W5: Vision Flashcards
define
Retinofugal Projection (RFP)
the neural pathway that leaves the eyes, beginning with the optic nerve
suffix “-fugal” = to flee, move away; move away from retina
define
Decussation
Crossing og a fiber bundle from one side of the brain to the other.
For reasons we do not understand, decussations are common in visual and motor systems.
Where is the decussation in the RPF?
Optic Chiasm
Here, the axons originating in the nasal retinas cross from one side or the other. Because only the axons cross, we say there is a PARTIAL DECUSSATION of the RFP at the optic chiasm.
The axons of the RFPs form the optic tracts which run just under the pia.
define
Full Visual Field
The entire region of space (measured in degrees of visual angle) that can be seen with both eyes looking straight ahead.
define
Binocular Visual Field
The region of space, the central portion of both visual hemifields viewed by both retinas.
define
Lateral Geniculate Nucleus (LGN)
Relay stations between the eye and the visual cortex, responsible for efficiently transmitting signals in a linear fashion to the central visual system for processing information related to shape, colour motion
Eye -> LGN -> V1 -> LGN -> …
define
Optic Radiation
Projections of LGN axons to the PVC
state
Two Major Targets of the 2 Optic Tracts
Right LGN & Left LGN in dorsal thalamus
Arrangement of LGN & Arrangement of Input from the Eyes
6 layers of cells, numbered 1-6, starting with most ventral layer
- stacked on top of one another like pancakes, except they bend around the optic tract like a knee joint (geniculatus = “like a little knee”)
- At the LGN, in the different layers, input from the 2 eyes is kept separate
Components of LGN
moving dorsally
Layers 1 - 2: Magnocellular LGN layers, containing larger neurons (receiving input from M-type cells)
Layers 3 - 6: Parvocellular LGN layers, containing smaller neurons (receiving input from P-type cells)
Layers K1 - K6: Koniocellular LGN layers, lying ventral to each of the 6 principal layers (receiving input from nonM-nonP cells)
Properties of Magnocellular Cells
- Large center surround
- Transient bursts of APs
- Low resolution
- Insensitive to different wavelengths
Properties of Parvocellular Cells
- Small center surround
- Sustained increase of AP frequency
- Many exhibit colour opponency
define
Colour Opponency
tbd
state its various names
Primary Visual Cortex
- PVC
- striate cortex
- Brodmann’s Area 17
- V1
define
Retinotopy
Organisation whereby neighbouring cells in the retina feed information to neighbouting places in their target structures
e.g. in this case, LGN and V
in this way the 2D surface of the retina is mapped onto the 2D surface of the subsequent structures
list
3 Points to Remember ab/ Retinotopy
- Mapping of visual field onto a retinotopically organised structure is often distorted (rep. of visual field distorted in the central few degrees as they are overrepresented / magnified with overlapping input from several sources)
- When the retina is stimulated by a point of light, the activity in the V1 is a broad distribution with a peakat the corresponding retinotopic location
- There are no pictures in the PVC – don’t be misled by word “map”. Perception is based on the brain’s interpretation of distributed patterns of activity(electrical / chemical)
Cytoarchitecture of the Striate Cortex
2mm full thickness, made up of at least 9 distinct layers
I, II, III, IVA, IVB, IVC(α, β), V, VI (+ White Matter)
this anatomical separation into layers suggests there is a division of labour in the cortex, similar to the one seen in LGN
list relationships (primary 2, secondary 1)
Projection from LGN to Striate Cortex
Primary Projections:
Magnocellular LGN Neurons -> IVC alpha
Pervocellular LGN Neurons -> IVC beta
Secondary Projection:
Koniocellular LGN Neurons -> I & III
Binocular Receptive Fields
When neurons have 2 receptive fields, on in the ipsilateral eye, on in the contralateral eye.
Retinitopy is preserved so both receptive fields are precisely placed on the retinas such that they are “looking” at the same point
=> stereoscopic vision + depth perception
