Vision Flashcards
Non-stretchy sclera plus intra-ocular pressure
Keep eye rigid, back surface smooth and stable, and distances between optics and retina correct
Iris
Controls how much light enters the eye via the pupil.
Cornea
Primarily responsible for focusing light rays
Lens
Provides additional, variable “fine” focus
Rod photoreceptors
Night vision
Cone photoreceptors
Day vision
Cone photoreceptors
Membrane - Outer segment
Nucleus and Axon - inner segment
Synaptic terminal (the end)
Retinal processing
You can only see in detail with the very centre of your visual field
Loss of peripheral vision
- Glaucoma
* Retinitis Pigmentosa
Loss of central vision
Age-related macular degeneration
Peripheral vision
Majority of the retina serves only coarse vision:-
• the visual image is optically blurred.
• the cone photoreceptors are large and widely spaced (separated by larger number of rods).
• the signals from many cones converge onto single ganglion cells.
Central vision is very special
The fovea is specialised for high resolution:-
• good focus – overlying layers are absent
• only cone photoreceptors, primarily red and green
• which are narrow and closely packed
• the signals from the photoreceptors are kept separate throughout the primary visual pathway
Off centre retinal ganglion cells
- central photoreceptor depolarised (red) by decreased illumination
- bipolar and ganglion cells depolarised by excitatory synapses
On centre retinal ganglion cells
- central photoreceptor hyperpolarised (blue) by increased illumination
- bipolar cell depolarised by inverting synapse excites ganglion cell
Parvocellular
- small field with strong surround
- fine resolution
- accurately follows changes in light
- needs stable image
Magnocellular
- large field with weak surround
- coarse resolution
- transient responses to change
- responds well to fast movement
Parvocellular
- selective inputs from “red” or “green” photoreceptors
- by comparing these responses they can encode wavelength
- RED vs GREEN
Bistratified
- elective inputs from “blue” or “red+green” photoreceptors
- by comparing these responses they can encode wavelength
- BLUE vs YELLOW
