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Physiology > Visual Acuity > Flashcards

Visual Acuity Flashcards

1
Q

visual acuity depends on

A

optical requirements
neural prociessing of visual stimuli

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2
Q

all objects in the visual field

A

subtend the visual angle at the retina

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3
Q

define visual acuity

A

measure of the ability to discriminate between 2 stimuli
it is the spatial limits of vision

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4
Q

what is visual threshold

A

how large the stimulus is
approximately 50% of stimuli wll be detected

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5
Q

arcminute and arcsecond

A

arcminute - 1/60th of a degree
arcsecond - 1/60th of an arc minute

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6
Q

minimum visuble oject

A

possible at 1 second of arc of less

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7
Q

angle subtended y a single photoreceptor

A

20 arcmintues

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8
Q

minimum for identification of smal objects

A

30-60 seconds of arc

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9
Q

hyperacuity

A

minimum discriminable angle
at 3 seconds of arc

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10
Q

when does vernier acuity reach matuiry

A

14 years

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11
Q

how is vernier acuity achieved

A

retinal neuraonl synaptic organisation

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12
Q

how is vernier acuity measured

A

sweep mode visual evoked potenitals

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13
Q

neonatal VA

A

approximated from maximum resolution is about 1 cycle per degree
is <6/60

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14
Q

at 8 years

A

neural pathways to the visual cortex are developed

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15
Q

unit of light

A

quanta

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16
Q

simulation of a sinlge rod

A

does NOT produce visual sensation

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17
Q

how many rods needed for visual sensation

A

10-15 (as stimulate 1 ganglion)

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18
Q

how many quantas of light need to hit the cornea

A

50-150
10% reaches the back of the eye

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19
Q

what is light detection influenced by

A

illumination
spatial frequency
summation
darak adaption
optical qualities

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20
Q

Ricco’s law

A

threshold intensity of light stimulus is inversely proprotional to the area of stimulus

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21
Q

angle alpha

A

angle between the optical and visual aces

describes the tile of the eye relative to the optical axis

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22
Q

what is the typical angle of alpha

A

5.2 degrees horzontally

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23
Q

angle kappa

A

angle between the pupillary axis and the visual axiss

determined by the angle between the centre of the pupil and the light reflex

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24
Q

what is the tyical angle of kappa

A

2.6 degrees horziontally

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25
Q

what is the VA 5 degree’s away from the fovea

A

1/4 of the central VA

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26
Q

what factors affect VA

A

optical
physiological
anatomical

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27
Q

describe the optical factors in VA

A
  • distance between two objects
  • diffraction (e.g. through the pupil)
  • aberration
  • scatter
  • absorption
  • background illumination
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28
Q

how does diffraction change VA

A

causes loss of contrast
– creates a diffraction pattern: central Airy disc surrounding light and dark concentric rings
spread of light is proportional to wavelenght/aperture

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29
Q

how does aberration change VA

A

light is not focussed
also increases with increasing pupil size

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30
Q

what is the opitmum pupil size

A

2.8mm

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31
Q

describe the physiological factors affecting VA

A
  • crowding pehnomena
  • target orinteation
  • stimulus duration
  • stimulus brightness
  • contrast
  • adaptation
32
Q

crowding phenomena

A

maximal with seperations of 2-5 arcminutes

33
Q

stimulus duration decreases

A

VA decreases

34
Q

how does contrast influence VA

A

resolution of blue tragers on a red background is poorer than red targets on a blue background

35
Q

describe the anatomical factors affecting VA

A
  • position on the retina
  • temporal responsiveness
  • troxler phenomenon
  • saccadic suppression
36
Q

why does VA decrease away from the fovea

A
  • change in spacing and density of cone photoreceptors
  • change in photoreceptor and ganglion ration (greater convergence of cones to ganglion cells at the periphery)
  • change in cortical representation of the ganlgione cells
37
Q

what is temporal responsiveness

A

in low light - priority is detection
in high light it is differenetion and diffrentionw

38
Q

what is temporal summation

A

retina stimulated in rapid sucession is equal to it stimulated continously over the same length of time

39
Q

timing for temporal summation

A

0.1 seconds for detection
0.4 seconds for discrimination
0.25 seconds for blue target
0.1 seconds for red target

40
Q

bloch’s law

A

the intensity of the threshold stimulus is inversely proprotional to it’s duration

for the first 0.1 second

41
Q

what happens after 0.1 seconds in bloch’s law

A

does not apply
get partial summation

42
Q

Broca-sulzer effect

A

brightness produced by a flash of a given luminance depends on it’s duration

shorter flashes can appear brighter than longer one

43
Q

troxler phenomenon

A

light stimulus hed stationary on an area of retina will fade and disappear

44
Q

saccadic suppression

A

tkae 10-80 milliseconds to occur, and occur 3-5 times per second
vision is swithced off during rapid movements to avoid confusion

45
Q

testing visual acuity in a child 0-2 years

A
  • preferential looking cards with gratings (teller, keeler)
  • perferential looking cards with imagery (cardiff)
46
Q

testing visual acuity in a child 2-3 years

A

picture optotypes (Kay)

47
Q

testing visual acuity in a child 3+ years

A

picture +/- letter optotypes
(sheridan-gardiner, sonksen-silver, keeler logMAR)

48
Q

what is an optotype test

A

based on the Landhold C test
the gal in a C or E substends 1 arminute while the whole letter subtends 5

49
Q

the average person can resolves

A

two points seperated by 1 arcminutesn

50
Q

snellen VA

A

1 / minmum angle of resolution

51
Q

snellen fraction

A

distance at which the test is read / distance at which optotype subtends 5 arcminutes at the eye

52
Q

catford drum

A

uses osillating dots
for babies

53
Q

sonksen-silver

A

crowded optoptyc
deterecting reduced VA in amblyopia

54
Q

objective measurements of VA

A

visual evoked potenitals can approximate VA
flash VEP can be used for babies

55
Q

what is contrast sensitivty

A

changes in luminance at the edge of an onkect

contrst - sharpness

56
Q

how does wavelength affect contrast sensitivity

A

higher sapatial frequencey less coloyr

57
Q

what changes contrast sensitivity

A

luminance
bar width
lenght
wavelenght
grating motion

58
Q

how to measure contrast sensitivity

A

pelli-robson and vistech

59
Q

what is spectral sensitivty

A

relative efficnecy to detect different wavelenghts

60
Q

where is the max density of blue cones

A

~7-10 degrees from the fovea
fovea has poor sensivity for blue light

61
Q

the eye can detect flicker

A

of certain frequence but not beyond a certain level

62
Q

crictical flicker frequencey increases

A

porpoortional to the luminace
ferry-porter law

63
Q

threshold flicker frequnecny

A

frequencey at whcih flicker is persiceved 50% of the time

64
Q

flicker sensitivty decreases

A

with retinal eccentricity

65
Q

what is the optimum flicker

A

10 Hz
0.1 sceonds between stimuli

66
Q

what changes when the eye is dark adapted

A

responds to increasinglg weaker stimuli

67
Q

how long does dark adapation take

A

20 minutes
bipartite reponse

68
Q

initial period of dark adapation

A

due to cone regneration
resulting in an increase in cone sensitivity
RAPID 5-10mints

69
Q

second period of dark adapation

A

slower
where rods reach maximum sensitivty
15-20 minutes

70
Q

threshold for light intensity increases by

A

10,000 fold

71
Q

which part of the retina adapts more quickly

A

peripheral

72
Q

what is the pulfrich phenomenon

A

illusion of abnormal motion - 2D objects percieved as 3D
reflects delyaed conductions (usualyl optic neuritis)
pendulum swining in a straight line is perceieved as following an elliptical path

73
Q

what is entopic imagery

A

visual phenomenon created by structures in the eye (either nomral or patholigcal)
e.g. AC cells, viterous cells, reitnal vessel cells

74
Q

what patholoigical process does not cause entopic imagery

A

asteroid hyalosis

75
Q
A

Physiology (3 decks)

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