Visual Processing (Dr. Bianco) Flashcards
What is receptive field (RF) ?
A RF is the region in space (part of body, visual field or auditory space) in which the presence of a stimulus will affect the electrical activity (change in MP or firing rate) of a neuron.
How are visual RFs measured ?
In degrees angle subtended by incident light that stimulates the neuron.
θ=2arctan(h/2D)
What discovery about GCs did Stephen Kuffler make in 1953 ?
How are their RFs referred to ?
That there were “ON centre” and “OFF centre” GCs.
The center and surround have opposite ON vs OFF responses.
Ganglion cell RFs are therefore referred to as Centre Surround Antagonistic Receptive Fields (CSARFs)
How are centre-surround receptive fields formed ?
By lateral inhibition from HCs.
How does lateral inhibition work ?
When does this system work best ?
GABA release from horizontals cells increases the hyperpolarization of the photoreceptor that it connects to, enhancing the response of the postsynaptic bipolar cells to light.
GABA release is max when surround region is DARK.
When is RGC activity maximal ?
Why is this the case ?
When the difference in light levels in centre compared to surround is greatest. This is contrast enhancement.
What are the 2 major population of RGCs ?
What are there properties ?
Parvocellular/midget RGCs:
1. Can be either ON or OFF centre
2. Have sustained responses to light
3. Small physical size and small receptive fields
4. Occupy mostly the centre of the visual field (fovea)
5. Carry information about colour
6. Center and surround sum linearly
Magnocellular/parasol GGCs:
- Can be either ON or OFF centre
- Have transient responses to light
- Large physical size and large receptive fields
- Are distributed across the whole retina
- Center and surround do not always sum linearly
- Receives info from many photoreceptors (via BPs)
Which type of RGCs exhibit color opponency ?
Parvocellular RGCs exhibit color opponency (red-green or M-L, and blue-yellow or S (L+M).
These is no such opponency in magocellular RGCs: M, L (and rare S) are mixed only to detect difference in luminance levels
What is the opponent “color opponent” theory of color vision ?
The opponent-process theory of color vision suggests that our ability to perceive color is controlled by three receptor complexes with opposing actions. These three receptors complexes are red-green, blue-yellow and black-white.
According to the opponent-process theory, these cells can only detect the presence of one color at a time because the two colors oppose one another. Hence we cannot see greenish-red because the opponent cells can only detect one of these colors at a time
Are RGCs uniformly distributed across the retina ?
No, they from mosaics.
What are bistratified RGCs ?
Bistratified retinal ganglion cells have been identified only relatively recently. About 10% of all retinal ganglion cells are bistratified cells, and these cells go through the koniocellular pathway. They receive inputs from intermediate numbers of rods and cones. They have moderate spatial resolution, moderate conduction velocity, and can respond to moderate-contrast stimuli. They may be involved in color vision. They have very large receptive fields that only have centers (no surrounds) and are always ON to the blue cone and OFF to both the red and green cone.
Where do the axons of RGCs directly project to ?
Superior Colliculus: orienting and avoidance responses, eye movements
Pretectum: pupillary reflex
Lateral Geniculate Nucleus (90% of axons in primate), the main route to visual cortex: visual perception
Which RGCs from the retina cross the optic chiasm to go the the contralateral visual cortex ?
RGCs on the nasal retina cross the optic chiasm and project to the contralateral visual cortex while the RGCs on the temporal retina project to the ipsilateral visual cortex.
How are the optic pathways to the visual cortex organized ?
All functionally parallel pathways are kept separate on their path to the visual cortex and are anatomically organized by R-eye/L-eye origin, by RGC type and by their location on the retina.
Where to RGCs synapse ?
How well is info transmitted ?
RGCs synapse in the LGN w/ LGN neurons (the only synapse in this pathway).
LGN neurons inherit their receptive field properties directly from their ganglion cell inputs, they are thus good relay cells.
How is the right LGN structured ?
6 layers:
1: L-eye, magnocellular
2: R-eye, magnocellular
3: R-eye, parvocellular
4: L-eye, parvocellular
5: R-eye, parvocellular
6: L-eye, parvocellular
In which cortical layer do most LGN neurons project ?
Layer IV in V1 (striate cortex or area 17 or primary visual cortex).
What are ocular dominance columns ?
How can this be proved ?
Ocular dominance columns refers to the axons from thalamic relay cells innervating layer IV in patches that are segregated according to eye origin. Right and left eye inputs alternate.
This can be proved by injective of radio-labelled proline into one eye and observing an autoradiograph obtained from a slice through layer IV of V1. We can observe that bright stripes are innervated by LGN axons from the injected eye.
If there are ocular dominance columns, where and how are inputs from both eyes mixed ?
Above and below layer IV via intracortical circuitry.
What is retinotopy ?
Retinotopy is retinal mapping, and refers to the fact that adjacent areas of visual space are mapped to adjacent physical areas on the retina and cortex (can be seen in monkey by activity dependent labeling of V1 while a visual stimulus is presented).
What is “orientation selectivity” in V1 ?
Orientation selectivity in V1 was a phenomenon discovered by Hubel and Wiesel in 1959-61. This refers to regions in V1 that respond selectively to the orientation of an edge or light bar. These regions have “elongated” ON and OFF sub-regions.
These can form orientation columns, w/ each column having its preferred orientation.
What 2 main types of cells have been discovered in V1 and what are their properties ?
Simple cells: 1. Respond best to edges and bars 2. Orientation matters 3. Can show direction selectivity 4. Precise location of stimulus is important Complex cells: 1. Respond well to bars, edges 2. Have orientation preference but location of light stimulus not important 3. Cannot be mapped into fixed ON and OFF regions 4. Can be direction selective 5. Formed by inputs from simple cells?
How is orientation selectivity in cortical neurons generated if the inputs from retina via LGN are not orientation tuned ?
That’s a an idea for your PhD if you’re really into vision.
What model did Hubel and Wiesel propose to explain visual cortex RF properties ?
That there was a convergence of the ON and OFF patterns in V1.
Overlapping ON and OFF centre RGC-LGN inputs could thus contribute to the RF properties of a cortical simple cell.
What is direction selectivity ?
Direction selectivity refers to the property of certain neurons in V1 who fire more AP when an object/image is moved in a particular direction. This can lead to the creation of complex direction maps.
What are muliple columnar representations ?
These are maps of V1 representating:
- orientation selectivity
- direction selectivity
- ocular dominance
- visual space (retinotopy)
