Vision definitions Flashcards
Illuminance
Light from a light source
Measured in lux
Luminance
Light reflected from objects
Measure in cd/m^2
Pointspread function
Angular diameter, δ = λ/D radians
Because of diffraction (causes waves to spread when they pass through a small aperture), the image of a point source, even through a perfect lens will be a blurred circle- the pointspread function.
Emmetropic
Object at infinity is sharply focussed
Ametropic
Object at affinity is not sharply focused
Myopia
Short sight, ~20% pop
Corrected with converging lens.
Presisposes to retinal detachment & glaucoma
Hypermetropia
Long sight, ~30% pop
Corrected with diverging lens
Presbyopia
Reduction in accommodating power of the lens.
Occurs as lens elasticity decreases with age.
Power of a lens
1 / focal length (m)
Measured in dioptres
To a first approximation the power of a compound lens system is given by the sum of the powers of the individual components.
Aqueous humour
Supplies the avascular cornea & lens with metabolites.
Secreted by the epithelium of the cilliary body, drained through the trabecular meshwors and Canal of Schlemm. Reduction in rate of outflow = primary cause of glaucoma
Near reflex or ‘Triple response’
- Lens accommodation
- Constriction of the pupil to improve depth of focus
- Convergence of the 2 eyes to fixate on new target
Argyll-Robertson pupil
Characteristic of neurosyphillis
Pupil does not react to light, but does react to accommodation.
Foveola
Central ~1degree of the fovea with the highest acuity.
Avascular, completely rod-free.
Parafoveal region
Area of most sensitive vision under mesopic & scotopic conditions.
Rod density peaks ~20degrees either side of the fovea.
Papilloedema
Optic disk can appear swollen with raised intracranial pressure
Night blindness
Can be caused by
- deficiency of vitamin A (of which retinal is a derivative)
- hereditary retinitis pigmentosa
Negative internal transmitter
e.g Phototransduction G protein-coupled cascade that culminated in the destruction of cGMP.
Hereditary retinitis pigmentosa
~1/3000, Progressive hereditary retinal degradation
Gradual onset of night blindness in adolescence, loss of all peripheral vision in adulthood, in extreme cases total blindness.
5-10% cases are caused by mutations in rhodopsin gene.
Dark current
Continuously circulating in dark.
Na+ and Ca++ ions flowing into outer segment of photoreceptor, depolarizing to ~- 30mv. Circuit completed by outward current in the inner segment, mainly through K+ channels.
Photoreceptor light adaption
= a reduction in sensitivity as the steady light intensity increases.
Believed to be alrgely mediated by calcium ions.
Principle of univariance
Although each cone absorbs maximally at a particular wavelength, alone it provides no colour information.
e.g although a green cone might absorb 10x fewer red photons than green photons, 100 incident red photons would be indistinguishable from 10 green photons
Young-Helmholz trichromacy theory
Colour is defined by the ration of excitation in the three cone classes.
Colour vision is achieved by comparing the outputs of different cones. A trichromatic system can distinguish over 2m colours.
Protanopia
Type of dichromatic vision (red-dichromacy)
No red cones/ no red pigment.
Gene is on X chromosome- sex linked
Deuteranopia
Type of dichromatic vision (green-dichromacy)
No green cones/ no green pigment
Gene is on X chromosome- sex linked
Tritanopia
Type of dichromatic vision
No blue cones/ no blue pigment.
Rare.
However, in normal patients blue cones are excluded from the foveola- resulting in foveal tritanopia.
Anomalous trichromacy
Shifted red/green cone.
Results when hybrid photopigments are formed by unequal intragenic recombination with shifted spectral sensitivities.
Unequal homologous recombination
When DNA strands align incorrectly during recombination, resulting in erroneous recombination.
Unequal intergenic recombination
= unequal homologous recombination between genes.
Results in a loss or duplication of genes.
Unequal intragenic recombination
= unequal homologous recombination within genes.
Results in the generation of hybrid genes.
In photopigments- some may form novel rhodopsins with shifted absorption spectra.
Deuteranomoly
Anomolous trichromats result when hybrids are formed by unequal intragenic recombination with shifted spectral sensitivities.
Hybrid green-red gene may give rise to deuteranomoly
Protanomoly
Anomolous trichromats result when hybrids are formed by unequal intragenic recombination with shifted spectral sensitivities.
Hybrid red-green gene may give rise to protanomoly
Presynaptic ribbon
Photoreceptor synapses have a characteristic presynaptic ribbon.
Modified presynaptic density characteristic of synapses that transmit graded signals.
Sign-inverting synapses
Cones to on-centre bipolar cells.
Invaginating contacts
mGluR6 receptors
Sign-preserving synapses
Cones to off-centre bipolar cells.
Flat contacts
Ionotropic AMPA receptors
Purkinje shift
Shift in overall spectral sensitiviry from 560nm to 500nm (rod peak sensitivity) associated with shift from photopic to scotopic conditions and consequent adaptation.
Cortical magnification
Orderly retinotopic representation of the retina is maintained in pathways to V1. However the fovea is grossly overrepresented.
Achromatopsia
Cortical colour blindness.
Caused by lesions in V4
Prosopagnosia
Inability to recognise faces.
Caused by lesions in the inferotemporal cortex.
