Visual system Flashcards
What 3 stimuli the visual system tuned to recognise and localise?
- Food
- Mate
- Predator
What defines how the visual system is organised?
Example?
The behaviour of the animal
Predators have eyes on the front of their head
Prey have eyes on the side
What is the morphology of a sensory neuron largely defined by?
The function of that neuron
What 3 things can happen at the processing level?
- Positive feedforward
- Negative feedforward
- Negative feedback
What is negative feedforward?
Feedforward but with an inhibitory neurotransmitter
What is a hemifield?
The range of vision seen by one eye focussing straight ahead
What side of the brain does the left hemifield activate? (which LGN/primary visual cortex)
Why?
The right side (right LGN and right primary visual cortex)
Due to the organisation of the connections in the brain
Where is the image focussed to in the eye?
The retina
What processes the visual information?
Why is there a need for it to be processed?
The retina
Need to be processed to determine what is important for the brain to know and what isn’t
As the optic nerve only has a certain LIMIT
Where does the information from the eye go to in the brain?
To the LGN (lateral geniculate nucleus)
What are the properties of the neurons in the LGN similar to?
Ganglion cells
Where does the information from the LGN go to?
Where in this structure does processing start?
The primary visual cortex
Starts in the V1 cortical area
After the V1 cortical area, where does the information go to?
Splits into 2 streams:
- Dorsal stream
- Ventral stream
What is the dorsal stream important in?
Processing spatial information (WHERE)
What does ‘spatial’ define?
POSITION and SPEED of stimulus (motion)
What is the ventral stream important in?
Object recognition (WHAT)
What parts of the brain are involved in the dorsal stream?
Posterior parietal
What parts of the brain are involved in the ventral stream?
V2
V4
Inferior temporal cortex
What structure regulates how much light falls on the retina?
The pupil
What is the fovea?
What is the structure?
Part of the retina where the light is focused
Has the highest visual activity (clearest vision)
Densely packed with photoreceptors
Contains mostly cones
How is the rest of the retina different to the fovea?
Has a smaller visual acuity
Contains primarily rods
Are rods or cones more sensitive to light?
Rods
What focusses the light onto the fovea?
Lens
What are Muller cells and why are they needed?
Cells found in the retina that allows light to travel through the retina (which is opaque)
(Act as a light guide through the retina)
Needed so that the whole retina isn’t illuminated when exposed to light
What are the first cells in the retina to respond to light?
The photoreceptors
How many layers of neurons are there in the retina?
What are they?
3 layers:
- Photoreceptors
- Bipolar cells
- Ganglion cells
How many layers of synapses are there in the retina?
What are they?
2 layers:
- PhotoR, bipolar and horizontal
- Bipolar, ganglion and amacrine
What are the feedforward neurons in the retina?
- PhotoR
- Bipolar
- Ganglion cells
What are the feedback neurons in the retina?
- Horizontal cells
- Amacrine cells
Where are horizontal cells present?
Between the photoreceptors and the bipolar neurons
Where are the amacrine cells present?
Between the bipolar neurons and the ganglion cells
What are the ‘plexiform’ layers?
The layers of synapses:
Inner and outer
What synapses are contained in the inner plexiform layer (IPL)?
- Ganglion cell
- Bipolar cell
- Amacrine cell
What synapses are contained in the outer plexiform layer (OPL)?
- Photoreceptor
- Bipolar cell
- Horizontal cell
Which plexiform layer is the most dense?
IPL
How many layers of synapses are in the IPL?
5-6 layers
Which photoreceptors are activated at dim light?
Bright light?
Dim light - rods
Bright light - cones
How are photoreceptors polarised?
They have different structures on either side of the cell
What is phototransduction and where does it occur?
The conversion of light into an electrical signal
Happens in the outer segment of the photoreceptor
What happens to the photoreceptors when they are activated by light?
They HYPERPOLARISE (decrease in the membrane potential)
What can happen if you sensitise the retina?
The retina can respond to individual photons
What are the key molecules in light phototransduction?
cGMP and CMP
What channels does the membrane of the outer segment of the photoreceptor contain?
Channels that are permeable to Ca and Na
In the dark, what is there high levels of in the cytoplasm?
What does this cause?
cGMP
- Na and Ca selective channels in the outer segment of the photoreceptor to open
- Membrane to depolarise
What happens in the photoreceptor outer segment when activated by light?
Signalling cascade which activates photodiesterase:
- PDE converts cGMP to GMP
- Less cGMP in the cytoplasm
- Na and Ca selective channels close
- Hyperpolarisation of the membrane
What receives input from the photoreceptors and where?
The bipolar cells and horizontal cells
In the outer plexiform layer
What receives input from the bipolar cells and where?
Ganglion calls and amacrine cells
In the inner plexiform layer
When do photoreceptors release glutamate?
In the dark, when the photoreceptors are depolarised
In the dark, what does the release of glutamate cause?
Activation of OFF cells (release more neuroT)
Inhibition of ON cells (release less neuroT)
In the light, what happens to the level of released glutamate from photoreceptors and why?
It decreases
Na and Ca channels are closed
Photoreceptors are hyperpolarised (inactive)
What happens to ON and OFF cells in the light?
ON cells are activated (by inhibiting inhibition)
OFF cells are inhibited (by inhibiting activation)
What glutamate channels do ON bipolar cells express?
Metabotropic (inhibitory)
What glutamate channels do OFF bipolar cells express?
Ionotropic (excitatory)
What is the receptive field of a visual neuron?
Area of sensory space that will activate (modify firing rate) of the visual neuron when illuminated
(Collection of the entire area which causes activation of the neuron when stimulated with light)
Do all light inputs cause the same activation of a single neuron?
No
How is the receptive field of the visual neurons arranged?
What does this mean?
Centre-surround organisation
Means that illumination of the centre and the surround leads to responses in opposite polarities
What happens if you illuminate the centre of the ON cell receptive field?
The ON cell is activated (depolarisation)
What happens if you illuminate the outside of the ON cell receptive field?
The ON cell in inactivated (hyperpolarisation)
What is used to stimulate the outside of the receptive field?
What is this?
Annulus of light
Ring of light
How many photoreceptors converge onto one bipolar cell, NOT in the fovea?
MANY photoreceptors converge onto ONE bipolar cell
In the fovea, how many photoreceptors converge onto one bipolar cell?
One
How do the photoreceptors in the centre of the receptive field activate the bipolar cell?
Directly (not via horizontal cell)
How do the photoreceptors in the surround of the receptive field activate the bipolar cell?
Via horizontal cells
What happens if you illuminate the centre of the OFF cell receptive field?
OFF cell is hyperpolarised
What happens if you illuminate the outside of the OFF cell receptive field?
OFF cell is depolatised
In the retina, after the bipolar cells have processed the information, where does it go to?
What is present here?
To the inner plexiform layer
Contains:
- Bipolar cell (presynapse)
- Ganglion cell (postsynapse)
- Amacrine cells (pre and postynapse)
What do amacrine cells do?
Shape the information from the bipolar cells to the ganglion cells
What type of neurons are amacrine cells?
Inhibitory interneurons
What are the differences between different ganglion cells?
Why?
Many types with different dendritic tree morphologies:
- Some are small and dense
- Some are larger and more diffuse
- Some have asymmetrical dendritic trees
Differences in morphology - perform different functions
What is the use of small, dense dendritic trees?
Part of pathway responsible for visual acuity and object recognition
What is the use of large, diffuse dendritic trees?
Responsible for positional information
How is the receptive field of ganglion cells arranged?
Center-surround
What happens if you stimulate (with light) the centre of the receptive field of an ON CENTRE cell?
Increase in spike rate
What happens if you stimulate (with light) the centre of the receptive field of an OFF CENTRE cell?
Decrease in spike rate
What happens if you stimulate (with light) the surround of the receptive field of an ON CENTRE cell?
Decrease in spike rate
What happens if you stimulate (with light) the surround of the receptive field of an OFF CENTRE cell?
Increase in spike rate
What does the illumination of the whole receptive field lead to in the ganglion cells?
Why?
NO activation of the ganglion cells
Ganglion cells are designed to respond to DIFFERENCES in illumination between the centre and the surround
What is the centre-surround of the ganglion cells a result of?
Inhibitory feedback from the amacrine cells
What are the 2 classes of ganglion cells?
What are the subtypes of these cells
Parvocellular (P-type)
Magnocellular (M-type)
Subtypes: ON, OFF
Which type of cells have subtypes ON and OFF?
Bipolar cells
Ganglion cells
What are P-type ganglion cells responsible for?
Processing information about SHAPE and COLOUR
What are M-type ganglion cells responsible for?
MOTION detection
How are P-type ganglion cells different to M-type ganglion cells?
- Smaller dendritic tree
- Smaller receptive field
- Slow conductance velocity
How are M-type ganglion cells different to P-type ganglion cells?
- Larger dendritic tree
- Larger receptive field
- Faster conductance velocity
When does adaptation of the visual system occur?
When stimulate with the SAME stimulus OVER TIME
OR
A CONTINUOUS stimulus
How does adaptation in the visual system occur?
2 different ways:
1) Desensitisation - decrease sensitivity to stimulus (decrease spike rate)
2) Sensitisation - increase sensitivity to stimulus (increase spike rate)
What is the reason for visual illusions?
Adaptation in the visual system
