L3 - LGN to Cortex Flashcards
1
Q
What is the structure of the early visual system?
A
- Optic nerve
- Optic chiasm
- Optic tract
- LGN: first relay station where neurons in retina make their way to, they become activated here
- Optic radiation
- Primary visual cortex (also known as V1)
2
Q
How is the visual field split?
A
- Left visual field is projected onto the lateral part of the right eye and the nasal part of the left eye and vice versa
- Once stimulus has been projected onto the retina
- Nasal parts of retina close to close project to LGN on other side as they project contralaterally (project to LGN on the other side)
- Lateral part of retina projects ipsilaterally - same side of brain (away from nose)
- Right LGN has 6 different layers containing a map of the left part of the visual space and vice versa for the right eye.
- Each LGN gets input from both eyes, but each layer gets inputs from only one eye - cells are monocular
- Retinotopic map: neighbouring relationships existing in the retina are retained in the LGN, neurones in retina that sit next to each other, they project to neurones in the LGN that also sit next to each other - neighbouring relation is retained
3
Q
What is the physiology of LGN?
A
- Each layer contains monocular cells that form a retinotopic map of half a visual field
- Layers 1 & 2: Large cell bodies, magnocellular layers, receive input from M ganglion cells (rods), high contrast sensitivity and low spatial res (receives from RODS), process coarse features in env and motion
- Layers 3&6: small cell bodies, parvocellular layers, receive input from P ganglion cells (cones), low contrast sensitivity and high spatial resolution, process fine features and colour
- Koniocellular cells between the 6 layers: receive input from S-cones = important for colour vision
4
Q
What is the function of LGN?
A
- Layers in LGN respond similarly as their input ganglion cells
- Also have centre-surround receptive fields
- Richly connected to many other parts to the brain
- Locus at which retinal info can be modulated by brain areas
- More than just a relay station
5
Q
What is the process from the LGN?
A
- LGN projects to v1, the primary visual cortex, e.g left LGN to right hemisphere
- Most neurons in V1 are binocular = receive input from both eyes
- V1 organised in a retinotopic map = neighbouring relations maintained
- Map is distorted of input = cortical magnification because there are more neurons that are dedicated to processing the part of the stimulus over other parts of vision
6
Q
What are the consequences pf cortical magnification?
A
- That we have the same number of neurons processing the stimuli e.g small letter has more neurons processing but big letter has less this is why they are all equally hard/easy to see
- When they are all the same size, it becomes harder to identify the letters in the periphery due to less neurons processing it
7
Q
Why do we not use the same amount of neurons for the whole visual field?
A
- Trade-off between having a large visual field and having some areas of high acuity (how good can you resolve details? Like resolution in bio)
- If we want the same activity of our fovea, we would need eyes and brains multiple times the size of our skull
- Highlights importance of eye-movements
8
Q
What are receptive fields in V1?
A
- Retinal ganglion cells and LGN neurons have centre-surround receptive field = respond best to spots of light - allows high contrast very locally
- Neurons in V1 have elongated receptive fields that respond best to lines, bars and edges
9
Q
What happens when we look at an array of ganglion/LGN cells that connect to a V1 simple cell that sits in the primary visual cortex?
A
- If a bar of light changes only one of five, it is not enough to trigger the threshold to reduce production in the V1 cell
- If you shine a light all over the ganglion cell = it stays at baseline as all processes cancel out
- If you have a bar of light that only shines in the centre of the ganglion cells = V1 responds
10
Q
What are simple cell receptive fields?
A
- Some have a centre that is inhibitory and faciliatory
- They all respond to different orientations because they respond to bars of light
11
Q
What is orientation tuning?
A
- Have neuron with receptive field, you get a maximum response from it depending on how the light shines on it.
- As you rotate the bar, more light shines on the inhibitory parts, so responding becomes lower until it does not respond at all
- Neurons have one orientation they respond best to
12
Q
What are feature detection and population codes?
A
- Neurones look at orientation AND intensity of stimulus
- Present a perfect stimulus but less intensity = will have same reaction as = Okay stimulus but very intense
- Do not know if the neuron responds because of the orientation of the neuron or the brightness of the stimulus
13
Q
How to overcome feature detection (ambiguity)?
A
- Consider activity of neurons compared to other neurons
- Brain looks at relative activity of neurones that are tuned to different values of the same parameter e.g tuned to slightly different orientations and measures the relative activity
- Populations of cells tuned to different orientations allows to disambiguate intensity and orientation - features are coded in a population code
- Relative activities matter not absolute activity
14
Q
What are simple cells?
A
- Depending on where you shine the light, the activity goes up/down depending on if it shines in fac/inhib sections, when shone on everything = no response as they cancel out
- Clearly defined fac/inhib
15
Q
What are Complex cells?
A
- Another cell in V1
- Just responds to a certain orientation, no matter where it shows up in its receptive field
- When shine across all of their receptive field = no change in baseline
