Lecture 2 - visual pathways 1 Flashcards
What are the three types of anatomically defined ganglion cells?
- form around 90% of all optic nerve fibres
- midget, parasol, and bistratified ganglion cells
Midget cells
Input area: small
polarity: ON and OFF
Number: 70%
Inputs: L and M cones
Parasol cells
Input area: large
Polarity: ON and OFF
Number: 10%
Inputs: L and M cones
Bistratified cells
Input area: from 2 areas
Polarity: ON
Number: 5-10%
Inputs: S cones (and L and M)
what are the types of physiologically defined ganglion cells
- magnocellular cells
- parvocellular cells
- koniocellular cells
magnocellular cells
- 10% in total
- have a large receptive field (similar to parasol cells)
- produce fast responses
- not sensitive to colour of light (just care about difference between center and surround)
- some are sensitive to directions of visual motion
- sensitive to low contrasts but they saturate when the contrast is high
Parvocellular cells
- 70% in total
- smaller receptive field than magno (similar to midget cells)
- more sensitive to the form and fine details of visual stimuli
- respond poorly to low contrast but do not saturate at high contrasts
- sensitive to differences in wavelength of light
- most have colour opponent center-surround receptive field for red-green
koniocellular cells
- 8-10%
- similar to bistratified cells
- they are the only cell class known to carry the S-cone signal
- low contrast sensitivity
Rod pathways
- pathway starts with many rods converging information to the rod bipolar cell
- rod bipolar cells do not contact ganglion cells directly
- they contact amacrine cells which spread out the rod information before conveying on ganglion cells
what does the optic nerve do
- transmits visual information to the brain
- exits the eye at optic disk
the two optic nerves come together at the optic chiasm
what is the thalamus
- an essential link in transfer of sensory information to the cerebral cortices
- the main visual component of the thalamus is the dorsal lateral geniculate nucleus
what are the layers of the lateral geniculate nucleus
- 6 layers
- Layers 1 and 2 = magnocellular (large cells)
- Layers 3 to 6 = parvocellular (small cells)
koniocellular cells in between layers
General info about LGN
- each layer contains excitatory and inhibitory neurons
- each cell receives most of its retinal input from single retinal ganglion cell
- 90% of retinal outputs terminate in the LGN
where do the magno-, parvo- and konio-cellular cells project inputs to in V1
magnocellular to 4Ca
parvocellular to 4Cb
koniocellular to CO blobs in V1
cerebral cortex
- the outer layer of neural tissue in humans
- its divided into two cortices which are highly convoluted
the primary visual cortex is also known as the…
striate cortex
where does the information leaving the lateral geniculate nucleus go to
the primary visual cortex
what are the layers of the cortex
- 6 layers
- main input is layer 4
- main projection is layer 2/3
- main feedback layer is layer 6
what are the output layers of the visual cortex
1) extrastriate cortex - different pathways predominantly from layers 2/3 and 4b
2) LGN - feedback comes primarily from layer 6
3) superior colliculus- predominantly from layer 5
what are the basic organisations of V1
- retinotopic organisation
- ocular dominance
- orientation selectivity
- direction selectivity
- depth selectivity
- colour selectivity
retinotopic organisation
- nearby cells in V1 represent nearby locations in the visual field in other words
- neighbouring regions of the retina are represented by neighbouring in v1
Ocular dominance
- Axons from the different layers of the LGN (eyes) go to separate “zones” in the cortex, forming “ocular dominance columns”
- responsible for integration of visual information from both eyes. they play a crucial role in binocular vision and depth perception.
orientation selectivity
- certain cells are more responsive to horizontal, vertical or diagonal orientations.
what are simple cells
1) elongated antagonistic RFs
2) highly selective for size of stimulus (SF) and orientation
3) small RFs -> likes high SFs
4) large RFs -> likes low SFs
5) RFs sizes increase with eccentricity
6) sensitive to spatial phase
Complex cells
- complex cells sum up multiple simple cells to produce a receptive field that has overlapping ON and OFF responses
- larger RF size
- insensitive to phase
- usually more transient than simple cell
Hypercomplex cells
- Hypercomplex cells are length tuned – they respond maximally to a defined length and are inhibited if the stimulus is longer than this
direction selectivity
- Direction selectivity mostly found in layer 4B in V1.
- This layer projects out of V1 to V2 and also directly to MT – two regions involved in motion processing
