lecture 7 Flashcards
Vision 2: central processing of vision - targets of ganglion cells - visual pathway - extrastriate processing of visual information -- dorsal pathway -- ventral stream
1
Q
Why can’t David see properly?
A
- David, 67 year old retired journalist.
- driving a few days ago, when he noticed a translucent white curtain obscuring the upper part of his vision
- he felt his motor perception was off
- when he bumped into an old friend in the street, the friend started walking next to him but looked unfamiliar, only recognised the person when he spoke
- couldn’t recognise his wife or family until they spoke
- couldn’t recognise a picture of the prime minister
2
Q
What are ganglion cells?
A
Output neurons of the retina
- M (parasol) ganglion cells
- magnocellular = large
- large receptive fields;
- motion detection, flicker and analysis of gross features
- not very good at fine detail
- P (midget) ganglion cells
- parvocellular = small
- more numerous
- visual acuity and colour vision
- about 80% of ganglia in the retina
Output targets of ganglion cells
- many brain regions
- majority target the lateral geniculate nucleus (thalamus)
3
Q
What is the visual pathway?
A
- retina
- optic nerve
- LGN
- Optic radiations
- visual cortex
4
Q
What is the optic chiasm?
A
- the fibres from right and left optic nerves combine to form the Optic chiasm
- lies at the base of the brain, anterior to the pituitary
- nasal fibres cross at the optic chiasm
- right visual hemifield as “viewed” by the left hemisphere
- left visual hemifield is “viewed” by the right hemisphere
- partial decussation of the visual pathway
5
Q
What is the lateral geniculate nucleus?
A
- functional streams of information passed to LGN
- six layers (numbered 1-6)
- contain 2 types of cells
- magnocellular layers = layers 1, 2: receive input from M (parasol) ganglion cells
- parvocellular layers = layers 3 - 6: receive input from P (midget) ganglion cells
- roughly alternating which eye to which layer = i.e. each layer only receives info from one eye
6
Q
What are optic radiations?
A
- axon tracts going from LGN to visual cortex
- very expensive white matter tracts
- wrap around ventricles
7
Q
What happens in the primary visual cortex?
A
- LGN neurons project to the primary visual cortex
- area 17
- occipital lobe around the calcarine fissure
- located right at the back of the brain
- each half of the visual field is represented on the contralateral visual cortex
- retinotopic organisation:
- neighbouring cells within the retina project to neighbouring cells in the LGN and Vis cortex
- central vision is interpreted by the part of the brain at the very very back
- as you go along the calcarine fissure towards the front of the head, those are the parts that are interpreting more peripheral fields of vision
8
Q
Where is the input to the primary visual cortex?
A
- like all parts of the cortex (a half centimetre surface of the brain), the primary visual cortex has six layers, histologically speaking
- the most important layer in the sensory cortices is layer 4
- in particular layer 4C
- throughout the entire sensory cortices layer 4C is the input layer from the thalamus
- the primary visual cortex therefore gets it information from the LGN in layer 4C
9
Q
What do cortical neurons best respond to? How does this demonstrate their function?
A
- orientation selectivity: neurons respond best to bars moving in a particular orientation
- through the layers of neurons they have overlapping receptor fields
- they are all encoding for the same place in your retina
- each of these neurons (in a vertical line) respond to bars of the same orientation
basically:
- the primary visual cortex is made up in a very ordered way
- you have layers of neurons that all respond in the same way - different slabs respond in different ways
- part of this idea that you are breaking down an image and your brain has to slowly put it back together
10
Q
What are ocular dominance columns?
A
- Input from the LGN is segregated into small regions of the primary visual cortex - called an ocular dominance column
- information input is roughly right, left, right, left etc in 4C
11
Q
When do we get mixing of information from each eye?
A
- segregation of information from different types of LGN cells
- M type GC/LGN input to layer 4C-alpha
- P type GC/LGN input to layer 4C-beta
- mixing of information from each eyes occurs in IVB and Layer III
- processing of information occurs AFTER the primary visual cortex
12
Q
What was the problem with the german woman, (desola?)?
A
- 43 yo woman presents because she is having difficulty crossing the road – cars appear out of “nowhere”
- pouring coffee is very difficult – appears frozen until cup overflows
- visual acuity and colour vision normal
- visual fields: normal
13
Q
What are the two large cortical streams of visual processing?
A
- a dorsal pathway (where?)
- a vental pathway (what?)
14
Q
what is area MT?
A
- in the dorsal stream
- Area MT: middle temporal lobe is an area specialised for processing object motion
- receives retinotopic information from a number of cortical areas including V2 and V3
- receives input from cells in layer IVB of the primary visual cortex (i.e. M-type GCs/LGN)
15
Q
How does ‘comparison’ affect what we see?
A
- the ganglion cells (in particular P cells) detect comparisons in colour - not do I detect a colour, but what is the colour in the context of its surroundings important comparisons are red vs green and blue vs yellow
