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Visual Science > Retinal imaging processing & receptive fields > Flashcards

Retinal imaging processing & receptive fields Flashcards

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1
Q

list the retinal organisation in a vertical pathway (connections which run vertically across the retinal layers) series (step by step) processing

A
  • photoreceptors
  • bipolar cells
    &
  • ganglion cells
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2
Q

what does light travel through before it strikes the outer segment of the photoreceptor

A

all the layers of the retina

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3
Q

where is the visual pigment rhodopsin located

A

outer segment of the photoreceptors

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4
Q

when light activates the rhodopsin, what does it activate

A

the photo transduction cascade

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5
Q

which direction does the processing of the visual information go

A

the opposite direction:

  • photoreceptors to the
  • bipolar cells to the
  • ganglion cells
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6
Q

so when light passes in one direction, it processes…

A

in the other direction

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7
Q

what are the connections between

A

the synapses of the photoreceptors found in the outer plexiform layer

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8
Q

what are the connections with

A
  • dendrites that belong to bipolar cells whose cell bodies are in the inner nuclear layer
  • and from axons and synapses of bipolar cells which are found in the inner plexiform layer
  • and the dendrites of the ganglion cells in the inner most part of the retina
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9
Q

list the retinal organisation in the lateral pathways

A
  • horizontal cells

- amacrine cells

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10
Q

what is lateral inhibition important in

A

constructing receptive fields of bipolar cells in the inner nuclear layer

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11
Q

what type of dendrites do horizontal cells have

A

wide

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12
Q

what do horizontal cells do

A

modify signalling between photoreceptors and bipolar cells

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13
Q

where are amacrine cell bodies found

A

inner nuclear layer

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14
Q

what do amacrine cells do

A

modify signalling between bipolar cells and ganglion cells

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15
Q

what do all retinal neurons have

A

a receptive field

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16
Q

what do neurons in the retina only respond to

A
  1. a specific light stimulus featured in…

2. a spatially restricted region of visual field

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17
Q

what are photoreceptor RFs determined by

A
  • light sensitivity of their visual pigment (the rhodopsin that they have in their outer segment)
  • and position of their outer segment in the retina (where it looks)
  • which determines what wavelength of light gets them going
  • and their position in the retina determines the spatial location of their receptive fields
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18
Q

what are all RFs of all other retinal neurons (bipolar and ganglion cells) determined by

A
  • synaptic inputs and connectivity received from photoreceptors (i.e. bipolar cells received from photoreceptors and ganglion cells from bipolar cells)
  • and or other retinal cells in the vertical or lateral pathways (e.g. horizontal cells)
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19
Q

how much region of space is there in a photoreceptor where light can cause a membrane potential

A

<0.01 degrees (tiny)

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20
Q

what shape and appearance are photoreceptor receptive fields

A

circular shape

uniform

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21
Q

what are the photoreceptor RFs interested in

A
  • luminance (only interested in level of brightness which is the amount of light picked up by the visual pigment)
    &
  • wavelength dependent responses
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22
Q

what do the photoreceptor RFs respond by

A

graded changes in membrane potential

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23
Q

describe the RFs of bipolar & ganglion cells

A
  • larger than photoreceptor RFs
  • circular
  • non-uniform (increase in complexity)
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24
Q

what pattern of synaptic inputs do bipolar and ganglion cell RFs get and from where

A

convergence and spatial summation of synaptic inputs from several photoreceptors

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25
Q

what is the term used for as ganglion and bipolar cell RF increases complexity

A

concentric

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26
Q

what is it meant by the ganglion and bipolar cell RFs being antagonistic

A

centre and surround regions

which is a two part RF which photoreceptors don’t have

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27
Q

describe what happens with antagonistic ganglion and bipolar RFs

A

where light is shone on each region (centre & surround) depends on the response on the response of ganglion and bipolar cells

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28
Q

what are contrast detectors in bipolar and ganglion RFs

A

luminance change/difference in luminance between centre and surround

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29
Q

what type of response are contrast detectors in bipolar and ganglion RFs

A

dependent response

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30
Q

which cell fires action potentials and why

A

ganglion cells

the only cell neurons which fire action potentials as they have axons which signal to the brain

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31
Q

how are contrast dependent RFs constructed

A
  • the centres of RFs from photoreceptors input to bipolar cells
  • surrounds from photoreceptor- horizontal cell (lateral) interactions (inhibition) then are relayed to bipolar cells
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32
Q

what are contrast dependent RFs functional significance

A
  • for retinal ganglion cell signalling to the brain

- different types of contrast sensitivity in midget/parvo compared to parasol/magno ganglion cell types

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33
Q

what is the cellular basis of concentric centre-surround RFs of bipolar cells centre

A

has direct photoreceptor inputs which determines the connections it receives in the vertical pathway directly from photoreceptors to the dendrites of the bipolar cell

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34
Q

what is the cellular basis of concentric centre-surround RFs of bipolar cells surround

A

is from the photoreceptor-horizontal cell interactions (lateral pathway)
it is determined by input from the surrounding photoreceptors with horizontal cells then back to the central photoreceptors and then back to the bipolar cell

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35
Q

what does the location of bipolar cell RF depend on

A

where the surround and centre photoreceptors above the bipolar cell are looking

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36
Q

what do photoreceptors do in the dark

A

depolarise (cell is excited)

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37
Q

what do photoreceptors do in response to light

A

hyperpolarise (cell is unexcited)

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38
Q

what is intracellular recording

A

when a micro electrode is inserted into the outer segment of the photoreceptor and a response of the change in membrane potential due to light stimulus is recorded

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39
Q

what type of pattern is shown in intracellular electrophysiological recordings in relation to hyper-polarisation with increasing light intensity (brightness)

A

gradual

but non linear increase

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40
Q

what are the two major +ve charged ion channels

A

sodium
&
potassium

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41
Q

what does the sodium channel allow

A

sodium to enter the cell

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42
Q

what does the potassium channel allow

A

potassium to leave the cell

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43
Q

once the potassium channel allows potassium to leave the cell, where do these potassium ions go

A

into extracellular space

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44
Q

in the dark, what occurs with the ions through the channels

A

sodium influx
&
potassium efflux
through open channels in the outer segment

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45
Q

in the dark, is the inside of the photoreceptor cell positively or negatively charged

A

not very negatively charged

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46
Q

what is the condition of the photoreceptor cell when the inside is not very negatively charged in dark conditions

A

depolarise/excited

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47
Q

what flows in and what flows out when both channels are open when its dark

A

sodium flows in
&
potassium flows out

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48
Q

what is the membrane potential when sodium flows in and potassium flows out when both channels are open when it is dark

A

0 membrane potential so the cell is slightly depolarised

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49
Q

in the light, what is the state of the ion channels

A

sodium channel in outer segment closes
but
potassium channels remain open

50
Q

as a result of sodium channels in the outer segment closing and potassium channels remaining open, when it is light, what happens with the ions

A

potassium efflux continues via the open channels

51
Q

what is the condition of the photoreceptor cell as a result of potassium efflux continuing via the open channels in light conditions

A

the inside of the photoreceptor cells becomes more negatively charged than the outside, hence the cell becomes, hyper polarised/inhibited

52
Q

what happens to an on bipolar cell in response to cone input

A

depolarises

53
Q

what type of synapse occurs when an on bipolar cell depolarises in response to cone input

A

sign inverting synapse

54
Q

what happens to an off bipolar cell in response to cone input

A

hyperpolarises

55
Q

what type of synapse occurs when an off bipolar cell hyper polarises in response to cone input

A

sign conserving synapse

56
Q

as the cone input is the same for both bipolar cells, what is the difference

A

the response to the outer segment of the cone photoreceptor

57
Q

what happens to the cell to an off response

A

cell in depolarised
excited by light off/darkness in its RF
just like photoreceptors

58
Q

what happens to the cell to an on response

A

cell is depolarised
excited by light on/brightness in its RF
opposite to photoreceptors

59
Q

why do different bipolar cells respond in opposite ways to the input from the same cone

A

neurotransmiiter release

60
Q

when do neurons increase neurotransmitter release

A

when they are depolarised/excited

61
Q

when do neurons stop releasing neurotransmitter

A

when they are hyper polarised/inhibited

62
Q

what does the effect of neurotransmitter release depend on

A

the type of receptor it activates in the post-synaptic cell/type of neurotransmitter found in the membrane of the post synaptic cell

63
Q

what do off an on bipolar cells have different in their dendrites

A

receptors in their dendrites to the transmitter released at cone synapses

64
Q

what is the name of the amino acid neurotransmitter of off and on bipolar cells

A

glutamate

65
Q

off and on bipolar cells are not just functionally different but also…

A

anatomically different

66
Q

name the two morphological cone bipolar cell types

A
  1. sign-reversing (ON) synapses

2, sign-conserving (OFF) synapses

67
Q

what type of dendrites do sign reversing ON cone bipolar cells synapse onto

A

long invaginating dendrites

68
Q

what type of dendrites do sign conserving OFF cone bipolar cells synapse onto

A

short, non-invaginating dendrites

69
Q

explain why the sign reversing ON synapses of cone bipolar cells, produces an opposite response which happens to a cone

A

in the light the dendrites stop releasing neurotransmitter which no longer binds to the APB receptor so sodium slows in the open membrane and the cell becomes depolarised

70
Q

what do the dendrites go into on sign reversing ON synapses of cone bipolar cells

A

the groove of the pedicle called invagination synapses of ON bipolar cells

71
Q

what does the APB neurotransmitter not have in them

A

no pores in them to allow ions to come in and out

72
Q

if the APB neurotransmitter has no pores in them, how do they ions manage to come in and out

A

the ions are coupled to a G-protein

73
Q

what do the G-proteins influence

A

sodium channels nearby

74
Q

what happens when glutamate binds to the APB receptors

A

it closes sodium channels

75
Q

what happens when the glutamate binds to the APB receptors and closes the sodium channels

A

sodium doesn’t flow into the dendrites and the cell becomes negative inside - hyper polarises

76
Q

explain why the sign conserving OFF synapses of cone bipolar cells produces the same response which happens to a cone

A

in the light, the cell will stop releasing glutamate as it is hyper polarised.
so no neurotransmitters are binding to the AMPA receptor
so sodium channels in the dendrites of the bipolar cell is closed
so no sodium enters into the cell therefore the inside is negative/hyperpolarised

77
Q

what happens in the dark to cone and off bipolar cell

A

both hyperpolarise

78
Q

what does the AMPA receptor open

A

the sodium channel in the receptor

79
Q

when does the AMPA receptor open the sodium channel in the receptor

A

when the neurotransmitter binds to it

80
Q

what occurs when the neurotransmitter binds to the AMPA receptor and opens the sodium channels

A

sodium flows into the dendrites of the OFF bipolar cell and sodium influx makes the inside less negative/depolarised

81
Q

what does the glutamate bind to of the OFF bipolar cell

A

the dendrites

called non-invagitnating synapses of the off bipolar cells

82
Q

what causes a morphological as well as functional difference between sign reversing ON and sign conserving OFF bipolar cells

A

the dendrites occupy different locations of the cone pedicle

83
Q

horizontal cells have reciprocal…

A

synaptic mechanisms
&
bipolar cell RF surrounds

84
Q

what type of connections do cones make with a single horizontal cell

A

excitatory, glutamatergic connections

85
Q

how do cones make excitatory, glutamatergic connections with the same single horizontal cell

A

in the outer plexiform layer with the extensive horizontally orientated dendrites

86
Q

how do horizontal cells make connections in order to help with surround properties of bipolar cells

A

makes synaptic contacts back to the cone pedicles, to back and forth between cone pedicle and horizontal cell
i.e. the same horizontal cell makes inhibitory (reciprocal) connections with the same cones supplying its excitatory input

87
Q

what is the outcome of the reciprocal processing of horizontal cell making inhibitory connections with the same cones supplying its excitatory input

A

outcome if relayed to the cones involved in the bipolar cell RF centre

88
Q

what does light ON in the receptive field of surrounding cones i.e. darkness in the centre region cause

A
  • hyperpolarizes = inhibits the activity of horizontal cells, so horizontal cells stop receiving the neurotransmitter glutamate so it hyperpolarizes as the synapse is sign conserving = excited/depolarised
  • to which these cones are directly connected to the outer plexiform layer via conventional sign conserving synapse
  • horizontal cells release an inhibitory neurotransmitter on the cones in the dark called GABA, but this now stops releasing inhibitory neurotransmitter so - + - = + which causes the centre to be more depolarised as if its even more darker, so the bipolar cell OFF is more depolarised
  • i.e. this disinhibition results in depolarisation of the central cones as if it just got dark in the centre so that the:
    OFF bipolar cells are now excited (their surround response is ON)
    ON bipolar cells are now inhibited (their surround response is OFF)
    thus both types of bipolar cells become contrast detectors
89
Q

what are bipolar cells mediated by

A

its surround by the input from surrounding photoreceptors with the horizontal cell which conveys the outside of that processing to the central photoreceptors which then convey that information to bipolar cell

90
Q

what do retinal ganglion cells receive convergent inputs from

A

several cone bipolar cells of the same morphological/functional types

91
Q

what do retinal ganglion cells receive convergent inputs from several cone bipolar cells of the same morphological/functional types via

A

excitatory sign conserving glutamate-AMPA synapses on their dendrites in the inner plexiform layer

92
Q

the two types of bipolar cells make connections with two types of ganglion cells creating…

A
  • ON-centre/OFF surround bipolar cells generate ON-centre/OFF surround ganglion cells, via synapses in the inner zone of the IPL
  • OFF-centre/ON surround bipolar cells generate OFF_centre/ON-surround ganglion cells, via synapses in the outer zone of the IPL
  • so the OFF-centre/ON surround bipolar cells have shorter dendrites and axons than the ON-centre/OFF surround bipolar cells
93
Q

what are contrast borders described as

A

differences in illuminances between adjacent objects

94
Q

what is the generation between detection of bipolar cells and ganglion cells is important for

A

detecting contrast borders

95
Q

when do ON-centre retinal ganglion cells fire/respond maximally

A

when there is a higher luminance at the centre of their RF and less light in the surround = best case scenario

96
Q

when do ON-centre retinal ganglion cells fire least

A

when theres a higher luminance in their RF surround and less light in the centre = the worst case scenario

97
Q

when do ON-centre retinal ganglion cells fire at a intermediate rate

A

when luminance levels are identical (wither low or high) in both their RF centre and surround

98
Q

what is the ON-centre retinal ganglion cell intermediate rate fire when luminance levels are identical in both RF centre surround due to

A

mutual antagonism between their ON & OFF zones
(diffuse illumination, no contrast, which the cell will not like as it wants contrast and not diffuse, so it will only respond a little bit)

99
Q

when do OFF/centre ON-surround retinal ganglion cells respond maximally

A

when theres a higher luminance in their RF surround and less light in the centre = best case scenario

100
Q

when do OFF/centre ON-surround retinal ganglion cells fire least

A

when there is a higher luminance at the centre of their RF and less light in the surround = worse case scenario

101
Q

when do OFF/centre ON-surround retinal ganglion cells fire at a intermediate rate

A

when luminance levels are identical (wither low or high) in both their RF centre and surround

102
Q

what is the major advantage of ganglion cell contrast enhancement

A

perceptual constancy

103
Q

describe how the major advantage of ganglion cell contrast enhancement is perceptual constancy

A

our visual system always tries to extract information about the physical realities of the world irrespective of how bright things are or what angle we’re viewing them from
eg a text which appears black on white can be read with equal case under all lighting conditions. this perceptual constancy is a product of post-receptoral retinal processing. it occurs despite the fact that white page reflects less light in a dimly lit room than the black print does in bright sunlight

104
Q

what really matters for visual perception

A

not the absolute light intensities entering the eye and that are detected by our photoreceptors under these two environmental conditions, but that the relative contrast between the page and print remains invariant

105
Q

what are seen by photoreceptors at twilight

A

not much light is shone on to space so not much is shone of it

106
Q

what are photoreceptors not

A

contrast detectors

107
Q

what is the minor disadvantage of contrast enhancement

A

is perceptual illusion as the visual system is fooled by light verses dark surround i.e. the illusion created by contrast detectors in the retina

108
Q

how many types of image contrast are detected by the 2 major retinal ganglion cell sub classes

A

three

109
Q

list the three types of image contrast detected by the two major retinal ganglion cell sub classes

A
  1. brightness/luminance = form
  2. chromatic/wavelengths = colour
  3. temporal = motion
110
Q

what type of response is the brightness/luminance (form) of midget (parvocellular) ganglion cells

A

sustained responses to stationary contrast

111
Q

what type of response is the brightness/luminance (form) of parasol (magnocellular) ganglion cells

A

transient responses to stationary contrast

112
Q

what are the most common cells of brightness/luminance (form) image contrast

A

midget cells ~80%

113
Q

what are the least common cells of brightness/luminance (form) image contrast

A

mango cells ~10%

114
Q

what do both midget and magno ganglion cells have in relation to brightness/luminance (form) image contrast detection

A

both types of contrast detectors i.e.
ON and OFF midget cells and OFF and ON midget cells
ON and OFF magno cells and OFF and ON magno cells
so both with different polarities are engaged in luminance and contrast detection which is important for recognising forms i.e. squares and rectangles etc

115
Q

what do chromatic/wavelength (colour) image contrast produce with midget and parasol ganglion cells

A

sustained responses to red/green colour contrasts
also midget/parvocellular ganglion cells and parasol/magno cells are not interested in colour, but is interested in transient changing contrast or moving stimuli to the presence of contrast to receptive fields

116
Q

describe temporal (motion) image contrast detected by midget and parasol ganglion cells

A

transient (changing contrast) responses to light change/flicker = parasol/magno ganglion cells

117
Q

name a way to measure colour contrast with OFF and ON ganglion cells

A

antagonistic wavelength-selective responses at chromatic contrast borders

118
Q

what will be the best case scenario of colour contrast

A

Red ON in centre, very light green in surround

119
Q

what will produce a weak response/no contrast (diffuse illumination on centre and surround)

A

Red equal luminance in centre & surround

120
Q

what will be the worse case scenario (mutual antagonism) which makes the cell inhibited

A

Red ON in centre & Green ON in surround

121
Q

explain why in terms of temporal contrast, why midget ganglion cells produce sustained (to presence of contrast to receptive fields) response

A
  • off surround, on centre
  • still responding (firing) to the first spot, so subsequent stimuli were invisible to this cell
  • so it won’t respond to another stimulus whilst still dealing with the previous one
122
Q

explain why in terms of temporal contrast, why parasol ganglion cells produce transient responses to each stimulus (brief)

A
  • off surround, on centre
  • brief burst of firing to each stimulus, so able to follow temporal frequency (flicker) on after the other
  • Good at detecting motion

Visual Science (14 decks)

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