Retinal projections: parallel processing pathways (to the thalamus & midbrain) Flashcards
which two major classes of retinal ganglion cells have concentric surround receptive fields that turns them into luminance contrast detectors
midget/parvo
&
magno/parasol
where do midget & parasol retinal ganglion axons travel down and end up
travel down the optic pathway
end up at LGN of thalamus
what is the third cell in addition to midget and parasol, that end up at the LGN
konio-cellular systems
what is the divisions of labour/specialisations for visual perception
functional specialisations that divide different aspects of our visual perception processes.
what does the divisions of labour/specialisations for visual perception begin with
the three types of retinal ganglion cells found in the retina
list the three main sub-classes of retinal ganglion cell
- midget/parvo
- parasol/magno
- small bi-stratified/konio
what are the properties of the midget/parvo sub-class of RGC
detailed form processing/high acuity & colour processing (red/green) distinguish green from yellow
what are the properties of the parasol/magno sub-class of RGC
global (not fine) form/low acuity vision & motion (flicker, good a detecting change)
what are the properties of the small bi-stratified/konio sub-class of RGC
colour (blue/yellow)
what do RGC’s that are not involved in perception send their axons to in not to the LGN
different non-geniculate nuclei outside the thalamus
what are the further divisions of labour/specialisations mediating visual reflexes
- circadian rhythms
- pupillary light responses
- eye movements
how much % does the midget/parvocellular sub-class of RGC represent of all ganglion cells
majority - 80%
what are the anatomical features/structure of the midget/parvocellular sub-class of RGC
- small cell bodies
- short dendrites
- small diameter axons
where are the midget/parvocellular sub-class of RGC concentrated
concentrated in the central retina
fewer peripherally
which of the LGN layers do the midget/parvocellular sub-class of RGC project
to the parvocellular layers 3-6
how much % does the parasol/magnocellular sub-class of RGC represent of all ganglion cells
10%
what are the anatomical features/structure of the parasol/magnocellular sub-class of RGC
- large cell bodies
- wide dendritic trees
where are the parasol/magnocellular sub-class of RGC concentrated
even distribution across central to peripheral retina
which of the LGN layers do the parasol/magnocellular sub-class of RGC project
to the magnocellular layers 1 & 2
how much % does the small bi-stratified or koniocellular sub-class of RGC represent of all ganglion cells
5%
what are the anatomical features/structure of the small bi-stratified or koniocellular sub-class of RGC
- small cell bodies
- wide dendritic trees
- very thin axons
where are the small bi-stratified or koniocellular sub-class of RGC concentrated
concentrated in the central retina
very few peripherally
which of the LGN areas do the small bi-stratified or koniocellular sub-class of RGC project
the inter-laminar zones of the LGN
which RGC has wider dendrites than parasol magno cells
small bi-stratified RGCs
why is the name given to small bi-stratified RGCs
have two strata which go into the inner plexiform layer to receive input from bipolar cells
which sort of shape do the dendrites of parasol magno RGCs resemble
come out like an umbrella in a 3D form
where do the dendrites of the parasol magno RGCs go to and why
go to the inner plexiform layer where it receives synaptic input from bipolar cells
what are the parasol magno cell RGCs RFs larger than
midget cells
what do the parasol/magno RGC axons have around them and why
myelin for faster conduction
where are konio cells scattered in the LGN
scattered/dotted about in the inter laminar zones between the major (P&M) cell layers 1-6
what are the white bands between the cells in the LGN
white matter which are axons of the RGC trying to get neurons they want to make connections with and are also axons of LGN that want to send information further up the pathway to the primary visual cortex
what do the white band of the axons of the RGC separate
the grey matter neurone cell bodies from each other
what do the darker dots on the LGN show
the bigger cells/cell bodies of the LGN
what type of RFs do midget parvocellular RGC sub-classes have
small RFs
what type of responses do the RFs of the midget parvocellular RGC sub-classes have
sustained/long responses
what type of centre-surrounds do midget parvocellular RGC sub-classes have within their RFs
ON/OFF luminance and red/green contrasts
what type of luminance: spatial frequencies do midget parvocellular RGC sub-classes have
high spatial frequencies (high acuity and fine detail, interested in contrast)
due to the high spatial frequencies that midget parvocellular RGC sub-classes have, what type of contrast sensitivity do they have
low contrast sensitivity
need high contrast to keep them going as they are not sensitive to lower contrast
what type of acuity is midget parvocellular RGC sub-classes associated with
high acuity (detailed form)
as well as high acuity and detailed form, what else is midget parvocellular RGC sub-classes associated with
colour
what type of RFs do parasol magno cellular sub-classes have
larger RFs
what type of responses do parasol magno cellular sub-classes have
transient (brief) responses
what type of centre-surrounds do parasol magno cellular sub-classes have
ON/OFF centre surrounds luminance only (achromatic)
what type of luminance: spatial frequencies do parasol magno cellular sub-classes have
low spatial frequencies
due to the low spatial frequencies that parasol magno cellular sub-classes have, what type of contrast sensitivity do they have
high contrast sensitivity (stimulate to low contrasts)
swell as high contrast sensitivity, what else do parasol magno cellular sub-classes have
also high temporal frequencies (flicker)
what type of acuity is parasol magno cellular sub-classes associated with
low acuity (global form)
as well as low acuity and global form what else is parasol magno cellular sub-classes associated with
motion
what type of RFs do small bi-stratified or koniocellular sub-classes have
small RFs
what type of responses do small bi-stratified or koniocellular sub-classes have
sustained responses
what do small bi-stratified or koniocellular sub-classes not have
centre-surround or luminance
what are small bi-stratified or koniocellular sub-classes only associated with
blue-yellow contrasts only
what are removed except at the block borders of global forms
filtered images with fine detail (high spatial frequencies) as seen by the lower acuity parasol or mango-cellular system
what can global forms (low acuity vision) still be used to
roughly identify things
what do both midget & parasol RGCs possess
ON-OFF or OFF-ON centre-surround concentric type RF organisations & generate responses that emphasise luminance (brightness) differences at contrast borders in the visual image
what different contributions do midget & parasol RGCs make
to the retinal contrast sensitivity function (CSF)
what is the normal human visual system most sensitive to in the contrast sensitivity function
3-8 cycle per degree (cod)
describe the properties of the pallie robson chart
a contrast chart with:
- high contrast at top and low contrast at bottom
- but all letters have the same spatial frequency (1cpd = low spatial frequency)
what is 1 cod equivalent to
in one degree of visual field there is one black bar and one white bar
how many black and white bars are in one degree of visual field
30
where does the low contrast and highest spatial frequency cut off
where no one can tell the contrast around 30 cpd
wha va’s is 30cpd equivalent to
6/6 & 6/4.8 = 40cpd
what type of contrast is a low spatial frequency
high contrast
at which point of spatial frequency in cpd is the highest sensitivity
5 cpd
which va’s is 5 cpd equivalent to
9/60 or logmar 0.7
what is the preferred spatial frequency of both RGC types magno and parvo
match the size of their ON & OFF sub-regions of their RFs
which RGCs can the ON-centre OFF-surround RF belong to
both midget and parasol RGCs
when will a RGC like a particular stimulus and fire a large response
e.g. when the ON-centre is filled by the light bar and OFF-surrounds covered by dark bars i.e. the spatial phase of the contrast borders maximally excites both the light ON-centre & light OFF-surround zones of the cells RF
because centre-surround RF sizes influence spatial frequency selectivity, what effect is there with midget cells RFs
- smaller receptive field
- centres & surrounds are more responsive to higher spatial frequencies
because centre-surround RF sizes influence spatial frequency selectivity, what effect is there with parasol cells RFs
- larger receptive field
- centres & surrounds are more responsive to lower spatial frequencies
so what is the difference if RF sizes responsible for
the tuning of midget & parasol RGCs
which RGC likes lower spatial frequencies/high contrast sensitivity
parasol/magno cells
what is the whole contrast sensitivity function made of
separate channels with smaller curves makes up the whole anatomy
what are the individual channels which make up the whole contrast sensitivity function a response property of
individual neurons of ganglion cells in the retina
what are the midget and parasol RGCs sensitive to within the contrast sensitivity function
the peak/middle range = 3-8 cpd where both RGCs contribute
what do midget and parasol ganglion cells decrease in with retinal eccentricity
density
what do midget and parasol ganglion cells increase in with retinal eccentricity
size
how much of visual angle in degrees is 1mm equivalent to
2.2 degrees
how much of visual angle in degrees is 10mm equivalent to
22 degrees
at 2.2 degrees (foveal 1 degree angle) of visual angle, how much larger does this value make the dendritic fields of midget & parasol RGCs
20x and 10x larger respectively
as a result of dendritic sizes of the midget and parasol RGCs increasing with eccentricity, what else increases as a consequence
RF sizes
what does the increasing RF sizes with eccentricity contribute to
the reducing acuity (high spatial frequency cut off) of the CSF in peripheral 22 degrees verses foveal 1 degrees
which visual angle out of 22 and 1 degrees does not have good acuity
22 degrees
what type of cone inputs do midget cells receive
selective cone inputs
what are the midget cells responses influenced by, as they receive selective cone inputs
influenced by the wavelengths of the luminance contrast borders
what type of cone inputs do parasol ganglion cells receive
mixed cone inputs
what are the parasol ganglion cells responses influenced by, as they receive mixed cone inputs
ONLY influenced by the luminance differences
what cones do the bipolar cells which supply to the parasol RGCs get
mixture of both red & green cones
so both are responsible for its centre and surround
and the mixture of red and green cones so will like the red and green stripes in its RF as the ON response is mediated by both cones so can respond to either
if a midget cell RF = red-ON green-OFF what type of response is given
maximal response to red ON/dark with minimal or no response to green ON/dark gratings and some response to white ON/dark as theres some red in white light
what does a parasol RF respond to
just ON-OFF
so doesn’t care if ON is white red or green
which RGC have sustained responses
midget cells
which RGC have transient/detect flicker responses
parasol cells
what is the peak temporal frequency of a midget/parvo RGC
10Hz
what is the cut off temporal frequency of a midget/parvo RGC
20Hz
what is the peak temporal frequency of a magno/parasol RGC
10Hz
what is the cut off temporal frequency of a magno/parasol RGC
50Hz
what is the significance of a magno/parasol RGC having a cut off temporal frequency of 50Hz
- high temporal contrast = can detect flicker (50 flashes a second)
- better for detecting image motion
does does critical flicker fusion frequency mean
highest flicker that the visual system can follow (=50Hz)
which RGCs are concentrated at the retina
midget
which RGCs have large, heavily myelinated axons
parasol
which RGCs are particularly sensitive to blue/SW light
bi-stratified
which RGC gives transient responses to light stimulations
magno/parasol
what are the RFs of parvo, magno or konio LGN cells similar to
those of the (P, M or K) retinal ganglions that supply their specific synaptic inputs
what is the difference between the RFs of parvo, magno or konio LGN cells compared to the P, M or K retinal ganglions that supply their synaptic inputs and why
they are slightly larger than the ganglion cells that supply them input
due to convergence from several ganglion cells of one type & even less responsive to diffuse light (due to stronger ‘lateral’ inhibition in the LGN itself)
The LGN acts mainly to organise the synaptic inputs into…
- function-specific layers (including the interlaminar zone)
before sending the information on, still in parallel, to the primary visual (v1) cortex in the occipital lobe
required for visual perception
i.e. one of the main functions of the LGN is not to change the receptive field properties of the inputs, but is to do this
what do the parallel image processing in the primary visual pathway send information of different LGN layers into
parallel pathways up to different neurons in the visual cortex
where do parallel pathways beginning continue through to
begin in the retina and continue into the LGN which then continued from LGN and end up into the primary visual cortex
what happens to the other RGC sub-classes which are not involved in visual perception who’s axons don’t project to the LGN
- either stop off earlier (above the optic chiasm) in the hypothalamus
or - go down the tract and continue past the LGN (go into the special fibre pathway called the superior brachium) and ends up in separate nuclei of the midbrain, behind the thalamic
what do the non-perceptual parallel processing pathways include
- (1) projections from specialised photosensitive (melanopsin-containing) cells to the
suprachiasmatic nucleus of the hypothalamus
& - 3 parallel pathways to the upper midbrain
from other photosensitive-type ganglion cells to:
> olivary pretectal nucleus (OPT): for the pupillary light reflex (constriction)
& from other members of the small bi-stratified sub-classes to:
> superior colliculus (SC): for fast eye movements to novel stimuli
> accessory optic nuclei (AON): for optokinetic reflex during head motion
what is melanopsin
visual pigment which transduces light from a wide region of space during a long time & only interested in day time & night time as they are radiance detectors where the illuminance of day & night regulates our biological clock, which regulates our circadian rhythm
what do the ganglion cell axons which end up in the hypothalamus contain
melanopsin
what does melanopsin contain
contain ganglion cell dendrites which span over wide regions of the retina containing melanopsin
what does melanopsin not do
dont transduce light from local regions of space
