33 - Higher Visual Processing Flashcards
Outputs of V1
1
2
3
Layer 3 and 4B : outer cortical areas
Layer 5: Superior colliculus and pons
Layer 6 LGN
Inputs to V1
1
2
3
1) Segregation of M and P pathways.
2) M cells terminate in layer 4Calpha, then go to 4B.
3) P cells terminate mainly in layer 4Cbeta.
Where do M cells from LGN project to?
Layer 4Calpha
Types of cells in layer 4B
Some cells show orientation selectivity.
Some neurons show preference for the direction of motion of the target
‘Directional selectivity’
How do cells show directional selectivity?
Fire (in layer 4B) when a visual stimulus moves in a certain direction.
Cell doesn’t fire when visual stimulus moves in the opposite direction.
Proportion of cortex involved in vision
~40%
Two parallel visual streams
1) Dorsal pathway (Where?)
2) Ventral pathway (What?)
Area of the brain specialised for processing object motion
Middle temporal lobe (part of the dorsal stream).
From where does the middle temporal lobe receive input?
V1 and layer 4B (cells here show directional sensitivity)
Characteristics of neurons in the middle temporal lobe 1 2 3 4 5
1) Neurons have large receptive fields.
2) Respond to stimulus movement.
3) Almost all the cells in area MT are directional selective.
4) Respond to different types of motion
Eg drifting spots of lights.
5) This area has direction-of-motion columns
Response of cones to light
All will respond to wavelengths of light, but will respond maximally to one particular wavelength.
Relative amounts of activation of R/G/B cones informs colour
Neural mechanism of colour vision 1) 2) 3) 4)
‘Colour opponency’
1) P ganglion cells exhibit a colour opponent centre-surround.
2) Some P ganglion cells are excited by red falling on their centre and inhibited by green following on the surround.
3) Others are excited by blue or yellow lights falling on their receptive field centre.
4) Colour perceived is determined by the activity of ganglion cells.
How can a red centre, green surround ganglion cell receptive field be maximally stimulated?
Red light on centre, green light on surround.
Red and green cells are hyperpolarised to light.
Horizontal cells are hyperpolarised too, leading to less GABA release onto red centre cells.
Releases more glutamate onto ganglion cell, leading to increased or decreased firing to brain from GC (depending on whether it is an ON or OFF GC).
Optimal stimulus for brain to interpret colour
Either maximal firing from GCs or minimal.
Gradations are harder to interpret.
Ventral stream path
V1 -> V2 -> V4
