Central Visual Pathways and Vision Flashcards
What is the normal visual pathway?
Eye Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Optic radiation Visual cortex
How is an image seen by our eye?
An inverse image
Describe how the light travels into the eye and get transformed into electrical energy.
Light is focused by cornea and lens
Traverses the vitreous humour
Travels through layers of retinal neurons
Before reaching photoreceptors
What are the different types of photoreceptors?
Rods Not present in central retina Photosensitive Dark adapt Many rods converge into one single bipolar cell
Cones Concentrated in fovea High acuity Day vision Colour vision Three types of cones (blue, red, green)
What are the different classes of neurons in the retina?
The retina has three major functional classes of neurons
Photoreceptors (rods, cones)
Interneurons (bipolar, horizontal and amacrine cells)
Combining signals from photoreceptors
Ganglion cells
Magnocellular (M) and parvocellular (P)
What are the different sensitivities of the different ganglion cell pathways of the retina?
Stimulus features : Magnocellular \/ Parvocellular Colour contrast : No \/ Yes Luminance Contrast : Higher \/ Lower Fine detail : lower \/ higher Motion : Higher \/ Lower
Tell me about the optic chiasm.
Axons from ganglion cells pass through optic disc to optic nerve and optic chiasm
Fibres from nasal retina cross to opposite site of brain
Temporal axons from ganglion cells do not cross
What is the fovea?
A depression in the back of the eye, with a high concentration of cones
It is opposite the lens and is responsible for sharp vision in the centre of your visual field.
Tell me about the optic tract.
Right optic tract
Fibres from right half of each retina
Responsible for the left hemifield of the visual field
Left optic tract
Fibres from left half of each retina
Responsible for the right hemifield of the visual field
The optic tracts are from the optic chiasm
Tell me about the lateral geniculate nucleus (LGN).
90% of retinal axons terminate in lateral geniculate nucleus (part of thalamus)
Major input to LGN from other centres
(Reticular formation, brain stem, cortex)
Feedback connections
Tell me about the primary visual cortex.
Each half visual field is represented in contralateral primary visual cortex
Each half of the visual field is split into 6 sections
The inner sections, where the fovea picks up the light, has the largest amount of neurones and space of brain, and so gives a sharper image than the rest.
Tell me about the ventral and dorsal pathways.
P pathway > ventral stream > V2 and V4 > inferior temporal cortex (Object recognition and colour)
M pathway > dorsal stream > V3 and V5 > Posterior parietal cortex (object location and motion)
How does vision develop?
New born can barely see anything
At 4 weeks slightly better, very general idea of shapes
8 weeks slightly better again, better shape recognition
3 months, much sharper image
6 months better colour and sharper
What is amblyopia?
Lazy eye caused by abnormal binocular input early in life
Strabismus (having a squint)
Anisometropia (refractive difference between the two eyes)
Deprivation (congenital cataract, ptosis, media opacities)
What are the different types of strabismus?
Esotropia (one eye goes inwards compared to the other)
Exotropia (One eye goes outwards)
Left Hypertropia (One eye goes upwards) (left eye)
Left Hypotropia (one eye goes downwards) (left eye)
