retinal image processing and receptive fields Flashcards
what does light reflected from surfaces pass through ?
. light reflected from surfaces and objects in our field of view , passes through all the transparent media and crosses all the retinal layers, before the light is captured by visual pigment ( rhodopsin ) in outer segment
what is the direction of information processing?
. information processing direction is opposite to the direction of light
. cell bodies of rods and cones occupy the outer nuclear layer , they have synapses in the inner plexiform layer which they apply into dendrites of bipolar cells
. bipolar cells , cell body resides in the inner nuclear layer
. bipolar cells have synapses , which they form in the inner plexiform layer with the ganglion cells
what is vertical pathway?
- serial processing
. starts from photoreceptors to bipolar cells to ganglion cells
what is lateral pathway?
- lateral inhibition
- mediated by horizontal and amacrine cells
what is the function of horizontal cells?
- modifying the pathways between photoreceptors and bipolar cell
what is the function of amacrine cells?
- modifying the pathways between the bipolar cell and ganglion cell
what is a receptive field?
- all retinal neurons have a receptive field (RF)
. they only respond to specific light stimulus features in a spatially restricted region of the visual field
what are photoreceptors receptive field determined by?
- light sensitivity of their visual pigment and position of their outer segment in the retina
what are all other retinal neurons receptive field determined by?
- synaptic inputs received from photoreceptors and/or other retinal cells in the vertical and lateral pathways
what are the properties of receptive fields of photoreceptors?
- tiny(<0.01 degrees), circular and uniform
- luminance (brightness) and wavelength-dependent responses
- graded changes in membrane potential is their response to capturing light
what are the properties of bipolar and ganglion cells receptive fields ?
- larger, circular and non-uniform( more complex)
- due to convergence and spatial summation of synaptic inputs
- concentric , antagonistic , they have centre and surround regions
- luminance difference (contrast)-dependent responses- interested in intensity falling in centre than surround
- ganglion cells fire action potential
what are the centres of bipolar cells RF mediated by?
. the centres of bipolar cells receptive fields are mediated directly by photoreceptor input
what are the surrounds of bipolar cells RF mediated by?
. surrounds from photoreceptor-horizontal cell ( lateral ) interactions, then relayed to bipolar cells
what is the functional significance of contrast - dependent RF?
. for retinal ganglion cell signalling to the brain
. different types of contrast - sensitivity in midget/parvo compared to parasol/mango ganglion cell types
what is the difference between bipolar cells RF centre and surround?
. RF centre: direct photoreceptor inputs
(vertical pathway)
. RF surround: from photoreceptor-horizontal cell interactions ( lateral pathway)
where is the bipolar cell RF location?
. same region in space as where the photoreceptors are looking
what is the polarisation of photoreceptors ?
. photoreceptors are depolarised in the dark and hyperpolarize in response to light
why are photoreceptors are excited in the dark and inhibited when they catch light ?
. in the dark a current flows through both into the outer segment
. in the dark there are cation channels in the membrane of photoreceptors outer segment which allow sodium ions in the extracellular fluid to enter the cells outer segment ]
. at the same time potassium ions exist into the extracellular fluid
. there is a simultaneous influx of positively charged sodium ions and efflux of positively charged potassium ions - overall resting membrane potential is close to 0
what happens when photoreceptors capture light ?
. in the light , when photoreceptors capture the light through their rhodopsin , this results in closure of sodium channels in their outer segment
. when photoreceptor catches light , it leads to an enzyme cascade which reduces the concentration of cGMP which is converted into GMP
. GMP can’t hold the sodium channels open
. as a consequence in the light, there is no sodium influx into the outer segment
. potassium efflux continues
. cell becomes more negative
what is the difference between the two functional cone bipolar cell types? when responding to light
- OFF-BIPOLAR cell
- in the dark, the cell is depolarised with a membrane potential off -20mV, when light is on the cone hyperpolarizes
- response of OFF-BIPOLAR cell is the same as the cone which is providing its input - ON-BIPOLAR cell
- has a resting membrane potential , when light is one the on-bipolar cell depolarizes , when light is off , it goes back to its resting membrane potential
what is the difference between synapses in the on and off bipolar cells?
. the synapses that the on and off bipolar cells receive from the cone have different receptors associated with them and have different responses to the neurotransmitter
what is an OFF response?
- cells is depolarized = excited by light OFF or darkness in its RF ( like a photoreceptor)
what is an ON response?
- cells is depolarized= excited by light on or brightness in the RF
how can different cone bipolar cells respond in opposite ways to the input from the same cone?
. to do with neurotransmitter release
what is the concept of neurotransmitter release ?
. neurons increase neurotransmitter release when they are depolarized/excited and stop releasing neurotransmitter when they are hyperpolarized/inhibited
. the effect of neurotransmitter release depends on the type of receptor it activates in the post-synaptic cell
. OFF and ON bipolar cells have different receptors in their dendrites to the transmitter ( glutamate) released at cone synapses
what are the 2 morphological cone bipolar cell types?
. sign-reversing ON synapses on long, invaginating dendrites
. sign-conserving (OFF) synapses on short, non-invaginating dendrites
what happens in sign- conserving OFF synapses on short, non-invaginating dendrites ?
. cone pedicle makes connection with the dendrites of OFF-BIPOLAR cell
. dendrites of the OFF-BIPOLAR cell are found in the bumps of pedicle
. in the dark, the cone is depolarized , and releases neurotransmitter( glutamate ) onto the dendrite of 0FF-BIPOLAR cell
. glutamate is binding to a particular receptor found within the bipolar dendrite ( AMPA ) receptor
. when glutamate binds to AMPA receptors it opens sodium channels in the membrane of bipolar cells
. sodium flows into the bipolar cell and becomes depolarized
. when light is captured by cone , cone hyperpolarizes and stops releasing neurotransmitter
. no glutamate is binding to AMPA receptor
. sodium channel close
what is AMPA ?
.AMPA receptor are ion channels , when glutamate binds to AMPA receptors it opens sodium channels in the membrane of bipolar cells
what happens in sign-reserving (ON) synapses on long, invaginating dendrites?
. the dendrites of ON-BIPOLAR cell go in the groove of cone pedicle and are called long, invaginating dendrites
. in the dark, cone is depolarized and releases glutamate into the dendrite of the ON-BIPOLAR cell
. glutamate binds onto the receptor found on the ON-BIPOLAR
. that receptor is called (APB)
. this results in closure of sodium channels within the dendrites
. failure of sodium ions to enter the bipolar cells
. cone hyperploarizes
. in the light, cone is hyperpolarised and sodium channels open
. the cell becomes depolarized
what do horizontal cell have ?
. horizontal cells have long horizontal oriented dendrites , they receive inputs from many cones
what do horizontal cells do?
. horizontal cells make inhibitory connections with the same cones supplying its excitatory input
. the outcome of this reciprocal processing is relayed to the cones involved in the bipolar cell RF centre
. horizontal cells also connect to rods but no to bipolar cells
what do cone do in the OPL?
. many cones make excitatory, glutamatergic connections in the OPL with the extensive horizontally-oriented dendrites of the same horizontal cell
how is antagonistic surrounds of a bipolar cell is mediated via lateral inhibition involving photoreceptor to horizontal cells?
. light ON in the RF of surrounding red cones
. horizontal cell hyperpolarizes
. because these cones are directly connected to it via conventional, sign-conserving synapses
. horizontal cells normally release an inhibitory neurotransmitter in the dark back onto cones in the dark=GABA: but this is now inhibited as the bipolar cell as hyperpolarizes
. this ‘disinhibition’ of the HC is equivalent to an excitatory effect
. so all the cones connected to it, including the central (blue) ones in the dark depolarize
. for the central cones connected to the bipolar cells, its as if it just got darker in the RF centre
. the blue cone depolarize even more
what does it mean when it gets darker in the RF centre for OFF-BIPOLAR cells?
. in the dark, when the cone are depolarized
. OFF-BIPOLAR cells depolarize , so excited by light ON in the surround
. they have OFF-CENTRE, and ON-surround
. they like darkness in the centre and light in the surround
what does it mean when it gets darker in the RF centre for ON-BIPOLAR cell?
. in the dark, when the cone is depolarized
. ON-BIPOLAR cells hyperpolarize
. ON-centre , OFF-surround
how are both bipolar cells contrast -detectors?
.ON-BIPOLAR CELL= ON-centre , OFF-SURROUND
. OFF-BIPOLAR CELL= OFF-CENTRE, ON-SURROUND
where do retinal ganglion cells receive convergent input from?
. 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 (IPL)
what do ON-CENTRE/OFF-SURROUND bipolar cell inputs generate?
. ON-centre/OFF-surround bipolar cell inputs generate ON-centre/OFF-surround ganglion cells, via their synapses in the inner zone of the IPL
what do OFF-CENTRE/ON-surround bipolar cell inputs generate ?
. OFF-centre/ON-surround bipolar cell inputs generate OFF-centre/ON-surround ganglion cells, via synapses in the outer zone of the IPL
what are the 2 main classes of retinal ganglion cell?
. midget and parsol
. both classes have different sub-types with either ON/OFF or OFF/ON receptive field organisations
what happens is ON-centre/OFF-surround?
. when lights go on centre , the ganglion cell gives a big response
. when light go on in surround, the ganglion cell doesn’t surround
. theses cells tell the brain that centre is brighter than surround ( no response to diffuse illumination)
what happens in OFF-centre/ON-surround?
. when light goes on in centre, ganglion cell gives no response
. ganglion cells respond when light goes on in the surround
. this cell tells brain my centre darker than surround ( no response to diffuse illumination )
what is the ON-centre/OFF-surround ganglion cell response to different luminance contrast borders?
1 . ON-centre retinal ganglion cell’s fire maximally when there is a higher luminance ON in the centre of their RF and a less light in the surround= the ‘best-case scenario’ enhancing this particular contrast
- ON-centre retinal ganglion cells fire least when there is a less luminance in their RF centre and more light ON in the surround = the worst case scenario
- ON-centre retinal ganglion cells fire at a intermediate rate when luminance levels are diffuse= identical ( either low or high ) in both their RF centre and surround , due to mutual antagonism between ON and OFF zones
what is the OFF-centre/ON-surround retinal ganglion cell response to different luminance contrast borders?
. they have opposite contrast preference for 1 and 2 in the ON-centre/OFF-surround
. they also fire weakly in the absence of any luminance contrast
what is the major advantage of ganglion cell contrast enhancement ?
. the ganglion cells are accentuating the difference between light and dark and reporting that to the brain
. this is important for perceptual constancy which is a product of post-receptoral retinal processing
. our brain ignores brightness
. ganglion cells are enhancing the contrast the darkness of text against the brightness of the page
what are photoreceptors responses determined by?
. determined by the brightness of the environment to which they are exposed
what really matter for visual perception?
. what really matters for visual perception is not the absolute light intensities entering the eye and that are detected by our photoreceptors under theses two environmental conditions, but that the relative contrast between the page and print remains invariant
what is the minor disadvantage of contrast enhancement mediated by retinal ganglion cells?
. distortion of the perception of light and dark by ganglion cell contrast enhancement
what are ON-centre/OFF-surround ganglion cells interested ?
ON-centre/OFF-surround ganglion cells are interested in contrast between their centres and surround and don’t like diffuse illumination
