The Eye and the Retina

Question 1

What is the direct pathway through the retina?

Answer 1

Photoreceptor → Bipolar Cell → Ganglion Cell

Question 2

What are the first neurones in the retina that respond to visual stimuli with action potentials?

Answer 2

Ganglion cells

Question 3

What does light focus onto in the retina?

Answer 3

The outer segments of the photoreceptors

Question 4

How do retinal ganglion cells receive input?

Answer 4

Retinal ganglion cell in central vision gets direct input from a single central cone photoreceptor via bipolar cells. This single cone is its receptive field centre.

Also gets indirect input from surrounding cones via horizontal cells (interneuron). Horizontal cells gather input from surrounding cones and feed it back into bipolar cell and then ganglion cell.

Question 5

What are interneurones?

Answer 5

Interneurones (e.g. horizontal cells and amacrine cells) in the middle extract detail from the photoreceptor signal

Question 6

Describe the Distribution of rods and cones in the retina.

Answer 6

Central retina is cone-dominated
-cones are closely spaced and rods fewer in number between the cones

Peripheral retina is rod-dominated

• 10x as many rods as cones
• cones are separated by pools of rods, therefore there are big gaps in the sampling array

Question 7

What is the ganglion’s receptive field centre?

Answer 7

The part of the visual world focused on a particular photoreceptor in the retina directly linked to the ganglion cell

Question 8

What is the relationship between receptive field centre and detail?

Answer 8

The bigger the ganglion receptive field centre, the less fine detail you see

Question 9

Why can’t the peripheral retina process fine detail?

Answer 9

Because the light has to pass through multiple structures such as dense capillary beds, nuclei and organelles of cells. So, the light is scattered before it hits the outer segment of the photoreceptor, and focus is never good (image blurs as light passes through retinal tissue)

Question 10

Describe the Response of a single cone as illumination increases/decreases

Answer 10

Photoreceptors report changes in illumination from one moment to another. You get a hyperpolarisation when illumination increases (brighter), and a depolarisation when illumination decreases (darker)

Question 11

What happens if the brightness remains the same for a prolonged period of time over a cone photoreceptor?

Answer 11

ADAPTATION

-photoreceptor adapts and resets itself back to its resting potential (-45mV)

Now the photoreceptor is ready to respond to another change in brightness. This can happen over an enormous range of absolute illuminations.

Question 12

What is the benefit of cone adaptation?

Answer 12

Adaptation allows us to respond sensitively to tiny changes in brightness very quickly without saturation because the cones constantly reset their membrane potentials

Question 13

Given that all photoreceptors are depolarised by decreases in illumination, how do retinal ganglion cells respond to increases in illumination?

Answer 13

Half of all retinal ganglion cells respond to decreases in illumination, and half respond to increases in illumination.

The RGCs which respond to increases in illumination are called “on-centre” RGCs. This is because the “on-centre” RGCs have an inverting synapse in the pathway. The central photoreceptor hyperpolarises to an increase in illumination, but the inverting synapse depolarises the bipolar cell, which excites the ganglion cell.

Therefore, the retina sends information about increases and decreases in illumination equally, even though the photoreceptors respond only in one direction

Question 14

What are receptive fields of bipolar and ganglion cells like?

Answer 14

They have an antagonistic centre-surround system, depending on on-centre cells or off-centre cells

Question 15

What are Off-centre cells?

Answer 15

Bipolar and ganglion cells in the retina, whose receptive field centres are inhibited
by stimulation (illumination)

Question 16

What are On-centre cells?

Answer 16

Bipolar and ganglion cells in the retina, whose receptive field centres are activated by stimulation (illumination)

Question 17

What is the Off-centre response to decrease in illumination?

Answer 17

Central photoreceptor depolarises

• releases glutamate (excitatory)
• glutamate binds ionotropic receptor on bipolar cell and depolarises it
• bipolar cell will release glutamate (excitatory)
• glutamate binds ionotropic receptor on the off-centre ganglion cell and depolarises it
• burst of action potentials in RGC

Question 18

What is the Off-centre response to increase in illumination (stimulation)?

Answer 18

Central photoreceptor hyperpolarises

• glutamate not released
• bipolar cells is hyperpolarised too
• action potentials don’t fire in RGC

*when light goes back to dark (decrease in illumination), RGC fires burst of action potentials again

Question 19

What is the problem in only having off-centre cells?

Answer 19

There would be a huge asymmetry in the ability to see increases and decreases in illumination.

You would have higher accuracy in identifying decreases in illumination than increases in illumination, because off-centre RGCs fire APs in response to decreases in illumination

Question 20

Why do we need on-centre receptive fields?

Answer 20

in order to have an equal ability to see brightening and darkening

Question 21

What is the On-centre response to an increase in illumination (stimulation)?

Answer 21

Central photoreceptor hyperpolarises

• BUT, bipolar cell depolarises
• ganglion cell depolarises and fires action potentials

Question 22

How does the on-centre bipolar cell depolarise in response to an increase in illumination even though the central photoreceptor hyperpolarise?

Answer 22

Glutamate is excitatory everywhere except in on-centre bipolar cells, because on-centre bipolar cells carry a unique inhibitory metabotropic receptor mGluR6 (not ionotropic)

This receptor acts via a G-protein to close Na+ channels

Therefore, even though the central photoreceptor hyperpolarises in response to an increase in illumination and glutamate is not released, the absence of glutamate means the inhibitory metabotropic receptors on the bipolar cells are not activated, and therefore Na+ channels remain open, causing for the on-centre bipolar cells to depolarise and subsequently produce action potentials in the on-centre RGC

Question 23

What are Ganglion cell receptive field surrounds?

Answer 23

RGC in central vision gets direct input from a single photoreceptor. This single cone is its receptive field centre.

But, this RGC also receives indirect input from inhibitory horizontal cells which are picking up information from the whole pool of cones in the inhibitory receptive field surrounding that one single central cone
-RGC surrounds are inhibitory, or more accurately described as antagonistic

Question 24

Explain what Centre-surround antagonism is.

Answer 24

A common arrangement found in receptive fields where the neural response to light placed in the centre is opposite to that in the surround.

An on-centre cell has an off-surround
-stimulated by increases in illumination

An off-centre cell has an on-surround
-stimulated by decreases in illumination

Question 25

What happens if you increase illumination over the central photoreceptor in an on-centre, off-surround cell?

Answer 25

Increasing illumination over central cone will depolarise bipolar cell (inverting synapse) and ganglion cell, resulting in a burst of action potentials.

Question 26

What happens if you increase illumination over the entire receptive field in an on-centre, off-surround cell?

Answer 26

Increasing illumination over central and surrounding cones, there is little or no response in the ganglion cell firing because the surround inhibits the central response

Question 27

What happens if you decrease illumination over the central photoreceptor in an off-centre, on-surround cell?

Answer 27

Decreasing illumination only over central cone will depolarise the bipolar cell and ganglion cell, resulting in a burst of action potentials.

Question 28

What happens if you decrease illumination over the entire receptive field in an off-centre, on-surround cell?

Answer 28

When you decrease illumination over central and surrounding cones

• central cone depolarising bipolar cell and RGC
• surrounding cones are exciting the inhibitory horizontal cells, and if enough surrounding inhibition produced, the action potentials from the single central cone is suppressed

Question 29

What happens if you increase illumination over the central cone, and decrease illumination over the surrounding cones in an off-centre, on-surround cell?

Answer 29

Central cone will hyperpolarise during increased illumination and no action potentials produced downstream by RGC.

Surrounding cones will depolarise and excite the inhibitory horizontal cells, and this inhibition is not opposed because the central cone is not excited, and the ganglion cell is therefore inhibited

Question 30

What happens if you decrease illumination over the central cone, and increase illumination over the surrounding cones in an off-centre, on-surround cell?

Answer 30

Central cone depolarises, depolarising bipolar cell and firing action potential in RGCs

Surrounding cones hyperpolarise, hyperpolarising the inhibitory horizontal cells, withdrawing inhibition from horizontal cell, allowing the bipolar cells to depolarise, resulting in firing of action potentials in RGCs

Question 31

What happens if illumination (brightness) in the central receptive field and the surrounding inhibitory receptive field is different?

Answer 31

The excitation and inhibition won’t cancel out and the ganglion cell will respond

Question 32

What happens if illumination (brightness) in the central receptive field and the surrounding inhibitory field is the same?

Answer 32

The excitation and inhibition will cancel out and the ganglion cell will not respond

Question 33

Describe the Response of off-centre ganglion cell.

Answer 33

Dark stimulus on the off-centre cell, you get a burst of action potentials

Bright annulus (surround), you get a burst of action potentials

*and if you use the opposite, you will get inhibition in both case

Question 34

Describe the Response of on-centre ganglion cell.

Answer 34

Light stimulus on on-centre cell, you get a burst of action potentials

Dark annulus (surround), you get a burst of action potentials

*and if you use the opposite, you will get inhibition in both cases

Question 35

In an on-centre cell, the grey surround is producing excitation. How is it, that a dark annulus, which is depolarising the surrounding cones, and therefore depolarising the horizontal cells and ought to be inhibiting, is actually exciting?

Answer 35

The grey surround produces excitation and therefore depolarises the inhibitory horizontal cell. However, the horizontal cell does not inhibit the bipolar cell, but inhibits the central cone. This prevents release of glutamate from central cone, and absence of glutamate in on-centre cell means excitation of the on-centre bipolar cells and on-centre ganglion cells

By feeding back onto the central cone, the horizontal cell ensures that the surrounding cones have the opposite effect to the central cone.

Another mechanism by which the grey surround of an on-centre cell produces excitation of the on-centre cell is:
-The grey surround produces excitation and therefore depolarises the inhibitory horizontal cell. Horizontal cells activate rods. Rods have no photopigment in bright conditions where cones are functioning, but decreased light on neighbouring cones hyperpolarises the rods via horizontal cells, and their signal feeds onto the same ganglion cells of the cones, adding to the antagonistic surround

Question 36

What do antagonistic receptive fields detect?

Answer 36

CONTRAST

-ganglion cells respond to differences in brightness (contrast), and not absolute brightness

Question 37

What process allows the antagonistic receptive fields to detect contrast?

Answer 37

Lateral inhibition

-allows retinal circuitry to detect changes in contrast from one place to the neighbouring place

Question 38

What process allows the antagonistic receptive fields to detect contrast?

Answer 38

Lateral inhibition

-allows retinal circuitry to detect changes in contrast from one place to the neighbouring place

Question 39

What are the perceptual benefits of lateral inhibition?

Answer 39

The benefits of looking at the contrast rather than the absolute brightness:

• Adaptation (whatever the ambience level of brightness, the cones will reset themselves, meaning the absolute brightness has been thrown away anyway by the responses in the cones)
• Reduced redundancy in the signal (instead of wasting energy sending a lot of action potentials to the brain to signal about the same homogenous space, the centre-surround receptive fields just report where things change/where the edges are).
• Increased dynamic range
• Facilitates “constancy” (e.g. checker shadow illusion, where 2 squares of identical brightness appear different shades)

Question 40

What is used to explore/probe visual receptive fields?

Answer 40

Sinusoidal grating stimuli

• they are powerful stimuli to use because their characteristics can be matched mathematically to features in natural scenes
• we can start to build a model of how cells would respond to a full visual scene

Question 41

In what manner can sinusoidal gratings be used to probe the visual field?

Answer 41

In a flashing or drifting manner to stimulate the receptor field

Question 42

What are the Characteristics of gratings?

Answer 42

Spatial Frequency

-how often the gratings repeat per unit of distance

Temporal Frequency

• how many cycles can pass over a particular point in a second
• the faster it goes, the more cycles will pass in a second

Orientation

Size

Contrast

Question 43

What can the majority of ganglion cells be divided into?

Answer 43

The majority of the cells in the retina can be divided into parvocellular and magnocellular types.

These cells form distinct, parallel systems. As you go out from the centre of the retina towards the periphery, sizes of both of those populations of cells get bigger and bigger but the differences remain.

Question 44

What is the Number of M cells and P cells in retina?

Answer 44

P cells constitute about 90% of the ganglion cell population, M cells constitute about 5%

Question 45

What are the Structural and visual response properties of M cells and P cells?

Answer 45

M cells:

• thick axons
• receive input from many photoreceptors via convergence
• larger receptive fields
• weak receptive field surround
• conduct action potentials more rapidly
• high contrast sensitivity and low spatial resolution, meaning they are involved in coarse vision
• fast and transient responses to rapid visual changes and motion detection
• unselective for chromatic stimuli

P cells:

• fine axons
• receive input from a small number of cones; less convergence
• smaller receptive field
• low contrast sensitivity and high spatial resolution, meaning they are involved in fine resolution
• slow and sustained responses on stimuli presentation
• involved in object identification, including colour and detailed form discrimination

Question 46

What are P cell and M cells also called?

Answer 46

Parvocellular ganglion cell (P cells) anatomists call the central ones “midget” cells.

Magnocellular ganglion cells (M cells) anatomists call the central ones “parasol” cells.

-can be confusing however when they are shortened to P cells and M cells

Question 47

What is the primary visual pathway?

Answer 47

It is the pathway for visual perception

• retina (photoreceptor-bipolar cell-ganglion cell)
• optic nerve
• optic chiasm
• optic tract
• LGN
• optic radiation
• primary visual cortex

Question 48

What is the lateral geniculate nucleus?

Answer 48

Layered structure in the thalamus which gets inputs from magnocellular and parvocellular ganglion cells

Layers 1-2:
-magnocellular input

Layers 3-6
-parvocellular input

Question 49

Why are there more relay cells for the parvocellular layers?

Answer 49

because P cells have very small receptive fields, so you need a large number of relay cells to tile the whole of the visual field

Question 50

What is the Contralateral and ipsilateral input to LGN?

Answer 50

Contralateral input to layers 1, 4 and 6

Ipsilateral input to layers 2, 3 and 5

Question 51

It is possible to selectively delete/kill components of the retinal input to the cortex at the level of the LGN. What happens if there is a loss of parvocellular/magnocellular pathway?

Answer 51

Loss of parvocellular pathway reduces acuity, colour discrimination

Loss of magnocellular pathway reduces low spatial and high temporal frequency vision (broad details and fast movement)