Week 3: Higher-Level Vision Flashcards
What is the Visual Pathway?
Optic nerve → optic chasm (contralateralization) → Optic Tract → LGN → Optic Radiation → V1
What is the Lateral Geniculate Nucleus?
- part of the thalamus
- Relay station of the brain
- Processes the contralateral visual field
- Left and right are information is kept separately in different layers
What are the layers of lateral Geniculate nucleus (LGN)?
Ventral (inner)
1: Magnocellular: contralateral eye
2: magnocellular : ipsilateral eye
3: parvocellular: ipsilateral eye
4: parvocellular: contralateral eye
5: parvocellular: ipsilateral eye
6: parvocellular: contralateral eye
dorsal (outer)
- also 6 koniocellularl layers between magnocellular and parvocellular layers
Where does the LGN project to?
- the V1
- Also to superior colliculi and extrastriate
What are different type of ganglion cells in the retina?
Parvocellular ganglion cells, magnocellular ganglion cells, koniocellular ganglion cells
what are parvocellular ganglion cells?
- Also „small“ cell, „midget“ cell
- receive input from midget bipolar cells that only integrate information coming from few photoreceptors
- Their axons land in the parvocellular layer of LGN
- Around 70% of all ganglion cells
- Accurate Color vision, antagonism for red-green
What are magnocellular ganglion cells?
- Also „large“ cell, „parasol“ cell
- receive input from diffuse bipolar cells from a large pool of photoreceptors (more from rods)
- Project to magnocellular layer of LGN
- 8-10% of ganglion cells
- Motion (fast moving stimuli) achromatic
What are koniocellular ganglion cells?
- also „sand“ cells
- Blue-yellow pathway
- Projects to koniocellular layer of LGN
What are the Color systems?
- light wavelengths are on a single dimension
- Perceived Color space is circular
- Photoreceptors code colors in trichromatic system (short-blue, medium-green, long-red)
- Opponent ganglion cells (blue-yellow, red-green)
What is the opponent Color system?
blue-yellow ganglion cell is getting input from both short (blue) cones and medium/long cones (green and red)
What is the V1?
The primary visual cortex = the striate cortex
What are the different cells in the V1?
Simple cells
Complex cells
End-stop cells
What does a neuron in the visual system respond to?
Tuning properties of the neuron
What are simple cells?
- Have tuning curves
- respond to preferred orientation of a line in receptive field in a position-specific manner
- Information from multiple Center-surround cells in LGN
- Formed by linking of adjacent LGN cells with circular receptive fields
- Linearity
- Also sensitive to edges
What are complex cells?
- Pool information from multiple simple cells that share a common orientation preference
- Activation no matter where the stimulus is in receptive field
- Activation to a specific orientation and movement in specific direction
- Spatial invariances (no linearity)
- Allows for lateral inhibition
What are end-stop cells?
- Have properties of both simple and complex cells
- Outside of the receptive field affects the firing of the cell
- Play important role in detecting luminance boundaries and discontinuities
- Decreasing firing rate with increasing length of stimulus beyond their receptive fields
What is a Gabor?
- great stimuli to activate V1 cells
- Used all The time in visual experiments
Why do we need models for responses in V1?
due to the scale of computation performed here
What is a Gabor filter?
- a model of simple cell responses in V1
- Constitutes a sinusoid multiplied with a Gaussain window
What does applying a Gabor filter mean?
- input image is convolved with all the Gabor filters
- Result: some patterns are highlighted/ enhanced
- Gives the highest response at edges and points where texture changes
What does applying a Gaussian filter mean?
- Center a kernel on a pixel
- Multiply the pixels under that kernel by the values in the kernel
- Sum all those results
- Replace the Center pixel with the sum
→ process is known as convolution
What is largescale topography of V1?
Hypercolumns consisting of:
- orientation columns
- Ocular dominance columns
What is the „icecube“ model?
- orientation columns: cells firing for a given orientation are grouped together
- Ocular dominance columns: cells for the same eye are grouped together
What is the map of orientation selectivity?
Color blobs
