Visual System Flashcards
Describe the visual pathways in humans
- Retinal ganglion cells
optic nerve - optic chiasm
optic tract - LGN
optic radiation - striate cortex
Describe the components of the LGN in primates
- 6 layers
- magnocellular LGN cells: large receptive fields, transient response, detect movement
- parvocelular LGN cells: small receptive fields, sustained response, detect edges, shape, contrast)
- koniocellular LGN cells: layers between M&P layers, nonM & nonP type RGCs, tiny, detect colour
Describe the organisation of the LGN
Info from the 2 eyes is kept separate at LGN
* M type to layers 1 & 2
* P type to layers 3-6
* crossed to layers 1, 4, 6
* uncrossed to layers 2, 3, 5
* nonM/nonP to K layers
Describe decussation in the visual system
the nasal half from each eye decussate at the optic chiasm
Results in decussation of visual fields & enables binocular vision
Explain post-to-point projection in the visual system
-each point in the visual field is kept discrete on the LGN and on the striate cortex (retinotopic mapping)
-lower half of visual field maps to medial LGN maps to Area 17 BELOW calcarine sulcus
-upper half of visual field maps to lateral LGN maps to Area 17 ABOVE the calcarine sulcus
What are the 2 pathways of the optic radiation?
- info from lower half visual field goes straight from LGN to V1
- info from upper half goes through Meyer’s loop then to V1
Where is the striate cortex?
Area 17/V1
gyrus surrounding the calcarine sulcus on medial occipital lobe, extending down the logitudinal fissure
Describe the cytoarchitecture of the striate cortex
- 9 layers - layer IV (main input layer) split into IVA, IVB, IVC-alpha and IVC-beta
- pyramidal cells in III, IVB, V, VI
- inhibitory neurons in all layers
What are the inputs to the striate cortex?
- magnocellular inputs to layer IVC-alpha
(then project to IVB) - parvocellular inputs to layer IVC-beta
(then project to layers II & III) - koniocellular inputs to layers III & some I
Describe intracortical connections of the striate cortex
- vertically arranged to preserve the retinotopic map - forms “columns”
- some lateral connections in III & V (start of perceptual binding)
What is meant by “ocular dominance columns”?
The organisation of cells in striate layer IV based on input
Columns for the same point in the L & R eyes will be adjacent
What is meant by “blobs”
The organisation of cells in striate layers II & III based on input
Each blob is centered on an ocular dominance column
Received koniocellular input for colour
What is a cortical module?
A section of striate cortex able to analyse every aspect of a portion of the visual field
ie all cortical layers, including multiple ocular dominance columns & blobs
What are efferents from the striate cortex?
Note: when info leaves striate cortex, some perceptual binding has taken place but info is still very raw - have not attached meaning yet
- From Layers II, III, IVB - to other cortical areas for higher processing & binocular integration (association & commissural tracts)
- From Layer V - to superior colliculi & pons for visual reflexes and cerebellar visual input (projection tracts)
- From Layer VI - back to the thalamus LGN
Where are the visual association areas?
occipital lobe surrounding V1
