Visual Processing Flashcards
What is the function of the retina?
- convert light energy into neural activity to detect differences in the intensity of light.
What is the lateral geniculate nucleus (LGN)?
the first synaptic relay in the primary visual pathway
How is the energy of light related to frequency?
energy is proportional to frequency - we see different wavelengths as different colours.
high energy - higher frequency - cooler colours -400-500nm
low enery - lower frequency - warm colours - 600-700nm
What is the direct vertical pathway in the retina, and the lateral connections that influence it?
photoreceptors
↓
bipolar cells
↓
ganglion cells
- horizontal cells (at the photoreceptor/bipolar cell synapse) - inhibitory
- amacrine cells (at the bipolar/ganglion cell synapse) - inhibitory.
How does light pass through the cells of the retina?
light passes through ganglion and bipolar cells before reaching photoreceptors
how many layers are in the retina?
3 layers of neurons and 2 layers of plexiform.
What are the types of photoreceptors?
Rod and cone
what are the 4 regions of a photoreceptor, and which part differs between rod and cone?
- Outer segment (membranous disks different between rod and cone)
- Inner segment
- Cell body
- Synaptic terminal
How do rods and cones differ structurally?
Rods outer segment is long and cylindrical with many disks, over 1000x more sensitive to light
Cones outer segment is short and tapering with fewer disks
What is meant by duplex retina?
2 visual systems - one of rods (dark conditions) and one of cones (light conditions)
Describe the distribution of photoreceptors across the retina
Cones are highly localised in the fovea, with much lower ratio in the periphery.
High ratio of rods to cones in the periphery, negligible amount in the fovea.
describe the regional histoarchitecture of the peripheral retina
- high ratio of rods to cones
- high ratio of photoreceptors to ganglion cells
- more sensitive to low light
What is the blind spot?
The optic disc where the axons of the ganglion cells make their way through the back of the eye and there is no photoreceptors.
Describe the regional histoarchitecture of the central retina (fovea centralis)
- mostly cones (much smaller than those in periphery)
- almost 1:1 photoreceptor to ganglion cells
- optimised for visual acuity - ganglion and bipolar layers are pushed apart to make a divot alloweing light to hit photoreceptors directly
How does transduction in cones differ from rods?
Different opsins - not rhodopsin
* Red (long wavelength)
* Green (medium wavelength)
* Blue (short wavelenth)
*They are not exclusive to a wavelenth but they are tuned
What is retinal?
The ligand within rhodopsin that responds & causes a conformational change when hit by electromagnetic radiation (goes from bent to straight)
How does the action of photopigment G protein (transducin) compare with NT gPCR system action?
in NT systems, G proteins increase second messengers (whether these are inhibitory or stimulatory).
Photopigment G proteins decrease second messengers and ALWAYS lead to a decrease in Na+ conductance.
What is the ion channel response when light hits a photoreceptor?
decrease in Na+ conductance
Explain phototransduction in rods and cones
- in the dark, Vm ~ -30mv (because cGMP keeps NA+ channels open)
- light hits, photon absorbed by opsin (rhodopsin in rods, red/green/blue opsin in cones), induces retinal conformational change, activates transducin (G Protein)
- transducin activates phosphodiesterase to convert cGMP to GMP
- falling [cGMP] closes Na+ channels
- hyperpolarisation (Vm moves towards Ek because K+ leak channels still open - Vm ~ -60mV)
In summary, light closes Na+ channels and hyperpolarises.
Depolarised in the Dark
What is the trichromacy theory of colour?
mixing of red, green & blue light = equal activation of the 3 types of cones = perceived as “white” light
Over what time period does adjustment between the visual systems occur? (ie between all-cone day vision & all-rod night vision)
minutes up to an hour
What are the factors in dark adaptation?
- dilation of pupils
- regeneration of unbleached rhodopsin
- adjustment of functional circuitry
What is notable about the cGMP-gated Na+ channel?
It is also permeable to Ca++
How does Ca++ impact cGMP levels?
Ca++ inhibits guanylyl cyclase enzyme
How is Ca++ regulated in a photoreceptor?
Intracellular Ca++ pumped out by exchange pump (4Na+ in/ Ca++ and K+ out)
What does it mean when a photoreceptor is saturated?
All cGMP-gated Na+ channels are closed causing maximal hyperpolarisation to around -70mV
What is the process of slow adaptation of photoreceptors when entering bright light?
cGMP levels drop very low
↓
All cGMP-gated Na+ channels closed (max hyperpolarisation -70mV)
↓
Ca++ influx stops (it uses the same channels)
↓
intracellular Ca++ levels drop
↓
Guanylyl cyclase activity increases (less Ca++ inhibition)
↓
cGMP levels increase
↓
cGMP-gated Na+ channels open
↓
depolarisation (no longer saturated)
What are the only true neurons in the retina?
ganglion cells
Why are non-ganglion cells not considered “true” neurons?
They do not generate an action potential. The generate a change in membrane potential (not a full AP) which is enough to result in NT release.
Why are non-ganglion cells not considered “true” neurons?
They do not generate an action potential. The generate a change in membrane potential (not a full AP) which is enough to result in NT release.
What is a receptive field?
An area of retina where light changes a particular neuron’s firing rate
How do the receptive field centre and surround interface with the bipolar cell, and what is their relationship to each other?
The receptive field centre photoreceptors synapse directly with the bipolar cell.
The receptive field surround photoreceptors input indirectly to bipolar cells via horizontal cells.
The receptive field centre will be either ON or OFF depending on the bipolar cell. The receptive field surround will always be the opposite to the centre.
What is the difference between ON and OFF bipolar cells?
ON Bipolar cells have mGLuR receptors and hyperpolarise in the dark, OFF bipolar cells have ionotropic glutamatergic receptors and depolarise in the dark.
Explain how OFF bipolar cells respond to light and dark
OFF bipolar cells have ionotropic glutamatergic receptors.
* in the dark, photoreceptors are releasing glutamate
* glutamate binds to ionotropic receptors and opens Na+ channels
* Na+ influx causes depolarisation
Therefore OFF Bipolar cell is depolarised in the dark.
In the light, no glutamate release from photoreceptors, Na+ channels closed, OFF bipolar cell is hyperpolarised.
Explain how ON bipolar cells respond to light and dark
ON Bipolar cells have mGluR receptors (inhibitory metabotropic glutamatergic gPCR receptor).
* in the dark, photoreceptors are releasing glutamate
* mGluR opens K+ channels
* K+ efflux causes hyperpolarisation
Therefore ON bipolar cell is hyperpolarised in the dark.
In the light, no glutamate release from photoreceptors, K+ channels closed, ON Bipolar cell is depolarised
What happens to an ON bipolar cell when light hits the receptive center?
The bipolar cell is depolarised
(photoreceptor hyperpolarised, direct synapse with bipolar)
What happens to an ON bipolar cell when light hits the receptive field surround?
The bipolar cell is hyperpolarised
(photoreceptors hyperpolrised, horizontal cell hyperpolarised)
what are the 2 types of ganglion cells?
- on-centre ganglion cell
- off-centre ganglion cell
Describe the center-surround organisation of receptive fields of ganglion cells
There is an RF centre (either ON or OFF) with an antagonistic surround (always the opposite to the centre).
How do populations of ganglion cells give rise to enhanced border detection?
It is about the border crossing the center-surround RFs of the ganglia. THe change in firing frequency as the border passes over the centre-surrounds gives more context than that response from a single ganglion cell.
How are primate retinal ganglion cells categorised?
by appearance, connectivity and electrophysiological properties
What are the 2 main types of RGCs?
M cells (10%)
P cells (the rest)
Compare M and P type ganglion cells
- M cells have larger RFs
- M cells have faster conduction velocity
- M cells more sensitive to low contrast
- M cells dont have colour opponent RFs (P cells do)
- M cells phasic reponse (on and off) while P cells are tonic response (steady)
- some M cells are movement/direction sensitive
M cells for our rod visual system, P cells for our colour cone visual system
What is meant by colour opponency in ganglion cell receptive fields?
the ganglion cell RF has opposing areas that respond to different colours rather than just light & dark. Colour opponent pairs are red/green and blue/yellow. found in P type RGCs.
What is the role of intrinsically photosensitive retinal ganglion cells?
THey project to the suprachiasmatic nucleus in the hypothalamus and control circadian rhythm in response to light signals.
What is the vision perception pathway?
optic nerve
↓
optic chiasm
↓
lateral geniculate nucleus (thalamus)
↓
optic radiation
↓
primary visual cortex & other higher order areas
What is the benefit of overlapping receptive fields?
binocular vision
explain decussation of visual fields
the visual fields are what decussate for processing, not the eyes. ie, not everything from R eye gets to L brain for processing. Everything in the R visual hemifield (from both eyes) foes to L brain.
What are the types of lateral geniculate nucleus (LGN) cells and how do they relate to RGCs?
- magnocellular LGN - large RFs with transient response (match with M-type RGCs)
- parvocellular LGN - small RFs with sustained response (match with P-type RGCs)
RFs for LGN neurons are almost identical to ganglion cells that feed them
What is the striate cortex?
primary visual cortex
What is the cytoarchitecture of the striate cortex?
6 layer neocortex
What is the differences and similarities between simple and complex cells in striate cortex
- Differences - simple have distinct ON and OFF regions, complex do not
- Similarities - both binocular, orientation selective
what are the 2 processing streams of visual information?
- dorsal: V1 → V2 (shape) → MT → MST
- Ventral: V1 → V4 (colour perception) → Inferior Temporal (recognition)
what is parallel processing?
the current prevailing theory of visual perception, based on groups of cortical areas contributing to the perception of colour, motion, and object meaning (like the sound produced by an orchestra rather than from individual neurons)
