment epithelial cells and enlarge the membrane surface. They’re removed regularly, which takes about 12 days to replace all the optic discs.

Question 13

What’s the difference between rods and cones in regard to light sensitivity?

Answer 13

Rods are very sensitive to light (absolute threshold), while the threshold for cones lies further. At indoor lighting, rods become saturated. Cones give the best resolution to size, shape and color and become bleached when light increases.

Question 14

Why is there regeneration of optic discs?

Answer 14

This is because photoreceptors become bleached and desensitized and to renew the receptors, the optic discs are regenerated.

Question 15

The distribution of rods and cones in our retina also differs. How are rods and cones distributed in the retina?

Answer 15

See picture → Cones have a low concentration in the peripheral part of retina, while they’re highly concentrated around the foveola. And vice versa with rods.

Question 16

Another difference between rods and cones are their connectivity to bipolar cells. How do they differ in this connectivity?

Answer 16

• Many rods can connect to one bipolar cell → large part of visual field is processed by one bipolar cell → low resolution.
• Cones have a one to one connectivity → they’re responsible for visual acuity → high resolution of visual field.

Question 17

How do rods and cones respond to light?

Answer 17

When light hits photoreceptor → cells respond with graded change in potential → hyperpolarization (exclusive feature of visual system).

Question 18

What is phototransduction?

Answer 18

Phototransduction is the process where light is converted into electrical signals in the rods, cones and ganglion cells of the retina of the eye.

Question 19

What happens during phototransduction in the absence of light? And what happens in the presence of light?

Answer 19

• In the absence of light → cation channels in rods and cones are kept open by intracellular cGMP and conduct an inward current carried largely by Na⁺ (but also Ca²⁺) and outward current of K⁺. This generates a depolarizing current.
• In the presence of light → conformational change of photoreceptors, which initiates an intracellular cascade → G protein transducin is activated which activates cGMP phosphodiesterase → this enzyme breaks down cGMP into 5-GMP → channels cannot be kept open anymore and close → inward current is reduced → hyperpolarization.

Question 20

Explain why the process of phototransduction seems such a complicated system, but in fact it is a really convenient system.

Answer 20

While the process of phototransduction seems complicated (and is complicated), it is a process that ensures no delay by causing an intracellular cascade that leads to the amplification of the signal. With this, phototransduction leads to the closing of hundreds to thousands of channels.

Question 21

How is the photoreceptor sensitivity modulated?

Answer 21

In the presence of light, Ca²⁺ is able to inhibit (the formation of) cGMP or is able to inhibit the rhodopsin kinase. By doing this, the sensitivity of light-sensitive receptors is modulated.

• So in the presence of light → the photoreceptor sensitivity is modulated by a decrease of the concentration of Ca²⁺, which ensures a good intracellular cascade.

Question 22

In line with the difference in connectivity of rods and cones, there’s also a difference in the response to light. Explain this difference.

Answer 22

• Many rods converge on one bipolar cell, therefore rods are more sensitive to light and have a longer and stronger response to light.
• Cones have a one-to-one connection to bipolar cells, therefore cones have much better visual acuity and response to color. They have a sharp and short response.

Question 23

To summarize → explain the differences between rods and cones in regard to:

• Light sensitivity
• Spatial resolution (visual acuity)
• Density in fovea
• Mediating color
• Connectivity
• Response to light

Answer 23

• Light sensitivity → rods have a high sensitivity, while cones have a low sensitivity.
• Spatial resolution (visual acuity) → rods have a low spatial resolution, while cones have a high spatial resolution.
• Density in fovea → rods have a low density in the fovea, while cones have a high density in the fovea.
• Mediating color → rods are not able to mediate color, while cones are able to mediate color
• Connectivity → rods have high connectivity (many rods to one bipolar cell), while cones have low connectivity (one to one).
• Response to light → rods have a long and strong response to light, while cones have a short and sharp response to light.

Question 24

There are three different cones for different wave lengths of light. Describe these three different cones.

Answer 24

• S cones → short wavelenght (blue)
• M cones → medium wavelength (green)
• L cones → long wavelenght (red)

Question 25

How are the different cones distributed in the retina?

Answer 25

Dispersed

• S cones make up about 5-10% of the cones and are absen in the fovea.
• The ratio of M to L cones differs largely from individual to individual.

Question 26

Name two diseases of the eye in regard to light sensitivity

Answer 26

• Dichomracy → 8% of males are color blind → red-green color blindness due to absense of either M or L cones
• Achromatopsia → no cones.

Question 27

What do we need in order to process color?

Answer 27

The role of context, look at picture.

Question 28

What is luminance contrast?

Answer 28

How much light is perceived from a certain visual field

Question 29

The visual field can be split into several receptive field. What is meant by this?

Answer 29

Each part of the visual field can be split into very small receptive fields. This means that each part of the visual field that you see, can be split into small pixels that only one cell can process.

Question 30

Describe the receptive field.

Answer 30

A receptive field is part of the visual field. This part of the visual field is monitored by one cell (one cone/one rod).

• A receptive field can be split into two parts → the center and surround. This means that different cells with the same receptive field will respond differently to the center and surround.

Question 31

What are on-center ganglion cells and off-center ganglion cells?

Answer 31

• On-center ganglion cells fire action potential when there’s light in the center of a receptive field.
• Off-center ganglion cells fire action potentials when there’s light off center, but when the light is at the center these cells will turn off.

In this way contrast is generated.

Question 32

Not (clearly) discussed in the lecture, but to make things clearer:

In a previous question, we’ve learned that the presence of light results in hyperpolarization of rods and cones. How does this lead to the firing of an action potential in on- and off-center ganglion cells?

Answer 32

For this it’s important to know that rods and cones connect to bipolar cells and that bipolar cells are connected to ganglion cells. The neurotransmitter that is important in this pathway is glutamate. Here, it’s important to remember that the concentration of glutamate is high when cells are depolarized and vice versa when cells are hyperpolarized. The following happens when light is present:

• The concentration of glutamate is in general low.
• Two types of bipolar cells respond to this low concentration of glutamate. The first type of bipolar cells responds to the low concentration of glutamate by closing their channels (OFF bipolar cells). Thus, also these bipolar cells stay hyperpolarized. The other type of bipolar cells responds to the low concentration of glutamate by opening their channels (ON bipolar cells)
• As a consequence of two different types of bipolar cells, there are also two types of ganglion cells. One responds to light with depolarization (ON-center ganglion cells) and one with hyperpolarization (OFF-center ganglion cells).

Question 33

Explain what happens to on- and off-center ganglion cells in the situation where there’s only a light spot in the center.

Answer 33

• On-center ganglion cells depolarize and fire action potentials
• Off-center ganglion cells do not fire

Question 34

Explain what happens to on- and off-center ganglion cells in the situation where there’s only a dark spot in the center.

Answer 34

• On-center ganglion cells do not fire
• Off-center ganglion cells depolarize and fire action potentials

Question 35

Explain what happens to on- and off-center ganglion cells in the situation where there’s light on both the center and surround.

Answer 35

• On-center ganglion cells → will fire less, since it’s less sensitive to surround.
• Off-center ganglion cells → will fire more, since it’s more sensitive to surround.

Question 36

To summarize:

What cells constitute retinal circuitry?

Answer 36

Rods and cones, bipolar cells, ganglion cell, amacrine cells and horizontal cells

Question 37

To summarize:

What happens to the cells of the retinal circuitry when the light falls on the center of their receptive field?

Answer 37

Center cone hyperpolarizes, bipolar on-center cell depolarizes, bipolar off-center cell hyperpolarizes, on-center ganglion cell fires action potentials, off-center ganglion cells shut down

Question 38

To summarize:

What happens when the light falls on the receptive field’s surround?

Answer 38

Center cone depolarizes, bipolar on-center cell hyperpolarizes, bipolar off-center cell depolarizes, on-center ganglion cell shuts down, off-center ganglion cells fires action potentials

Question 39

See if you understand this picture, if you do you also understand the previous questions.

(This picture depicts the response of on-ganglion cells to light in the center and surround).

Answer 39

Ok.

Question 40

What cells amplify this response in the previous question?

Answer 40

Horizontal cells that connect many cones with each other. They help increase the luminance contrast by secreting GABA onto other neurons and thus by inhibiting other neurons.

When light increases, horizontal cells become more hyperpolarized.

Question 41

What happens when light is only on the center of the receptive field (include horizontal cells)?

Answer 41

• Cones become hyperpolarized.
• Horizontal cells also become hyperpolarized (minimally) and maintain the hyperpolarization of cones by the secretion of GABA.
• On-center bipolar cells depolarize and increase in action potential firing.

Question 42

What happens when light is on the center of the receptive field and in the surround (include horizontal cells)

Answer 42

• Center cone is at first hyperpolarized.
• Horizontal cells become more hyperpolarized → results in less release of GABA.
• Center cones become less hyperpolarized and at last become depolarized.