Flashcards in lecture 7 Deck (18):
Why can't David see properly?
- 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
What are ganglion cells?
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)
What is the visual pathway?
- optic nerve
- Optic radiations
- visual cortex
What is the optic chiasm?
- 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
What is the lateral geniculate nucleus?
- 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
What are optic radiations?
- axon tracts going from LGN to visual cortex
- very expensive white matter tracts
- wrap around ventricles
What happens in the primary visual cortex?
- 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
Where is the input to the primary visual cortex?
- 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
What do cortical neurons best respond to? How does this demonstrate their function?
- 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
- 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
What are ocular dominance columns?
- 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
When do we get mixing of information from each eye?
- 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
What was the problem with the german woman, (desola?)?
- 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
What are the two large cortical streams of visual processing?
- a dorsal pathway (where?)
- a vental pathway (what?)
what is area MT?
- 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)
How does 'comparison' affect what we see?
- 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
What is the ventral stream?
- V1, V2, V4
- Area V4/IT
-- receives input from the blob and interblob regions of the primary visual cortex via V2
-- neurons in V4 have large receptive fields that are both orientation selective and colour selective
-- important for perception of shape and colour
What is Area IT?
- inferior temporal
- a major output area of V4
- neurons respond to a wide variety of abstract shapes and colours
- important for visual memory and perception
- important for perception of faces
- you get an additive effect as you go up the visual pathway beyond the primary cortex
- object recognition