Vision definitions

Question 1

Illuminance

Answer 1

Light from a light source

Measured in lux

Question 2

Luminance

Answer 2

Light reflected from objects

Measure in cd/m^2

Question 3

Pointspread function

Answer 3

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.

Question 4

Emmetropic

Answer 4

Object at infinity is sharply focussed

Question 5

Ametropic

Answer 5

Object at affinity is not sharply focused

Question 6

Myopia

Answer 6

Short sight, ~20% pop
Corrected with converging lens.

Presisposes to retinal detachment & glaucoma

Question 7

Hypermetropia

Answer 7

Long sight, ~30% pop

Corrected with diverging lens

Question 8

Presbyopia

Answer 8

Reduction in accommodating power of the lens.

Occurs as lens elasticity decreases with age.

Question 9

Power of a lens

Answer 9

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.

Question 10

Aqueous humour

Answer 10

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

Question 11

Near reflex or ‘Triple response’

Answer 11

• Lens accommodation
• Constriction of the pupil to improve depth of focus
• Convergence of the 2 eyes to fixate on new target

Question 12

Argyll-Robertson pupil

Answer 12

Characteristic of neurosyphillis

Pupil does not react to light, but does react to accommodation.

Question 13

Foveola

Answer 13

Central ~1degree of the fovea with the highest acuity.

Avascular, completely rod-free.

Question 14

Parafoveal region

Answer 14

Area of most sensitive vision under mesopic & scotopic conditions.
Rod density peaks ~20degrees either side of the fovea.

Question 15

Papilloedema

Answer 15

Optic disk can appear swollen with raised intracranial pressure

Question 16

Night blindness

Answer 16

Can be caused by

• deficiency of vitamin A (of which retinal is a derivative)
• hereditary retinitis pigmentosa

Question 17

Negative internal transmitter

Answer 17

e.g Phototransduction G protein-coupled cascade that culminated in the destruction of cGMP.

Question 18

Hereditary retinitis pigmentosa

Answer 18

~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.

Question 19

Dark current

Answer 19

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.

Question 20

Photoreceptor light adaption

Answer 20

= a reduction in sensitivity as the steady light intensity increases.
Believed to be alrgely mediated by calcium ions.

Question 21

Principle of univariance

Answer 21

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

Question 22

Young-Helmholz trichromacy theory

Answer 22

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.

Question 23

Protanopia

Answer 23

Type of dichromatic vision (red-dichromacy)
No red cones/ no red pigment.
Gene is on X chromosome- sex linked

Question 24

Deuteranopia

Answer 24

Type of dichromatic vision (green-dichromacy)
No green cones/ no green pigment
Gene is on X chromosome- sex linked

Question 25

Tritanopia

Answer 25

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.

Question 26

Anomalous trichromacy

Answer 26

Shifted red/green cone.

Results when hybrid photopigments are formed by unequal intragenic recombination with shifted spectral sensitivities.

Question 27

Unequal homologous recombination

Answer 27

When DNA strands align incorrectly during recombination, resulting in erroneous recombination.

Question 28

Unequal intergenic recombination

Answer 28

= unequal homologous recombination between genes.

Results in a loss or duplication of genes.

Question 29

Unequal intragenic recombination

Answer 29

= unequal homologous recombination within genes.
Results in the generation of hybrid genes.
In photopigments- some may form novel rhodopsins with shifted absorption spectra.

Question 30

Deuteranomoly

Answer 30

Anomolous trichromats result when hybrids are formed by unequal intragenic recombination with shifted spectral sensitivities.

Hybrid green-red gene may give rise to deuteranomoly

Question 31

Protanomoly

Answer 31

Anomolous trichromats result when hybrids are formed by unequal intragenic recombination with shifted spectral sensitivities.

Hybrid red-green gene may give rise to protanomoly

Question 32

Presynaptic ribbon

Answer 32

Photoreceptor synapses have a characteristic presynaptic ribbon.

Modified presynaptic density characteristic of synapses that transmit graded signals.

Question 33

Sign-inverting synapses

Answer 33

Cones to on-centre bipolar cells.
Invaginating contacts
mGluR6 receptors

Question 34

Sign-preserving synapses

Answer 34

Cones to off-centre bipolar cells.
Flat contacts
Ionotropic AMPA receptors

Question 35

Purkinje shift

Answer 35

Shift in overall spectral sensitiviry from 560nm to 500nm (rod peak sensitivity) associated with shift from photopic to scotopic conditions and consequent adaptation.

Question 36

Cortical magnification

Answer 36

Orderly retinotopic representation of the retina is maintained in pathways to V1. However the fovea is grossly overrepresented.

Question 37

Achromatopsia

Answer 37

Cortical colour blindness.

Caused by lesions in V4

Question 38

Prosopagnosia

Answer 38

Inability to recognise faces.

Caused by lesions in the inferotemporal cortex.