Physiology of Vision Flashcards
What two parts of the eye focus an image onto the retina?
Cornea and Lens
What can alter the focus of an image?
The shape of the lens
Where in the eye is unwanted light absorbed?
A pigment layer behind the retina
What can be used to view inside the eye of a patient, and what can be seen?
Ophthalmoscope
Optic disc and fovea can be seen
What is the optic disc?
Where the optic nerve leaves the eye and also where blood vessels enter/leave the retina.
What ratio of refraction is done by the different parts of the eye?
- 1/3 by the lens
- 2/3 by the cornea
What are refractive errors?
Problems with focusing
What are the two types of refractive errors?
Hypermetropia and Myopia
What is hypermetropia?
Long-sightedness which occurs when the eyeball is either too short or the lens system is too weak.
What is myopia?
Short-sightedness which occurs when the eyeball is either too long or too strong.
How can refractive errors be corrected?
Via spectacles
What is refractive power measured in?
Diopters
How is refractive power (diopters) calculated?
Reciprocal of focal length (1 / focal length)
How is the retina structured?
Back to front so ganglion cells and blood vessels are in the light path to the photoreceptors (except in the fovea)
How many rods and cones are there, and what type of cells are they?
120 million rods and 5 millions cones
They are receptors
What is the difference between rods and cones?
- Rods detect dim light
- Cones detect bright light and colours
How many types go cones are there?
3
What is the optic radiation?
What carries the output of the lateral geniculate nuclei to the striate cortex
What are the direct layers of the retina?
- Receptors
- Bipolars
- Ganglion cells
What are the transverse layers of the retina?
- Horizontal cells
- Amacrine cells
What is the ganglion cell response?
Because ganglion cells respond weakly to changes in overall light intensity, they instead respond to local contrast (light on dark background or dark on light background).
What does colourblindness result from?
A loss or modification of one (or more) of the three visual cone pigments.
The genes for which colour pigments are on the X chromosome?
Green and Red
The gene for the blue pigment is found in which chromosome?
Chromosome 7
Is blue colourblindness more rare or less rare than red/green colourblindness, and why is this the case?
More rare because it is on chromosome 7 and there are two of them, whilst there is only one X chromosome
What is central achromatopsia?
A much rarer form of colourblindness which is caused by damage to the cortical colour processing areas (V4)
What does the optic nerve from each retina divide into?
Into right and left halves
What is the optic chiasm?
X shaped structure where the two left halves and two right halves from the retinas combine to form optic tracts.
What do the optic tracts relay into?
The lateral geniculate nuclei of the thalamus
What do parts of each optic tract relay into?
Superior colliculus of the midbrain
Where does the output of each geniculate nuclei go to, and what then happens?
The striate cortex, where the input from each half is combined.
Where is the striate cortex located?
Occipital lobe
What are the three overlapping patterns in the primary visual cortex?
1) Occular dominance columns
2) Oriental columns (which are smaller)
3) Colour blobs
What is a hypercolumn?
An area of the primary visual cortex which contains a complete set of all three overlapping patterns
What is scotoma?
A partial loss of vision (blind spot)
What can cause scotoma?
- Lesion in the visual cortex
- Retinal damage
- Pressure from a tumour that is restricting the optic nerve, chiasm or tract
What are the two streams in the cortex?
- Dorsal Stream: Goes from the occipital lobe to the parietal lobe and is concerned with location, motion and action
- Ventral Stream: Goes from the occipital lobe to the temporal lobe and is concerned with object/face identity and conscious perception
What is prosopagnosia?
Inability to recognise familiar faces
What causes prosopagnosia?
Damage to specific parts of the temporal lobe (usually to fusiform sulcus which is located on the underside of the temporal lobe
What is a hemifield?
The 170 degree range of vision that one eye can see when facing directly forward
What is the vestibulo-occular reflex?
When the direction of the eyes remains constant whilst the head is moving
What is the optokinetic reflex?
When the eye fixes a gaze on a moving object
What is the pupillary reflex?
The contraction of both pupils when one eye gets illuminated.
This is because the pretactal nuclei and Edinger Westphal nuclei receive signals from both eyes.
What would prevent the pupillary reflex?
Damage to one of the oculomotor nerves, which would prevent the pupil in the affected eye from contracting.
What part of the eye is able to have the clearest vision, and why is this the case?
Fovea because it has the highest density of cone cells
