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Physiology > Visual Acuity > Flashcards

Visual Acuity Flashcards

(75 cards)

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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
what is the VA 5 degree's away from the fovea
1/4 of the central VA
26
what factors affect VA
optical physiological anatomical
27
describe the optical factors in VA
- distance between two objects - diffraction (e.g. through the pupil) - aberration - scatter - absorption - background illumination
28
how does diffraction change VA
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
29
how does aberration change VA
light is not focussed also increases with increasing pupil size
30
what is the opitmum pupil size
2.8mm
31
describe the physiological factors affecting VA
- crowding pehnomena - target orinteation - stimulus duration - stimulus brightness - contrast - adaptation
32
crowding phenomena
maximal with seperations of 2-5 arcminutes
33
stimulus duration decreases
VA decreases
34
how does contrast influence VA
resolution of blue tragers on a red background is poorer than red targets on a blue background
35
describe the anatomical factors affecting VA
- position on the retina - temporal responsiveness - troxler phenomenon - saccadic suppression
36
why does VA decrease away from the fovea
- 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
what is temporal responsiveness
in low light - priority is detection in high light it is differenetion and diffrentionw
38
what is temporal summation
retina stimulated in rapid sucession is equal to it stimulated continously over the same length of time
39
timing for temporal summation
0.1 seconds for detection 0.4 seconds for discrimination 0.25 seconds for blue target 0.1 seconds for red target
40
bloch's law
the intensity of the threshold stimulus is inversely proprotional to it's duration for the first 0.1 second
41
what happens after 0.1 seconds in bloch's law
does not apply get partial summation
42
Broca-sulzer effect
brightness produced by a flash of a given luminance depends on it's duration shorter flashes can appear brighter than longer one
43
troxler phenomenon
light stimulus hed stationary on an area of retina will fade and disappear
44
saccadic suppression
tkae 10-80 milliseconds to occur, and occur 3-5 times per second vision is swithced off during rapid movements to avoid confusion
45
testing visual acuity in a child 0-2 years
- preferential looking cards with gratings (teller, keeler) - perferential looking cards with imagery (cardiff)
46
testing visual acuity in a child 2-3 years
picture optotypes (Kay)
47
testing visual acuity in a child 3+ years
picture +/- letter optotypes (sheridan-gardiner, sonksen-silver, keeler logMAR)
48
what is an optotype test
based on the Landhold C test the gal in a C or E substends 1 arminute while the whole letter subtends 5
49
the average person can resolves
two points seperated by 1 arcminutesn
50
snellen VA
1 / minmum angle of resolution
51
snellen fraction
distance at which the test is read / distance at which optotype subtends 5 arcminutes at the eye
52
catford drum
uses osillating dots for babies
53
sonksen-silver
crowded optoptyc deterecting reduced VA in amblyopia
54
objective measurements of VA
visual evoked potenitals can approximate VA flash VEP can be used for babies
55
what is contrast sensitivty
changes in luminance at the edge of an onkect contrst - sharpness
56
how does wavelength affect contrast sensitivity
higher sapatial frequencey less coloyr
57
what changes contrast sensitivity
luminance bar width lenght wavelenght grating motion
58
how to measure contrast sensitivity
pelli-robson and vistech
59
what is spectral sensitivty
relative efficnecy to detect different wavelenghts
60
where is the max density of blue cones
~7-10 degrees from the fovea fovea has poor sensivity for blue light
61
the eye can detect flicker
of certain frequence but not beyond a certain level
62
crictical flicker frequencey increases
porpoortional to the luminace ferry-porter law
63
threshold flicker frequnecny
frequencey at whcih flicker is persiceved 50% of the time
64
flicker sensitivty decreases
with retinal eccentricity
65
what is the optimum flicker
10 Hz 0.1 sceonds between stimuli
66
what changes when the eye is dark adapted
responds to increasinglg weaker stimuli
67
how long does dark adapation take
20 minutes bipartite reponse
68
initial period of dark adapation
due to cone regneration resulting in an increase in cone sensitivity RAPID 5-10mints
69
second period of dark adapation
slower where rods reach maximum sensitivty 15-20 minutes
70
threshold for light intensity increases by
10,000 fold
71
which part of the retina adapts more quickly
peripheral
72
what is the pulfrich phenomenon
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
what is entopic imagery
visual phenomenon created by structures in the eye (either nomral or patholigcal) e.g. AC cells, viterous cells, reitnal vessel cells
74
what patholoigical process does not cause entopic imagery
asteroid hyalosis
75

Physiology (3 decks)

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