Visual perception Flashcards
What is the retina?
extension of the brain as it develops from a portion of the neural ectoderm that gives rise to the rest of the brain; it is a sheet of photoreceptors and connecting neurons
How is the retina organised?
- Photoreceptors – first link in visual processing, are situated being the neurons that connect them to the rest of the brain;
- There are different distribution of cones and rods in the retina
- Fovea - only cones; in the rest of the retina there is more rods than cons;
What is the role of rods?
mediate nocturnal vision – convergent pathway to ganglion cells, high sensitivity but poor spatial resolution (acuity)
What is the role of cones?
colour vision, direct pathway to ganglion cells; low sensitivity but good spatial resolution
What is the visual pathway?
Light–>photoreceptor (rod or cone)->bipolar cells ->ganglion cells ->optic chaiasm
Why are there different ganglion cells and photoreceprots concentrations in the eye?
- Processing different type of visual information and relaying it to different part of the btain
- Representation of particular point to the retina; axon streams toward the optic disc (out of the retinaàbecome malyenated-àform the optic nerve)
What is the optic chaism?
The crossover of the optic nerve
What is age-related macular degeneration?
- Progressive loss of central vision; main risk factor is age
- Progressive degeneration of photoreceptors due to leakage of blood vessels; gradual loss of retinal pigment; genetics
What is retinitis pigmentosa?
- Progressive loss of photoreceptors (rods then cones)
- Reduced night vision with progressive narrowing of the visual field; genetic
What is visual receptive field
the group of photoreceptors that converge onto a single ganglion cell; ganglion cell responds to stimulation of small circular areas if the retina, which define its receptive field.
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circular & size varies from small (in the fovea) to large (in the periphery)
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Two zones: OFF and ON, which produce different neural activity in response to retinal stimulation.
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two parallel pathways èON and OFF cells (crucial for contrast)
what are photoreceptors sensitive to?
Cells are sensitive to differences of light intensity reflected by the different surfaces rather than the absolute light intensity. For example, two patched that return equal amount of light are perceived as looking differently when placed at backgrounds with different luminosity
How is visual information processed?
- Each eye processes information from a different portion of the visual field
- Information is then combined in the visual cortex
What are the types of cells. Briefly describe them
- M cell (magnocellular) – large cell body, rich dendrites
- P cell (parvocellular) – small cell body, and limited dendrites
- K cell (koniocellular) – small as dust
- M and P cells consist of ON-center and OFF-center cells
- M cell à large receptive fields -à large objects, movement
- P cell -à small receptive fields -à small objects, details, colour
- K cellàvery large receptive fields-àonly center (no surrounds); involved in colour
What is the architecture of the lateral geniculate nuclei (LGN)
6 layers
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2,3 and 5 receive input from ipsilateral; retina
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1,4 and 6 from the contralateral retina
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Each layer has retinotopic representation; a stimulus in a certain position will activate cells within each layer
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Each layer has different cytoarchitecture – e.g. axons in the lower two layers are larger than in the other 4 layers.
M pathway à movement
P pathway -à colour and details
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Receptive fields in the cortex are more complex than those found in the retina and LGN
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Input from the LGN reaches the IV layer of visual cortex – cells show increased activity ONLY when the stimulus has linear properties (bar or line) but not to simple spot of light.
What is a simple cell?
It has discrete excitatory and inhibitory zones, larger than those found in the retina and LGN (rectangular size)
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Respond to different orientation – vertical, horizontal, oblique
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To some extend forms the input to complex cells
