CNS/sensory 4 - visual Flashcards
visual perception is dependent on…
context
bottom up modification
what is vitreous humour
clear jelly
what is retina
where transduction occurs
what is retinal pigment epithelium
layer of tissue
biochemistry of photoreceptors
what is optic nerve
axons of neurons in retina
sending visual info to brai
what is lens
can change shape
what is iris
changes size of pupil by constricting or dilating
what is pupil
behind cornea
can change size
Depends on light levels
what is cornea
part of eye
outer clear layer
what is sclera
white
becomes transparent in front = cornea
describe blood vessels of eye
part blocks photons
what is fovea centralis
highest visual acuity
like reading
what is optic disk
medial to fovea
blind spot - no photoreceptors here
brain fills it in
what does lens do
refracts
bends light to single point
describe how lens refracts light
happens when light travels to 2 diff substances
depends on angle of incidence of light and properties of substances
eye collects photons and refracts them - bend in a way where they occur at a single point on retina = focusing
what is light refracted by
cornea and lens
which refracts more light - cornea or lens
cornea refracts more light
image on retina is inverted
lens = flexible and controlled by some muscles that control its shape
describe accommodation for near vision
limited focal range - like if looking at something in distance = lens assumes a shape to focus on retina
if out of focus = ciliary muscles control lens shape
lens accommodates for changes in object location by changing shape
describe nearsightedness
eyeball too long
eye is myopic
image created in front of retina
can see objects up close fine but distance = hard
solution = squish eyeball or use corrective lens to compensate for too much refraction and focuses image on retina
describe farsightedness
eyeball too short
eye is hyperopic
pull eyeball longer or use lens
describe astigmatism
the lens or cornea are not spherical and smooth
describe presbyopia
lens gets stiff and is unable to accomodate for near vision
forms of farsightedness that happen after 45 = reading glasses
describe cataract
change in lens colour = becomes opaque and blocks photons entering eye
describe basic parts of organization of the retina
retinal pigment epithelium (cells interact with photoreceptors)–> transduction (rods and cones) –> change in neurotransmitter release–> processing and convergence (bipolar cells, horizontal cells, amacrine cells)–> becomes optic nerve (vitreous humour and ganglion cells)
describe fovea centralis (organization of the retina)
retinal circuitry is shifted out of the way
describe rods and cones (organization of the retina)
2 types of photoreceptors (~100mil/each eye)
1 type of rods = tend to be everywhere else, mediate low light vision, ~5mil in each retina
cones = 3 types of cones, mediate colour vision, mostly in fovea, central vision, highest acuity, small rf, ~90 mil in each retina
transduction occurs in photoreceptors
describe bipolar cells, horizontal cells, amacrine cells (organization of the retina)
circuitry does processes and extracts certain features from visual input
converges - optic nerve carries only 1 million
describe ganglion cells (organization of the retina) - gen
many photoreceptors drive a few ganglion cells
rf= bigger -driven by many rods and less cones
fire aps
axons = optice nerve
describe ganglion cells (organization of the retina) - details
in center of vision - highest acuity = ganglion cells driven by 1,2,3 or 4 cones, in fovea = small rf
in periphery = ganglion cells driven by hundred of rods, big rf
describe light (organization of the retina)
light entering causes closing of ion channels and photoreceptors
changes neurotransmitter release from photoreceptors
circuitry of horizontal, bipolar and amacrine cells
is retina part of cns
yes technically but also sends info to cns
name 2 portions of photoreceptor and describe
outer segment = stacked full of membrane disks = lipid membrane stacked,
inner segment = has nucleus - where neurotransmitter released
describe phototransduction while in dark
outer segment = lots of cyclic GMP in intracellular space of photoreceptor and lining outer segment = cyclic gmp gated sodium channels (if channels open and photoreceptor depolarizes = neurotransmitter released into circuitry in dark when no photons)
photoreceptors releasing neurotransmitters all the time
name 4 main steps of photo being released - transduction
light activates opsin molecule
g protein cascade
cGMP–>GMP
channels close
describe chromophore (phototransduction)
retinene - inside disk, related to vit a
molecule that sits in protein called opsin (captures photons)
describe specifically what happens during phototransduction
many disks stacked since photons hard to capture
since photon has billions of opsin proteins
rhodopsin - for rods
photons captured by opsin protein that contains chromophore
photon hits opsin molecule and chromophore and opsin separate and causes conformational change in opsin molecule = triggers g protein cascade
g protein cascade = motor, so triggers motor to start - can run for a while and converts cgmp to gmp
ion channels close - na channels close since less/no cgmp
light causes receptors to…
Hyper polarize
transmitter released in dark usually, so photons cause reduction in neurotransmitter response
how many different opsin molecules are there
4 diff opsin molecules
rhodopsin found in rods
(3 diff ones for 3 diff types of cones, sensitive to 3 different colours of light)
what does middle circuitry do (phototransduction)
doing an inversion - causes reducrion in neurotransmitter to produce an increase drive of ganglion cells fire ap
describe rods
high sensivity, night vision
more rhodopsin, captures more light
high amplification, single photon closes many na+ channels
slow response time - lasts longer and slower
more sensitivity to scattered light (animals = glowy eyes sine v reflective retinas, light reflected off retina so rods can capture scattered light)
describe cones
low sensitivity, day vision
less opsin
lower amplification since many photons
faster response time
more sensitive to direct axial rays
describe rod system
low acuity - not present in central fovea, highly convergent (many rods drive ganglion cels and rods)
achromatic = one type opsin
describe cone system
high acuity = concentrated in fovea, less convergent, few cones drive ganglion cells
chromatic = three types of opsin
describe dark adaptation
bright (rods are inactivated (opsin wiped out since hit with photons), cones are active) –> dark (temporary blindness until rods re-activate and take over, rods have to reconnect with chromophore, involves retinal pigment epithelium, 10-15 mins)
describe light adaptation
dark (cones inactive, rods active) –> bright light (rods initially saturated, temporary blindness untill rods inactivate (opsin and chromophores will separate and cannot function) and cones take over)
explain light and dark adaptation in more detail
once chromophore and opsin broken = cannot capture photons anymore
rods mediate light and dark adaptation
takes time to put the chromophore and opsin back together, slow process 10-15 mins
what does retina report
relative intensity of light
depends on what around it
does not tell you exactly how many photons are there, just how many is there compared to surroundings
computation done by circuits
what do ganglion cells of retina do
not same uniform thing
does computation compares middle of receptive field to center
what do retinal ganglion cells have
center surround receptive fields
name 2 types of ganglion cells
inhibitory surround and excitatory center
excitatory surround and inhibitory center
what do retinal ganglion cells signal
relative differences of the light - contrast across receptive fields
describe inhibitory surround and excitatory center ganglion cell response to lights
bright center dark surround = many aps in center
dark center bright surround = no aps in center
uniform light = same aps
describe inhibitory center and excitatory surround ganglion cell response to lights
bright center dark surround = no aps in center
dark center bright surround = many aps in center
uniform light = same aps
what are photoreceptors sensitive to
wavelength
blue, green, red
opsin molecule determine chromatic sensitivity of the photoreceptor
ganglion cells send info about dark and light and also sends info about colour, depends on which photoreceptors are converging
what is perception of colour dependent on
context dependent
what do retinal ganglion cells have - for colour
colour opponent receptive fields
describe colour opponent receptive fields
green vs red and yellow vs purple
output of retina encodes values of brightness and colour
colour in middle vs surround
describe colour blindness
more than 5% of population has deficit in colour perception
some opsins not functioning properly
could be due to genetics = perception of colour is different
mostly in males
could also be caused by damage or diseases and stuff
describe flow of visual info in brain
left visual field
right visual field
temporal retina and nasal retina - temporal retina does not cross
axons coming from retinal ganglion cells that are receiving inputs = do cross midline at optic chiasm
optic nerve = one eye with both visual fields
cross at optic chiasm
optic tract = both eyes with contralateral visual field –> lateral geniculate nucleus (thalamus) synapses –> optic radiations towards back of brain –> visual cortext in occipital lobe - both eyes with contralateral visual field
lesion in left optic nerve
loss of vision in ipsilateral eye
lesion in optic chiasm
bilateral loss of temporal visual fields
lesion in left optic tract
loss of contralateral visual field
lesion in left occipital lobe
loss of vision in contralateral visual field
what does polymodal mean
visual and other sensory modalities combined
describe parietal stream
where
spatial features and motion
large rfs
describe primary visual cortex
small rfs
simple image features like oriented line segments
start to extract basic features which can be put together to make visual image
in occipital lobe
describe temporal visual stream
what
large rfs
complex image features
selective to high level features
hands, places, buildings
jennifer aniston neuron
describe object recognition
faces
where does all parts of visual world come together
in frontal love
