from lgn to cortex Flashcards
Axons of ganglion cell
Very long
Make contact with a subcortical structure on their side or cross over
Vice versa representation
Left visual field represented in right LGN and vice versa
Contralateral projection
Nasal area
Ipsilateral projection
Away from nose
Same side of the brain
Retinotopic maps
Neighbouring relations that exist in the retina exist in the LGN
Next door neighbour neurons stay neighbours in the LGN
LGN
Lateral geniculate nucleus
Monocular cells in LGN
Each layer gets input from only one eye
Form a retinotopic map of half a visual field
Layers 1 and 2
Large cell bodies
Magnocellular layers
Receive input from M ganglion cells - rods
High contrast sensitivity and low spatial resolution
Process coarse features and motion
Coarse feature is what you can see when you squint
Layers 3 to 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
Fine details are seeing leaves on trees etc
Koniocellular cells
Between layers
Receive input from S-cones
Function of LGN
Richly connected to many other parts of the brain
Locus at which retinal information can be modulated by brain areas
Part of the brain where retinal informational processing can be modulated by information processing that happens in other parts of the brain
V1
Organised in a retinotopic map - spatial neighbouring relations are maintained
First stage of information processing where we combine info from both eyes
Map is distorted due to cortical magnification
Cortical magnification in V1
Much more space in the cortex so more neurons dedicated to processing some parts of the stimulus than others
Consequences of cortical magnification
More neurons the closer in to the visual field you go
Trade off between having a large visual field and some areas of high acuity
If we had same acuity that we have in the fovea across the whole visual field we would need a fucking huge brain.
Importance of eye movements
Move your eyes around to get the high resolution part of the visual system
Visual system patches up all the experiences in order to create a full picture of our visual field
Centre surround receptive field responds best to…
Spots of light
Neurons in V1
Elongated receptive field
Respond best to lines, bars and edges
Simple cell structure will connect to…
A large amount will connect to one V1 simple cell
One simple cell hit with light…
V1 cell may not respond
Beam of light across simple cells…
V1 will really strongly respond to light being beamed across the middle of the receptive fields
Simple Cell Receptive Fields
Similar to concentric receptive fields, with on centre and off centre
A lot more diversity, but they all have in common that they respond to longer bars of light
They respond to different orientations - orientation tuning
Orientation tuning
Will respond best when they fire to the orientation they are tuned to
Maximum response from the neuron when it is completely aligned with the faciliatory fields
Feature detectors and population codes
Neurons not only respond to orientation, but also to intensity of the stimulus
A weak optimal orientation stimulus will fire the same as a slightly slanted bright stimulus
Activity in context
Have to consider activity of all neurons in context to overcome the ambiguity of what kind of stimulus it is
Brain looks not just at one neuron but at relative activity of neurons that are attuned to different values of the same parameters
Complex cells
Complex cells would respond to certain orientations independently of where you present that orientation within the receptive field of that cell
Doesn’t matter where the orientation is, it will respond
If you shine light over the whole field, complex cells wont change their response rates
Hypercomplex cells
Hypercomplex cells are similar to complex cells in that they don’t have clearly defined inhibitory and facilitative regions
But if you extend the bar of light beyond the receptive field, that would reduce the firing rate - endstopping
