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OV2018 binocular vision > stereopsis > Flashcards

stereopsis Flashcards

1
Q

what is stereopsis?

A
  • depth perception

- when you look at an object you judge the distance of object to you

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2
Q

what are the two forms of depth perception ?

A
  • monocular ( one eye) cues

- binocular ( two eyes) cues

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3
Q

what is the best form of depth perception ?

A
  • binocular form - stereopsis ( BEST FORM)
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4
Q

what do people with monocular vision do to judge distances ?

A
  • they use monocular cues - it’s not as good as stereopsis but useful in helping px to judge distance
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5
Q

what are monocular pictorial cues ?

A

1 . relative size
- 2 objects equal in size, the one far away will take up less field of view, while the one closer to us has a bigger field of view

  1. occlusions ( interposition)
    - one object hidden, then the hidden object is considered further away
  2. relative height
    - below horizon: objects further away have higher bases
    - above horizon: objects further away have lower bases
  3. shadows:
    a. occlusion says blue glass in front
    b. contradicting cues of occlusion and relative height
    c. shadow removes the contradiction by saying vase is in front and elevated
  4. atmospheric perspective - due to light scattering by the atmosphere, objects that are a great distance away appear hazy
  5. linear perspective - with depth the distance between parallel line becomes smaller and they converge in 2D picture - come closer together
  6. familiar size - previous knowledge of the object’s size is used to determine the absolute depth of the object - e.g know trees are bigger than humans etc- but pics can make seem otherwise but we know.
  7. texture gradient- fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects
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6
Q

why does in the picture with two monsters, the monster at the back appears larger?

A
  • both monsters are the same size.Due to the background we perceive the upper one to be further away and therefore larger
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7
Q

why does the ames room picture give the impression that the lady is significantly smaller than the man?

A

. the lady looks smaller because she is much further away -
. the room has been designed so that the left hand corner is much further away than the right-hand corner - so all objects on left look much smaller

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8
Q

what are the monocular movement produced cues?

A
  1. motion parallax
    - near objects pass retinal image plane faster than distance objects- such as mountain remains in background but trees just go by
  2. kinetic depth effect
    - when a 3 dimensional form of an object can perceived when the object is moving
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9
Q

what is different oculomotor cues?

A
  • they are kinesthetic which means we can feel the sensations of theses changes
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10
Q

what are the oculomotor cues?

A

1- accommodation (monocular )

2-convergence ( binocular )

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11
Q

explain accommodation(monocular) as an oculomotor cues?

A

. when you bring an object closer-you can feel your eyes accommodating , you can feel ciliary muscle moving to ensure that the object that you’re bringing close to your eyes is not blurred

. kinesthetic sensations from ciliary muscles sent to the visual cortex where it is used for interpreting distance/depth

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12
Q

explain convergence (binocular) as a oculomotor cues?

A
  • if you were to bring an object close to your nose , you can feel those kinesthetic sensations from medial rectus contracting, allowing you to keep the object as single
  • kinesthetic sensations from those extraocular muscles also help in depth/distance perception
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13
Q

how does stereopsis arise?

A
  • arises from disparate objects, slightly different objects being fused together
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14
Q

when would we not be able to see in stereopsis ?

A
  • if information from both eyes was the same we wouldn’t be able to see in stereopsis
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15
Q

what allows us to see in 3D?

A
  • different objects stimulate each eye slightly differently
  • RE and LE get a slightly different view of the world
  • the fusion of the slightly different view of the world would allow us to see in 3D
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16
Q

explain binocular disparity cues?

A

. eyes located at different positions on the head
. binocular vision results in two slightly different (disparate) images are projected to the retina
. the disparities are processed in the visual cortex to yield depth perception ( stereopsis)
. the perception of depth and 3-dimensional structure is, possible with information visible from one eye alone however not as vivid as stereopsis

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17
Q

what are we using when looking at pictures in cinema ?

A
  • we are using the fovea of the right eye and the fovea of LE - they stimulate the same picture in both eye
  • when watching pictures in cinema we will not see in stereopsis
  • in cinema it’s all flat , so it will stimulate exactly the same corresponding retinal points
  • we need to create disparity by wearing glasses
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18
Q

when are binocular disparities naturally present?

A
  • binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes
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19
Q

what happens when viewing a 2D ?

A

when viewing a 2D scenes disparities need to be stimulated artificially by presenting two different images separately

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20
Q

what are the advantages of having stereopsis ?

A
  • you’re binocular summation is better
  • better space perception
  • better eye hand co-ordination
  • better reading
  • stereopsis
  • seeing objects in camouflage
21
Q

what jobs would be difficult to carry out without stereopsis?

A
  • cannot be a pilot

- job that you need to judge distances

22
Q

what happened in delta flight ?

A
  • pilot was corrected by monovision - one eye corrected for distance and one eye corrected for near - bad for stereopsis
  • pilot had problem judging how far the plane was away from runway
23
Q

what is stereo acuity ?

A
  • smallest depth difference we can detect

- the greater the disparity, the greater the depth effect

24
Q

what are the observed limits of stereopsis ?

A
  • best observes as good as 2 sec of arc

- on average 10 seconds of arc

25
Q

what are the factors affecting stereo thresholds?

A 
. practice effects
. luminance
. exposure duration
. retinal eccentricity 
. crowding effects
. motion in depth
. monocular blur is more detrimental to stereo acuity than binocular blur
26
Q

what is the development of stereopsis ?

A

. initially absent
. develops between 3-4 months
. reaches normal level at 6 months ( visual evoked potentials )

27
Q

what do we use to measure stereopsis ?

A
  • use books ( titmus) or (TNO)

- frisbee

28
Q

what is normal in stereo-tests?

A
  • depending on test we are using , dependent on what is considered normal
29
Q

what is considered normal when using a titmus in children?

A
  • children achieve 40 seconds of arc > 9years old
30
Q

what is considered normal when using a TNO in adults?

A
  • majority of adults achieve 60 seconds of arc (TNO) <70 years old
31
Q

how can we measure stereopsis ?

A
  • tested clinically
  1. quantitative - will get a number- lower number- better stereoacuity
    - evidence of stereopsis
  2. qualitative
    - a measure of stereoacuity
    - graded response( yes or no)
    . calibrated for a specific viewing distance
    . assumes an average p.d.
32
Q

what is total dissociation ?

A
  • the right and left eye gets totally different images
33
Q

what is the method for the synoptophore ?

A
  • each eye has a different viewing tube
    . two images presented; one to each eye
    . px reports if arrows are bending towards them or moving towards you as clinician
    . if they confirm they are bending - you have confirmed they have stereopsis and they have it qualitatively
  • major amblyoscope
34
Q

what happens in partial dissociation ?

A
  • the px gets the same image to both RE and LE
  • in this example , there are parts of the image that only the right can see and parts only left can see and parts that both eyes can see
35
Q

what is partial dissociation based on ?

A 
1- based on polarisation 
. titmus
.randot
. mallett unit
2. red/green
.TNO
3. optical
. Lang
36
Q

explain Titmus( Wirt)?

A
  • partial dissociation
    . polarised 3D glasses
  • relative depth
    . qualitative assessment (Fly - 3000 sec of arc)
  • quantitative
    . animals 500-100 sec
    . circles 500- 40 sec of arc

. viewing distance 16 deg ( approx 40cm)

. rotate card by 90 and 180 degrees

  • no depth 90
  • inversion of depth 180
37
Q

how to reduce monocular cues when carrying Titmus out ?

A

. monocular cues minimised if you put on glasses first

38
Q

what to avoid with titmus test?

A

. false positives - adults and children have a desire to have stereopsis

. rotate the plate by 90deg- to check for false positives - the fly should have no depth

39
Q

explain randot test?

A
  • partial dissociation
    . polarised 3D glasses
  • part contour stimuli
    . some monocular cues
  • part random dot principle
    . no monocular cues
    . simple geometric shapes
  • quantitative assessment
    . 500 to 20 of arc
    .viewing distance 16 ( approx 40 cm )
    problem : monocular cues
40
Q

explain mallet test?

A
  • polarid dissociation
  • two rows of symbols
    . fuse the two rows
    ( 3 base out prism )
  • symbols vary in disparity
    . 10’(600’’) to 30’’ of arc
  • viewing distance 36 cm
    . calibration given
    . will change with viewing distance
41
Q

explain the TNO test?

A
  • partial dissociation
    . red/green glasses ( red before LE)
    . initially you ask them to look at butterfly and ask them how many butterflies
    . if they got stereopsis they will see two butterflies
  • random dot principle
  • no monocular cues
    . 3 screening plates ( 1980 sec of arc )

. 3 quantitative plates

  • 4 presentations on each plate
  • 2 presentations at each level
  • 480-15 sec of arc

. viewing distance ( 4cm )

42
Q

what are the method of choice subject to in TNO test ?

A

. age
. understanding
. wearing glasses

43
Q

explain the Lang test ?

A

. mostly used for young children
. screening test - doesn’t go down very far

.2 plates
- Lang 1 ( cat, car, star)
. at 40 cm: 1200’’ to 550’‘seconds of arc

  • Lang 2 ( elephant, car, star, moon)
    . at 40 cm : 600’’ - 200’‘seconds of arc

. put the plate up and ask px what they see
. need to write down if child was able to identify object or point it out

. the control is the star ( to see of child is understanding what you are saying-(monocular cue-it is a control) )

  • inconclusive: don’t understand the test
  • negative : they can’t see it

. if they see the star and nothing else: suggests that they don’t have stereopsis

44
Q

what is no dissociation ?

A
  • no dissociation
  • perception of real depth
  • no 2D stimulation
  • the test is based on real depth - no 2D stimulation
45
Q

what are the two test of no dissociation ?

A

. Langs 2 pen

. frisby

46
Q

explain Langs 2 pen test?

A

. patient and examiner each hold a pen vertically
. patient brings points of pen into contact
. move pen to different location
. cover one of patients eyes
. repeat test

  • if accuracy poorer when using only one eye - indicates presence of stereopsis under binocularly conditions
  • equal monocular and binocular response
    . indicate absence of stereopsis
  • useful clinical tool
    . qualitative test
    . tests low grade BSV
47
Q

explain the frisby test ?

A

. random array of shapes
. no glasses required
.3 plates each got 4 squares of shapes per sheet
. one square contains a central circle
. observe behaviour or ask questions about circles
. flat button indicates where circle is sticking out
. turn around sheet ask again
. responses seen in infants as young as 6months
. 3 thickness of sheet (6mm, 3mm and 1mm)
. disparity 875’’ to 20’’
- sheet thickness
- viewing distance ( 30cm-80cm)

48
Q

which test would you use for steropsis?

A
  1. under < 2 years old-
    - Lang 1 or Lang 2 or frisbee - those tests are simple to describe to px
    frisbee- ask them to find ball
    Lang 1 or 2 - point where object is
  2. if about to decompensate:
    - frisbee or Lang 1 or Lang 2
    - because other test you need to put on glasses - act of putting on glasses could send them over edge and decompensate
  3. you require the most accurate test and they are 8 years old:
    TNO - goes down to 15 secs of arc
  4. child refusing to wear glasses:
    - Frisbee or Lang 1 or Lang 2
  5. home visit and you have NO stereo-tests:
    - Lang 2 pen
49
Q

what questions do you ask with each stereo-test?

A

. 2 years old

  • frisbee- can you see my ball
  • Lang 1 or Lang 2-

. 8 years old

  • TNO-
    ‘ can you point your finger to where you can see the picture’

OV2018 binocular vision (15 decks)

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