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NHB MVST 1b > Vision > Flashcards

Vision Flashcards

1
Q

What is the wavelength range of visible light?

A

390-700nm

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2
Q

Which variables of light are encoded?

A

Intensity
Wavelength
Position in space
Position in time

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3
Q

Contrast?

A

Delta I/I

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4
Q

Pointspread function? And what causes it to increase or decrease?

A

d = lambda//D. D = diameter of lens/aperture of pupil.
Increased by increasing pupil diameter, because even though diffraction decreases, chromatic and spherical aberrations increase.

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5
Q

Definition emmetropic?

A

If an object at infinity is sharply focussed

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6
Q

Definition ametropic?

A

Object at infinity not sharply focussed

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7
Q

Definition myopia? What does it predispose to?

A

Short sight, 20% pop

Retinal detachment, degeneration nd glaucoma

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8
Q

Definition hypermetropia

A

Long sight, 30% pop

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9
Q

Presbyopia?

A

Hardening of lens/loss of elasticity causing eye to focus light behind the retina when looking at close objects (loss of accommodating power)

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10
Q

What does two point resolution depend on?

A
  1. Pointspread function
    - Diffraction
    - Aberration
  2. Refractive errors
  3. Receptor spacing
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11
Q

How are the lens and cornea supplied with metabolites?

A

Aqueous humour - secreted by epithelium of ciliary body and drained off through trabecular network in Canal of Schlemm

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12
Q

What causes glaucoma?

A

Reduced rate of outflow through trabecular network or raised intraocular pressure

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13
Q

How do you measure power of a lens?

A

Dioptres - 1/focal length in metres
U = distance to lens from object
V = distance from lens to image
1/u+1/v=1/f = Power

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14
Q

How many optical surfaces are there for light to pass through and which has the highest power?

A

4: outside cornea, inside cornea, outside lens, inside lens

Front of cornea! 48.7 dioptres!

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15
Q

How does the lens accommodate?

A

Radial elastic ligaments (suspensory ligaments/zonule) and a circular ciliary muscle.
When ciliary muscle is relaxed, ligaments stretch the lens.
At rest, a normal eye is focussed on an object at infinity.

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16
Q

Near reflex

A

associated with accommodation and constriction of pupil. Convergence of the two eyes to fix on a close target.

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17
Q

Argyll-Robertson pupil

A

Characteristic of neurosyphilis.
pupil doesn’t react to light, but does to accommodation.
Maybe due to bilateral pretectal damage?

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18
Q

What do retinal glial cells do? What are they called?

A

Muller Cells

Act as optical waveguides to aid transmission of light

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19
Q

What size are the fovea and foveola and what is the difference?

A

Fovea central 1.5mm, foveola central 260micrometers. foveola higher acuity. avascular. completely rod free.

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20
Q

Where are there most rods and where are there most cones?

A

Most cones in foveola

Most rods in parafoveal region - 20 degrees either side of the fovea

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21
Q

What is the structure of the outer segments of the rods and cones and what are they used for?

A

Transduction!
Rods = stacked membranous discs
Cone = continuous folds of invaginating lamellae

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22
Q

What is rhodopsin? What does it bind?

A

GPCR pigment

binds chromophore 11-cis retinal

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23
Q

What is the peak absorption of retinal when it is bound to rhodopsin?

A

500nm

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24
Q

Discuss the amplification cascade for phototransduction

A

1 R+ –> 150 transducin –> 150 PDE –> 10^5 cGMP –> closes 500 channels

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25
Q

What mediates photoreceptor adaptation?

A

Calcium ions

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26
Q

Define colour vision

A

Ability to differentiate objects based on spectral reflectance independently of intensity

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27
Q

What is the maximum absorption wavelength of the 3 classes of cone?

A 
S = blue = 420
M = green = 534
L = red = 564
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28
Q

Explain the principle of univariance

A

The same photoreceptor can be excited by different combinations of wavelength and intensity, so the brain cannot know the colour of a certain point of the visual image just from one photoreceptor.
E.g. if a receptor absorbed 10x as much energy per red photon as green, couldn’t tell if incident was 10 green photons or 1 red photon.

29
Q

Definition protanopia

A

no red cones - X linked

30
Q

Definition deuteranopia

A

Green dichromacy. No green cones - X linked

31
Q

Definition tritanopia

A

No blue cones, rare

32
Q

Definition anomalous trichromat

A

Unequal intragenic recombination resulting in formation of hybrid genes with different spectral sensitivity

33
Q

Definition deuteranomaly

A

Anomalous trichromat of hybrid green-red gene, peak shifted away from yellow (reduced sensitivity to green light, requires more green to Rayleigh match)
most common form of colour blindness

34
Q

Definition protanomaly

A

Anomalous trichromat of hybrid red-green gene, peak shifted towards yellow (reduced sensitivity to red light, requires more red to match in Rayleigh match)

35
Q

Definition tritanopia

A

Loss of blue pigment gene. Rare.

36
Q

What is in the outer nuclear layer?

A

Cell bodies of photoreceptors

37
Q

What is in the outer plexiform layer?

A

Synapses between photoreceptors, bipolar cells and horizontal cells

38
Q

What is in the inner nuclear layer?

A

Cell bodies of bipolar cells, horizontal cells, and amacrine cells

39
Q

What is in the inner plexiform layer?

A

Synapses between bipolar cells, amacrine cells and ganglion cells

40
Q

What is in the ganglion cell layer?

A

Cell bodies of ganglion cells

41
Q

Where is there divergence and where is there convergence in the retina?

A

Fovea = divergence 1:3

Periphery convergence 16:1

42
Q

Define receptive field

A

Receptive field is the area of a retina (or its projection in the visual field) from within which a neuron’s activity can be influenced by light

43
Q

What type of synapse is between a cone and an off centre bipolar? What is the neurotransmitter and receptor?

A

sign conserving

Glu –> AMPA

44
Q

What type of synapse between a cone and an on centre bipolar? NT and receptor?

A

sign inverting

Glu –> mGluR6

45
Q

Discuss what is meant by parallel streams in the context of vision

A

Different aspects of the image are encoded by different cells with overlapping receptive fields - so message to brain = parallel neural images:
on v off, spatial detail, temporal detail, colour

46
Q

What is the purkinje shift?

A

In scotopic range, peak sensitivity shifts from the average of red and green cones (560nm) to the peak sensitivity of rods (500nm)

47
Q

What layer of V1 do LGN projections end on?

A

Spiny stellate neurons layer 4c. 4Calpha = magno, 4Cbeta = parvo

48
Q

Definition ambylophobia

A

Eye problems in early life lead to a permanent defect in cortical function.

49
Q

Definition achromatopsia

A

Cortical colour blindness caused by a lesion in V4

50
Q

Lesions where can lead to prosopagnosia?

A

Infratemporal cortex

Inability to recognise faces

51
Q

Lesions where can lead to akinetopsia?

A

V5/MT

52
Q

Definition colour constancy

A

Perceived colour of objects remains the same irrespective of illumination

53
Q

Describe the method of the increment threshold test

A

Green test spot on an orange background with an eccentrically-placed cross. Fixate on cross, so stim (which stimulates ble cones) falls on the parafoveal region. Stimulate threshold test spot intensity as a function of steady background intensity.

54
Q

Describe the results of the increment threshold test

A

Over a wide range of intensities, the log of the threshold intensity increases linearly with the log of the background intensity.
At very low intensities, threshold is independent of background intensity - an absolute threshold. set by dark light.
When the background becomes very bright, the rod system saturates and so there is a very steep increase in threshold.

55
Q

Weber’s law?

A 
Threshold contrast of delta I/I is constant. 
Io = dark light 
K = weber's law ratio
DeltaI/(I+Io) = k
if I >>> Io then Delta I/Io = k.
56
Q

Limits to sensitivity in vision?

A 
Quantal fluctuations (have to differentiate, during dim steady light, any stimulus as distinct from the random fluctuations)
Need to add more
57
Q

What is the critical fusion frequency?

A

Frequency above which a flickering light is perceived as steady.
At low freq, depends on rods, best stim by blue-green. Rod responses are slow, giving rise to a low flicker fusion frequency (<15hHz)
At high intensities, cones take over, exhibit far better temporal resolution, up to 60Hz.
As mean intensity increases, the visual system becomes progressively better at following fast changes.

58
Q

Size of fovea

A

1-1.5mm

59
Q

size of macula

A

6mm

60
Q

optic disk size

A

1.5mm = 5 degrees

61
Q

acuity at fovea

A

0.5 min = 30 s = 2.5micrometers

62
Q

eyeball diameter

A

17mm

63
Q

thickness of retina near fovea

A

418 micrometres

64
Q

thickness of retina peripherally

A

200 micrometers

65
Q

thickness of axon layer near fovea

A

30 micrometers

66
Q

thickness of axon layer near optic disk

A

200 micrometres

67
Q

blind spot size

A

4-6 degrees

68
Q

diameter of head of typical foveal cone

A

2.3 micrometers

69
Q

1 degree corresponds to what?

A

1cm object 57cm away

300micrometres on the retina

NHB MVST 1b (11 decks)

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