Retinal imaging processing & receptive fields Flashcards
list the retinal organisation in a vertical pathway (connections which run vertically across the retinal layers) series (step by step) processing
- photoreceptors
- bipolar cells
& - ganglion cells
what does light travel through before it strikes the outer segment of the photoreceptor
all the layers of the retina
where is the visual pigment rhodopsin located
outer segment of the photoreceptors
when light activates the rhodopsin, what does it activate
the photo transduction cascade
which direction does the processing of the visual information go
the opposite direction:
- photoreceptors to the
- bipolar cells to the
- ganglion cells
so when light passes in one direction, it processes…
in the other direction
what are the connections between
the synapses of the photoreceptors found in the outer plexiform layer
what are the connections with
- 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
list the retinal organisation in the lateral pathways
- horizontal cells
- amacrine cells
what is lateral inhibition important in
constructing receptive fields of bipolar cells in the inner nuclear layer
what type of dendrites do horizontal cells have
wide
what do horizontal cells do
modify signalling between photoreceptors and bipolar cells
where are amacrine cell bodies found
inner nuclear layer
what do amacrine cells do
modify signalling between bipolar cells and ganglion cells
what do all retinal neurons have
a receptive field
what do neurons in the retina only respond to
- a specific light stimulus featured in…
2. a spatially restricted region of visual field
what are photoreceptor RFs determined by
- 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
what are all RFs of all other retinal neurons (bipolar and ganglion cells) determined by
- 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)
how much region of space is there in a photoreceptor where light can cause a membrane potential
<0.01 degrees (tiny)
what shape and appearance are photoreceptor receptive fields
circular shape
uniform
what are the photoreceptor RFs interested in
- luminance (only interested in level of brightness which is the amount of light picked up by the visual pigment)
& - wavelength dependent responses
what do the photoreceptor RFs respond by
graded changes in membrane potential
describe the RFs of bipolar & ganglion cells
- larger than photoreceptor RFs
- circular
- non-uniform (increase in complexity)
what pattern of synaptic inputs do bipolar and ganglion cell RFs get and from where
convergence and spatial summation of synaptic inputs from several photoreceptors
what is the term used for as ganglion and bipolar cell RF increases complexity
concentric
what is it meant by the ganglion and bipolar cell RFs being antagonistic
centre and surround regions
which is a two part RF which photoreceptors don’t have
describe what happens with antagonistic ganglion and bipolar RFs
where light is shone on each region (centre & surround) depends on the response on the response of ganglion and bipolar cells
what are contrast detectors in bipolar and ganglion RFs
luminance change/difference in luminance between centre and surround
what type of response are contrast detectors in bipolar and ganglion RFs
dependent response
which cell fires action potentials and why
ganglion cells
the only cell neurons which fire action potentials as they have axons which signal to the brain
how are contrast dependent RFs constructed
- the centres of RFs from photoreceptors input to bipolar cells
- surrounds from photoreceptor- horizontal cell (lateral) interactions (inhibition) then are relayed to bipolar cells
what are contrast dependent RFs functional significance
- for retinal ganglion cell signalling to the brain
- different types of contrast sensitivity in midget/parvo compared to parasol/magno ganglion cell types
what is the cellular basis of concentric centre-surround RFs of bipolar cells centre
has direct photoreceptor inputs which determines the connections it receives in the vertical pathway directly from photoreceptors to the dendrites of the bipolar cell
what is the cellular basis of concentric centre-surround RFs of bipolar cells surround
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
what does the location of bipolar cell RF depend on
where the surround and centre photoreceptors above the bipolar cell are looking
what do photoreceptors do in the dark
depolarise (cell is excited)
what do photoreceptors do in response to light
hyperpolarise (cell is unexcited)
what is intracellular recording
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
what type of pattern is shown in intracellular electrophysiological recordings in relation to hyper-polarisation with increasing light intensity (brightness)
gradual
but non linear increase
what are the two major +ve charged ion channels
sodium
&
potassium
what does the sodium channel allow
sodium to enter the cell
what does the potassium channel allow
potassium to leave the cell
once the potassium channel allows potassium to leave the cell, where do these potassium ions go
into extracellular space
in the dark, what occurs with the ions through the channels
sodium influx
&
potassium efflux
through open channels in the outer segment
in the dark, is the inside of the photoreceptor cell positively or negatively charged
not very negatively charged
what is the condition of the photoreceptor cell when the inside is not very negatively charged in dark conditions
depolarise/excited
what flows in and what flows out when both channels are open when its dark
sodium flows in
&
potassium flows out
what is the membrane potential when sodium flows in and potassium flows out when both channels are open when it is dark
0 membrane potential so the cell is slightly depolarised
in the light, what is the state of the ion channels
sodium channel in outer segment closes
but
potassium channels remain open
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
potassium efflux continues via the open channels
what is the condition of the photoreceptor cell as a result of potassium efflux continuing via the open channels in light conditions
the inside of the photoreceptor cells becomes more negatively charged than the outside, hence the cell becomes, hyper polarised/inhibited
what happens to an on bipolar cell in response to cone input
depolarises
what type of synapse occurs when an on bipolar cell depolarises in response to cone input
sign inverting synapse
what happens to an off bipolar cell in response to cone input
hyperpolarises
what type of synapse occurs when an off bipolar cell hyper polarises in response to cone input
sign conserving synapse
as the cone input is the same for both bipolar cells, what is the difference
the response to the outer segment of the cone photoreceptor
what happens to the cell to an off response
cell in depolarised
excited by light off/darkness in its RF
just like photoreceptors
what happens to the cell to an on response
cell is depolarised
excited by light on/brightness in its RF
opposite to photoreceptors
why do different bipolar cells respond in opposite ways to the input from the same cone
neurotransmiiter release
when do neurons increase neurotransmitter release
when they are depolarised/excited
when do neurons stop releasing neurotransmitter
when they are hyper polarised/inhibited
what does the effect of neurotransmitter release depend on
the type of receptor it activates in the post-synaptic cell/type of neurotransmitter found in the membrane of the post synaptic cell
what do off an on bipolar cells have different in their dendrites
receptors in their dendrites to the transmitter released at cone synapses
what is the name of the amino acid neurotransmitter of off and on bipolar cells
glutamate
off and on bipolar cells are not just functionally different but also…
anatomically different
name the two morphological cone bipolar cell types
- sign-reversing (ON) synapses
2, sign-conserving (OFF) synapses
what type of dendrites do sign reversing ON cone bipolar cells synapse onto
long invaginating dendrites
what type of dendrites do sign conserving OFF cone bipolar cells synapse onto
short, non-invaginating dendrites
explain why the sign reversing ON synapses of cone bipolar cells, produces an opposite response which happens to a cone
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
what do the dendrites go into on sign reversing ON synapses of cone bipolar cells
the groove of the pedicle called invagination synapses of ON bipolar cells
what does the APB neurotransmitter not have in them
no pores in them to allow ions to come in and out
if the APB neurotransmitter has no pores in them, how do they ions manage to come in and out
the ions are coupled to a G-protein
what do the G-proteins influence
sodium channels nearby
what happens when glutamate binds to the APB receptors
it closes sodium channels
what happens when the glutamate binds to the APB receptors and closes the sodium channels
sodium doesn’t flow into the dendrites and the cell becomes negative inside - hyper polarises
explain why the sign conserving OFF synapses of cone bipolar cells produces the same response which happens to a cone
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
what happens in the dark to cone and off bipolar cell
both hyperpolarise
what does the AMPA receptor open
the sodium channel in the receptor
when does the AMPA receptor open the sodium channel in the receptor
when the neurotransmitter binds to it
what occurs when the neurotransmitter binds to the AMPA receptor and opens the sodium channels
sodium flows into the dendrites of the OFF bipolar cell and sodium influx makes the inside less negative/depolarised
what does the glutamate bind to of the OFF bipolar cell
the dendrites
called non-invagitnating synapses of the off bipolar cells
what causes a morphological as well as functional difference between sign reversing ON and sign conserving OFF bipolar cells
the dendrites occupy different locations of the cone pedicle
horizontal cells have reciprocal…
synaptic mechanisms
&
bipolar cell RF surrounds
what type of connections do cones make with a single horizontal cell
excitatory, glutamatergic connections
how do cones make excitatory, glutamatergic connections with the same single horizontal cell
in the outer plexiform layer with the extensive horizontally orientated dendrites
how do horizontal cells make connections in order to help with surround properties of bipolar cells
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
what is the outcome of the reciprocal processing of horizontal cell making inhibitory connections with the same cones supplying its excitatory input
outcome if relayed to the cones involved in the bipolar cell RF centre
what does light ON in the receptive field of surrounding cones i.e. darkness in the centre region cause
- 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
what are bipolar cells mediated by
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
what do retinal ganglion cells receive convergent inputs from
several cone bipolar cells of the same morphological/functional types
what do retinal ganglion cells receive convergent inputs from several cone bipolar cells of the same morphological/functional types via
excitatory sign conserving glutamate-AMPA synapses on their dendrites in the inner plexiform layer
the two types of bipolar cells make connections with two types of ganglion cells creating…
- 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
what are contrast borders described as
differences in illuminances between adjacent objects
what is the generation between detection of bipolar cells and ganglion cells is important for
detecting contrast borders
when do ON-centre retinal ganglion cells fire/respond maximally
when there is a higher luminance at the centre of their RF and less light in the surround = best case scenario
when do ON-centre retinal ganglion cells fire least
when theres a higher luminance in their RF surround and less light in the centre = the worst case scenario
when do ON-centre retinal ganglion cells fire at a intermediate rate
when luminance levels are identical (wither low or high) in both their RF centre and surround
what is the ON-centre retinal ganglion cell intermediate rate fire when luminance levels are identical in both RF centre surround due to
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)
when do OFF/centre ON-surround retinal ganglion cells respond maximally
when theres a higher luminance in their RF surround and less light in the centre = best case scenario
when do OFF/centre ON-surround retinal ganglion cells fire least
when there is a higher luminance at the centre of their RF and less light in the surround = worse case scenario
when do OFF/centre ON-surround retinal ganglion cells fire at a intermediate rate
when luminance levels are identical (wither low or high) in both their RF centre and surround
what is the major advantage of ganglion cell contrast enhancement
perceptual constancy
describe how the major advantage of ganglion cell contrast enhancement is perceptual constancy
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
what really matters for visual perception
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
what are seen by photoreceptors at twilight
not much light is shone on to space so not much is shone of it
what are photoreceptors not
contrast detectors
what is the minor disadvantage of contrast enhancement
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
how many types of image contrast are detected by the 2 major retinal ganglion cell sub classes
three
list the three types of image contrast detected by the two major retinal ganglion cell sub classes
- brightness/luminance = form
- chromatic/wavelengths = colour
- temporal = motion
what type of response is the brightness/luminance (form) of midget (parvocellular) ganglion cells
sustained responses to stationary contrast
what type of response is the brightness/luminance (form) of parasol (magnocellular) ganglion cells
transient responses to stationary contrast
what are the most common cells of brightness/luminance (form) image contrast
midget cells ~80%
what are the least common cells of brightness/luminance (form) image contrast
mango cells ~10%
what do both midget and magno ganglion cells have in relation to brightness/luminance (form) image contrast detection
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
what do chromatic/wavelength (colour) image contrast produce with midget and parasol ganglion cells
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
describe temporal (motion) image contrast detected by midget and parasol ganglion cells
transient (changing contrast) responses to light change/flicker = parasol/magno ganglion cells
name a way to measure colour contrast with OFF and ON ganglion cells
antagonistic wavelength-selective responses at chromatic contrast borders
what will be the best case scenario of colour contrast
Red ON in centre, very light green in surround
what will produce a weak response/no contrast (diffuse illumination on centre and surround)
Red equal luminance in centre & surround
what will be the worse case scenario (mutual antagonism) which makes the cell inhibited
Red ON in centre & Green ON in surround
explain why in terms of temporal contrast, why midget ganglion cells produce sustained (to presence of contrast to receptive fields) response
- 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
explain why in terms of temporal contrast, why parasol ganglion cells produce transient responses to each stimulus (brief)
- 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
