lecture 7

Question 1

Why can’t David see properly?

Answer 1

• 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

Question 2

What are ganglion cells?

Answer 2

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)

Question 3

What is the visual pathway?

Answer 3

• retina
• optic nerve
• LGN
• Optic radiations
• visual cortex

Question 4

What is the optic chiasm?

Answer 4

• 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

Question 5

What is the lateral geniculate nucleus?

Answer 5

• 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

Question 6

What are optic radiations?

Answer 6

• axon tracts going from LGN to visual cortex
• very expensive white matter tracts
• wrap around ventricles

Question 7

What happens in the primary visual cortex?

Answer 7

• 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

Question 8

Where is the input to the primary visual cortex?

Answer 8

• 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

Question 9

What do cortical neurons best respond to? How does this demonstrate their function?

Answer 9

• 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

Question 10

What are ocular dominance columns?

Answer 10

• 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

Question 11

When do we get mixing of information from each eye?

Answer 11

• 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

Question 12

What was the problem with the german woman, (desola?)?

Answer 12

• 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

Question 13

What are the two large cortical streams of visual processing?

Answer 13

• a dorsal pathway (where?)

- a vental pathway (what?)

Question 14

what is area MT?

Answer 14

• 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)

Question 15

How does ‘comparison’ affect what we see?

Answer 15

• 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

Question 16

What is the ventral stream?

Answer 16

• 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

Question 17

What is Area IT?

Answer 17

• 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

Question 18

So what was wrong with david?

Answer 18

• lesion affecting the fusiform gyrus (area IT)
• also caused a visual field defect
• • had to be from the chiasm back because it was the same side of each eye that had the vision problem