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C82COG - Perception > Retinal Processing > Flashcards

Retinal Processing Flashcards

1
Q

What is electromagnetic energy?

A

Electrically charged particles

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2
Q

What is light?

A

A wave/stream of photons, tiny particles that each consist of one quantum of energy.

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3
Q

Light can behave as both a wave and a particle. When is it best to use either approach?

A

Can consider it to be a wave when it is moving, or as photons when it’s captured.

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4
Q

What different things can happen to light?

A

It can be:

  • diffracted
  • absorbed
  • transmitted
  • reflected
  • refracted
  • scattered
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5
Q

What are degrees of visual angle?

A

A measure of object size determined by the angle between the lines of where the extremes of the object pass through the nodal point of the eye (no direction change here).

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6
Q

Where is the nodal point of the eye?

A

About 7mm from the corneal vertex.

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7
Q

What units is visual angle measured in?

A

Degrees, minutes of arc, and seconds of arc.

1 degree = 60 minutes, 1 minute = 60 seconds.

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8
Q

What types of nerves does the retina contain?

A

Photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells.

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9
Q

What parts of the eye focus the image on the retina?

A

The cornea and crystalline lens.

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10
Q

In what way does the retina act like the film in a camera?

A

Every point in the retina has a corresponding point in visual space from which it receives light – ‘visual field’.

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11
Q

How many degrees are there between the fovea centralis and the ora serrate?

A

Approx. 100 degrees.

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12
Q

Where is our blind spot?

A

The left, 7 deg V; 5 deg H.

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13
Q

How many distinct layers are there in the retina?

A

8.

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14
Q

What are the layers of the retina?

A

Choroid/layer of pigment cells, outer and inner segment, outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell, and nerve fibres.

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15
Q

What do the segment layers of the retina contain?

A

Photoreceptors.

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16
Q

What does the choroid layer/layer of pigment cells in the retina do?

A

Granules control the spread of light from one cell to another – in bright light they migrate into the cell processes between rods and cones, and in dim light they are confined to the cell body.

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17
Q

What is the fovea centralis?

A

The central fixation – the retina is thinner, forming the foveal pit, where the nerve fibres are displaced and there are lots of cones.

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18
Q

How large is the human fovea?

A

Small – 1mm2 or 2 degrees across.

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19
Q

How many cones does the fovea contain?

A

About 150 000.

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20
Q

How many connections are there between the fovea and the brain?

A

A lot – constitutes 1/7 of all connections between the retina and brain.

21
Q

What is the leading cause of blindness?

A

Age-related foveal degeneration.

22
Q

Describe the concentration of rods and cones in the retina.

A

Cones are concentrated at the fovea, and rods are absent from the fovea – reach a maximum density just nasal to the optic nerve head.

23
Q

What does the duplex theory of retinal processing refer to?

A

The use of rods and cones to enable the eye to deal with massive variations in the environmental range of luminance.

24
Q

How does the visual system operate in all likely light conditions?

A
  1. Pupil constriction – 8mm to 1mm, 64 fold change

2. Using the cone-driven photopic system or the rod-driven scotopic system

25
Q

How do rods work?

A
  • The outer segment contains flat discs containing the photopigment rhodopsin
  • The inside of the rod is electrically negative compared to surrounding fluid
  • When light strikes rhodopsin, ionic channels are blocked and the cell hyperpolarises (becomes more negative)
  • This electrical potential is then carried to later synapses with bipolar and horizontal cells.
26
Q

What is the difference between how rods and cones work?

A

Cones’ discs are continuous with the outer segment membrane, and the photopigment in cones is called iodopsin.

27
Q

What are bipolar cells?

A

The cells that form the inner nuclear layer and connect to the photoreceptors – they transfer information.

28
Q

What are the three basic types of bipolar cells?

A
  1. Rod bipolars – connect many rods to 1-4 ganglion cells.
  2. Flat bipolars – connect many cones to many ganglion cells.
  3. Midget bipolars – connect a single cone to a single ganglion cell, found only in the fovea.
29
Q

What do horizontal cells do?

A

Collect signals together from a number of different photoreceptors. Nt blocks hyperpolarisation of neighbouring bipolar cells = lateral inhibition.

30
Q

What do amacrine cells do?

A

Similar to what the horizontal cells do for photoreceptors, but for the ganglion cell layer – they transmit signals from one ganglion cell to another a short distance away.

31
Q

How many photoreceptors does the human eye contain, and why is this a problem for ganglion cells?

A

127 million, because there are only 1.25 million ganglion cells – they must collate and summarise all the information while retaining the essential features of the image.

32
Q

What can single cell recording in ganglion cells be used to study?

A

How ganglion cells collate and summarise information, by measuring the cell’s firing rate to certain stimuli.

33
Q

How is single cell recording in ganglion cells carried out?

A
  • A tiny microelectrode is implanted next to the axon of a ganglion cell, where it records electrical changes.
  • The eye is fixed in position through paralysis of the extra ocular muscles.
  • The position of light on a screen is moved to find the receptive field of the ganglion cell.
  • The cell either shows an on or an off response to the light.
34
Q

What is Centre-Surround Antagonism?

A

The arrangement in ganglion cells whereby ‘on’ (+) responses can be elicited from a restricted circular retinal region, and ‘off’ (-) responses from a surrounding ring.

35
Q

What purpose does Centre-Surround Antagonism serve?

A

It helps the retina find edges in images – objects are usually distinguished from their background by sudden changes in reflected light.

36
Q

How does Centre-Surround Antagonism in ganglion cells find objects’ edges?

A

When a uniform field is used, the excitatory response of the centre is cancelled out by lateral inhibition. However if an edge or line is positioned appropriately within the receptive field, the cell produces a net response regardless of orientation.

37
Q

How do ganglion receptive field sizes vary?

A

Systematically with retinal location:

  • smallest at the fovea (0.01mm) and have a low neural convergence factor - they provide high spatial resolution.
  • 10mm from fovea receptive field sizes show a 50 fold increase. They collect information from much larger areas of the retina (high neural convergence factor) - they provide poor spatial resolution but good light sensitivity.
38
Q

What are the ON and OFF systems?

A

Ganglion cell receptive fields with an excitatory centre and inhibitory surround are the ON system. The complementary array of ganglion cells with an inhibitory centre and excitatory surround are the OFF system.
They have about the same number of cells and cover the same retinal areas.

39
Q

What do the ON and OFF systems allow the visual system to do?

A

Detect incremental and decremental changes in light.

40
Q

Aside from ON and OFF, what other types of ganglion cells are there?

A

Magnocellular (M), parvocellular (P), and koniocellular (K).

41
Q

What proportion of ganglion cells are M, P and K cells?

A

10% M, 80% P, 10% K.

42
Q

Describe M cells in terms of receptive field size, conduction velocity, best objects, retinal position, colour sensitivity, and temporal response.

A 
Field size: large	
Velocity: fast
Objects: large, low contrast
Position: peripheral
Colour: no
Temporal: transient
43
Q

Describe P cells in terms of receptive field size, conduction velocity, best objects, retinal position, colour sensitivity, and temporal response.

A 
Field size: small
Velocity: slow
Objects: small, high contrast
Position: central
Colour: yes
Temporal: sustained
44
Q

Describe K cells in terms of receptive field size, conduction velocity, best objects, retinal position, colour sensitivity, and temporal response.

A 
Field size: medium
Velocity: slow
Objects: large, low contrast
Position: peripheral
Colour: yes
Temporal: sustained
45
Q

What information do M cells extract from an image?

A

Where the object is and how fast it moves.

46
Q

What information do P cells extract from an image?

A

What the object is and what colour it is.

47
Q

What information do K cells extract from an image?

A

How bright the object is and where it is.

48
Q

On what three factors can ganglion cells be subdivided?

A

Size, on/off, M/P/K.

49
Q

What are the consequences of Centre-Surround Antagonism in human vision?

A
  1. Illusory spots on Hermann grid due to less response for ganglion cells between squares causing dimming.
  2. Simultaneous contrast – different ends of a grey line on a graduated background appear to be different colours.

C82COG - Perception (9 decks)

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