The visual system Flashcards
What does the retina contain?
Photoreceptors
What do photoreceptors do?
Convert light to changes in membrane potential
What is the output of the retina via?
Ganglion cells
Axons in the optic nerve
What do bipolar cells do?
Link photoreceptors to ganglion cells
What do horizontal cells do?
Mediate lateral interactions
What are the two types of photoreceptors in the retina?
Rods
Cones
What are cones?
Less sensitive than rods
Operate optimally in daylight
What are rods?
More sensitive than cones (x1000)
Operate optimally in low light levels (twilight)
Saturated in daylight
Where are cones?
Concentrated at the fovea
Where are rods?
More numerous in the peripheral retina
What do rods and cones differ in?
Connectivity
Sensitivity
Distribution
What do cones connect?
1 to 1 with bipolar and ganglion cells
What do rods converge on?
Ganglion cells
How many types of cones does the retina contain?
3 types
What are these types of cones?
Maximally sensitive to a different wave length of light
Blue cones
Green cones
Red cones
What is the trichromacy theory?
The colour we perceive is largely determined by the relative activation of blue, green and red cones
What can be used to test for red-green colour-blindness?
Ishihara’s cards
Confusion of shades of red and green
How many people have red-green colour-blindness?
2% of the male population
What is missing in individuals with red-green colour-blindness?
Cones sensitive to red light are missing
What is a receptor field?
Area of retina it receives input
What is on centre retinal ganglion?
Light is stimulus that excites them
What is the off centre retinal ganglion?
Stimulated by shadow
How are ganglion cells organised?
Concentric centre surround (excitatory-inhibitory) receptive field organisation
What is the illumination responses to stimulation in the centre cancelled by?
The response to stimulation in the surround
What are maximum response to in luminance contrast?
Light-dark boundaries
How many M type cells are involved in luminance contrast?
10%
How many P type cells are there?
90%
What are P type cells sensitive to?
Different wavelengths
What do colour-opponent centre surround receptive fields respond to?
Light of one wavelength in the centre is cancelled by light of another wavelength in the suround
What colour pairs cancel?
Red-green
Blue-yellow
Where do the optic nerves of each eye join?
Optic chiasm
Where do fibres from the nasal retina meet?
Both eyes cross (partial decussation)
Cross at the optic chiasm
What do optic tracts carry information about?
Info relating to visual hemisfield
Where do optic tracts project on to?
Lateral genicualte nucleus (LGN) of the thalamus
What does the LGN consist of?
6 layers
What types of cells are in the LGN?
M-type and P-type
Where do M and P cells project?
M- magnocellular layers (1 and 2)
P- parvocellular layers (3-6)
What are the receptive field properties of the LGN neurones similar to?
Ganglion cells
What are LGN neurones primarily?
A relay to the cortex- information from different ganglion cells and different eyes is carried by segregated parallel lines
Where do thalamic relay neurones from the LGN terminate?
Layer IV of primary visual cortex
Where does information from 2 eyes project?
Alternating bands called ocular dominance columns
What do simple cells show?
Orientation selectivity
What are the different cortical areas that are specialised for different visual processing tasks?
V1
V5
V4
What does the V1 do?
Parcels out information to other specialised areas of cortex
Where do thalamic neurones terminate in V1?
Layer IV
What are cortical neurones like in V1?
More elaborate receptive fields- several relay neurones feed into them
What does V1 also extract?
Additional information
E.g. orientation, selectivity, direction selectivity
What does V1 parcel out?
Visual information to other specialised ares of the cortex
What is V5 specialised for?
Motion analysis- stimulated by viewing a moving pattern of black and white squares
What do lesions of V5 lead to?
Akinetopsia
Can’t see objects if they move
What is V4 specialised for?
Colour vision- stimulated by viewing an abstract pattern of colours
What do lesions of V4 lead to?
Achromotopsia
See only in shades of grey
