L3 - overview of the visual system Flashcards
3 types of stimuli for visual system
food
predator
mate
what is negative feedforward
when GABA neurons activate neurons downstream
what is meant by - the optic nerve is an information bottleneck
it cannot process everything that the retina acquires rejects a lot of information
functions of the retina
image acquisition
filters information and adapts
what is the function of the lateral geniculate nucleus (LGN)
preprocesses the visual information - deep in brain so difficult to research
function of visual cortex
place of main processing of information
2 main visual pathways in the cortex
ventral stream and dorsal stream
the two influence each other
ventral stream
used to process object identity (what), terminates in the inferior temporal complex
dorsal stream
used to process information about the location, speed, direction of the object (where). terminates in the posterior parietal complex
function of the pupil
regulates amount of light that falls on the retina
function of the lens
focuses the image on the fovea (part of the retina)
what is the fovea
part of the retina with the highest visual acuity
what does light travel through
transparent muller cells within the retina
layout pf the retina
3 layers of neurons and 2 layers of synapses
feedforward neurons - photoreceptors, bipolar cells and ganglion cells
feedback neurons - horizontal cells and amacrine cells
pathway through the retina
info starts in photoreceptors
goes to bipolar cells
goes to ganglion cells
goes through optic nerve to the brain
what are the two levels of synapses in the brain
inner and outer plexiform layers
what is the outer plexiform layer
contains synapses between photoreceptors, bipolar cells and horizontal cells
what is the inner plexiform layer
contains synapses between bipolar cells, amacrine cells and horizontal cells
what is active and dim light and what is active at bright light
rods active at dim light
cones active at bright light
where does phototransduction occur
in the outer segment of rods and cones
photoreceptors respond to light flashes by hyper polarisation
phototransduction cascade
1) shine light on photoreceptor, opsin change their conformation
2) activates the G-protein coupled receptors
3) G alpha subunit dissociates which activates phosphodiesterase
4) phosphodiesterase causes a drop in the concentration of cyclic GMP
5) decrease in cGMP causes a closing of the change;s
6) closing of the channels causes hyper polarisation of the membrane
what is the phototransduction cascade sensed by
hyperpolarisation causes a decrease in glutamate release
glutamate concentration sensed by bipolar and horizontal cells
less glutamate = less. activation of bipolar and horizontal cells
what are the two types of bipolar cells
ON cells - depolarise when light intensity increases
OFF cells - hyper polarise when light intensity increases
what does less glutamate lead to regarding OFF cells
less glutamate leads to less depolarisation of OFF cells therefore they hyperpolarse
what receptors do OFF cells express
ionotropic glutamate receptors (excitatory) coupled to G-proteins - leads to less closing of channels
what receptors do ON cells express
metabotropic glutamate receptors (inhibitory) - AMPA
what is a receptive field
an area in the retina which when illuminated activates a visual neuron
what is center-surround organisation
illumination of the centre and the surround leads to responses in opposite polarities
what cells have centre surround organisation of their receptive field
bipolar and ganglion cells
what occurs during stimulation of the centre of the receptive field of an ON cell
get a depolarisation and if stimulate away from the centre get a hyper polarisation
stimulation of an OFF cell with spot and annulus
spot = hyperpolarisation annulus = depolarisation
bipolar cells receive input from what 2 cells
photoreceptors activate directly and indirectly
direct - located in centre of receptive field, leads to hyperpolarisation of OFF cell
indirect - stimulation of surrounding photoreceptors causes inhibition of horizontal cell - no longer inhibits bipolar cell leads to depolarisation
what are located in the same subliminal in the IPL
axons of ON bipolar cells and dendrites of ON ganglion and amacrine cells
large number of dendritic trees =
more connections so may be responsible for higher sensitivity vision
larger size of dendritic tree =
larger receptive field, not as good for high acquit vision. more suited for change within a large area
properties of parvocellular ganglion cells
small field sustained reponse slow conduction velocity low sensitive function to detect form/colour
properties of magnocellular ganglion cells
large field transient response fast conduction velocity high sensitivity function - motion detection
