Lecture 10 - introduction to visual pathways and circuits Flashcards
What stream is concerned with the recognition of an object?
ventral (what) stream
What stream is concerned with spatial location?
dorsal (where) stream
What visible light range is our visual system sensitive to?
380nm (red) to 740nm (purple)
Describe the journey of light/information from entering the eye to the brain.
- information goes to the cornea
- anterior chamber
- pupil
- lens
- vitreous humour
- retina and fovea
- optic nerve
- brain
How is the retina organised?
- seven cell types
- three nuclear layers
- 2 synaptic layers
What are the three nuclear layers of the retina?
- outer nuclear layer
- inner nuclear layer
- ganglion cell layer
What type of cells does the ONL contain?
photoreceptors - rods and cones
What type of cells does the INL contain?
horizontal cells
bipolar cells
amacrine cells
What type of cell does the GCL contain?
ganglion cells
What are the 2 synaptic layers of the retina?
- outer plexiform layer
- inner plexiform layer
What does the OPL make connections between?
synaptic connections between photoreceptors, horizontal and bipolar cells
What does the IPL make connections between?
synaptic connections between bipolar, amacrine and ganglion cells.
What photoreceptors are responsible for peripheral vision?
rods (120 million) distributed through the retina
Describe rod cells.
- responsible for our ability to see in dim light (scotopic vision)
- high sensitivity
- monochromatic
- low resolution images
- good motion detection
What photoreceptors are responsible for central vision?
cones (6 million) concentrated around the fovea
Describe cone cells.
- work only in daylight (photopic vision)
- low sensitivity
- chromatic
- high spatial acuity
- narrow angle of coverage
How many types of cone cells are there?
3 - S, M and L
What are ganglion receptive fields?
each part of a scene is detected by many photoreceptors.
the receptive field is the part of the retinal ganglion cell that can gather information from the visual field
What do ganglion cells respond best to and why is this important?
they respond best to contrast not light.
this means they have antagonistic centre-surround receptive fields (the centre must be mainly light while the surround mainly dark vice versa)
Do ganglion cells on the fovea have a small or large receptive field?
small - can see more details
Do ganglion cells in the periphery have a small or large receptive field?
large - can see less details
Projections from the retina to central targets are organised in what fashion?
topographic
What does LGN stand for?
lateral geniculate nucleus
What is the role of the LGN?
- brings retinotopic maps from both eyes into register to make it easier for the cortex to combine inputs from the two eyes.
- other functions - not as well understood.
Describe the primary visual cortex V1.
Stria of Gennari (band of myelinated axon (from LGN cells)) running parallel to the surface of the cortex along the calcimine fissure of the occipital lobe.
How many layers does V1 have?
-6
- main input from LGN to layer IV
How is the striate cortex organised?
in ocular dominance columns, alternating between left and right eyes
What are the 3 main cell types in V1?
- simple cells
- complex cells
- hypercomplex cells
Describe simple cells.
- sum inputs from LGN with neighbouring RFs to build an elongated RF
- orientation selective
- monocular or binocular
- have separate on and off subregions
- perform length summation
Describe complex cells.
- receive input from simple cells
- orientation selective
- spatially homogenous RFs so no on/off subregions
- nearly all binoculars
- perform length summation
Describe hypercomplex cells.
- inhibitory flanks in the ends of RF
- response increases with increasing bar length up to some limit
- as the bar becomes longer, the response is inhibited.
Describe the pathway of the ventral (what) stream.
V1 > ventral peristriate cortex > inferotemporal cortex
Describe the pathway of the dorsal (where) stream.
V1 > dorsal peristriate cortex > posterior parietal cortex
