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
What is the Superposition of multiple maps?
- all visual areas provide a map of the external world, but the maps represent different types of information
- There are multiple maps in V1:
- Spatial frequency map
- Ocular dominance map
- Orientation map
- Map of Space (retiniotopic)
What is a retinotopic map?
a clear mapping between spatial location in the visual field and a neural representation → what is close to each other in the world, is close to each other in the brain
What are the basics of retinotopic mapping?
- polar coordinate system
- Left hemisphere → right visual hemifield
- Right hemisphere → left visual hemifield
- Eccentricity
- Mirror symmetry
- Fovea is overrepresented (makes up 0.01% of retina, but 8-10% of V1)
- Inverted: upper part of visual field is below the calcarine sulcus
- V2 and V3/VP each represent a quarter of the receptive field
How are boundaries between visual areas determined?
Functionally by topographic reversals in retinotopic maps
What are Cells in MT tuned to?
- Stimulus falls in receptive field
- direction of movement
- Speed of visual stimuli
What can lesions of MT lead to?
- cerebral akinetopsia
→ The inability to perceive (fast) movement, despite spared perception of static images
What is the color area of the brain?
V4/V8
What can lesions in the color area lead to?
Hemi-achromatopsia
What is hemi-achromatopsia?
The inability to experience Color in one of the visual hemifields
What are content-specialised regions in extrastriate visual cortex?
- MT+
- V8
- LOC
What is the MT+?
Motion Processing Complex
What is the V8?
Color area (also known as V4)
- neurons are active when coloured images are used
What is the LOC?
Lateral Object Recognition Complex (pFs+LO)
What are the different visual streams?
Ventral and dorsal
What is the ventral visual stream?
- determines „What is it?“
- From occipital lobe to inferior temporal (IT)
What is the dorsal visual stream?
- determines „Where is it?“ „How?“
- From Occipital Lobe to Parietal Lobe
What is the calcarine sulcus?
Where the primary visual cortex (V1) is concentrated
