Physiology of Vision Flashcards
What are the 3 stages of vision?
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Collection of information
- Eyeball - optical front and retina / optic disc at the back.
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Transfer of information
- Connections - optic nerve, chiasm, optic tract, LGN, radiations.
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Creation of conscious vision
- Brain - occipital, temporal, parietal and frontal lobes.
Fill in each part of the anterior segment of the eye.
Describe the refraction of the eye.
- Cornea
- Largest element - 40 Dioptres
- Lens
- Lesser element (20 Dioptres)
- But can vary in power (accommodation)
- Whole eyeball - ~60 Dioptres
What is presbyopia?
- Refractive error - age-related loss of ability to focus up close.
Describe the different types of refractive error and how they can be corrected.
What is contained within the anterior and posterior segments of the eye respectively?
Describe the structure of the retina.
- Information from the rods and cones is converged to ganglion cells, lateral cells and amacrine act to provide initial processing of the visual signal.
Compare and contrast rods and cones.
Describe the wavelength sensitivity of rods and cones.
- Visible light ranges from 400-700nm wavelength.
- Each opsin gives sensitivity to a different range of wavelengths of light.
- At their most sensitive wavelength, human eyes can detect as little as 5 photons of light.
- Contrary to expectation, photoreceptors are depolarised in the dark and hyperpolarise in the light.
What is the optic cup?
What causes the dimension of the optic cup to change?
- Optic nerve - central ‘cup’ is a space where there is no neuro-retinal tissue.
- If neuro-retinal tissue dies (loss of ganglion cell axons) the cup gets bigger - this is a characteristic finding in glaucoma.
Describe the ‘processing’ of vision.
- Happens at the level of the retina.
- Convergence - 100 million photoreceptors, but only 1 million ganglion cells.
- Ganglion cell axon transfers information to the brain.
- Optic nerve > chiasm > optic tract.
- First synapse is at the lateral geniculat nucleus.
- This is part of the thalamus.
- Major relay station for sensory information.
What happens at the optic chiasm?
Information from nasal retina crosses over.
Which visual field deformities would be caused by the lesions 1-7?
What are the main causes of the lesion at (3)?
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Chiasmal lesion, caused by:
- Pituitary tumour (pictured, left)
- Craniopharyngioma (pictured, right)
Describe the separate functional streams of visual information.
There are 2 separate functional streams of visual information that complement each other creating the overall sense of vision - the ‘what’ and the ‘where’ streams.
