stereopsis

Question 1

what is stereopsis?

Answer 1

• depth perception

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

Question 2

what are the two forms of depth perception ?

Answer 2

• monocular ( one eye) cues

- binocular ( two eyes) cues

Question 3

what is the best form of depth perception ?

Answer 3

• binocular form - stereopsis ( BEST FORM)

Question 4

what do people with monocular vision do to judge distances ?

Answer 4

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

Question 5

what are monocular pictorial cues ?

Answer 5

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

2. occlusions ( interposition)
   - one object hidden, then the hidden object is considered further away
3. relative height
   - below horizon: objects further away have higher bases
   - above horizon: objects further away have lower bases
4. 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
5. atmospheric perspective - due to light scattering by the atmosphere, objects that are a great distance away appear hazy
6. linear perspective - with depth the distance between parallel line becomes smaller and they converge in 2D picture - come closer together
7. 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.
8. texture gradient- fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects

Question 6

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

Answer 6

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

Question 7

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

Answer 7

. 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

Question 8

what are the monocular movement produced cues?

Answer 8

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

Question 9

what is different oculomotor cues?

Answer 9

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

Question 10

what are the oculomotor cues?

Answer 10

1- accommodation (monocular )

2-convergence ( binocular )

Question 11

explain accommodation(monocular) as an oculomotor cues?

Answer 11

. 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

Question 12

explain convergence (binocular) as a oculomotor cues?

Answer 12

• 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

Question 13

how does stereopsis arise?

Answer 13

• arises from disparate objects, slightly different objects being fused together

Question 14

when would we not be able to see in stereopsis ?

Answer 14

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

Question 15

what allows us to see in 3D?

Answer 15

• 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

Question 16

explain binocular disparity cues?

Answer 16

. 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

Question 17

what are we using when looking at pictures in cinema ?

Answer 17

• 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

Question 18

when are binocular disparities naturally present?

Answer 18

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

Question 19

what happens when viewing a 2D ?

Answer 19

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

Question 20

what are the advantages of having stereopsis ?

Answer 20

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

Question 21

what jobs would be difficult to carry out without stereopsis?

Answer 21

• cannot be a pilot

- job that you need to judge distances

Question 22

what happened in delta flight ?

Answer 22

• 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

Question 23

what is stereo acuity ?

Answer 23

• smallest depth difference we can detect

- the greater the disparity, the greater the depth effect

Question 24

what are the observed limits of stereopsis ?

Answer 24

• best observes as good as 2 sec of arc

- on average 10 seconds of arc

Question 25

what are the factors affecting stereo thresholds?

Answer 25

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

Question 26

what is the development of stereopsis ?

Answer 26

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

Question 27

what do we use to measure stereopsis ?

Answer 27

- use books ( titmus) or (TNO) | - frisbee

Question 28

what is normal in stereo-tests?

Answer 28

- depending on test we are using , dependent on what is considered normal

Question 29

what is considered normal when using a titmus in children?

Answer 29

- children achieve 40 seconds of arc > 9years old

Question 30

what is considered normal when using a TNO in adults?

Answer 30

- majority of adults achieve 60 seconds of arc (TNO) <70 years old

Question 31

how can we measure stereopsis ?

Answer 31

- 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.

Question 32

what is total dissociation ?

Answer 32

- the right and left eye gets totally different images

Question 33

what is the method for the synoptophore ?

Answer 33

- 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

Question 34

what happens in partial dissociation ?

Answer 34

- 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

Question 35

what is partial dissociation based on ?

Answer 35

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

Question 36

explain Titmus( Wirt)?

Answer 36

- 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

Question 37

how to reduce monocular cues when carrying Titmus out ?

Answer 37

. monocular cues minimised if you put on glasses first

Question 38

what to avoid with titmus test?

Answer 38

. 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

Question 39

explain randot test?

Answer 39

- 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

Question 40

explain mallet test?

Answer 40

- 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

Question 41

explain the TNO test?

Answer 41

- 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 )

Question 42

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

Answer 42

. age . understanding . wearing glasses

Question 43

explain the Lang test ?

Answer 43

. 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

Question 44

what is no dissociation ?

Answer 44

- no dissociation - perception of real depth - no 2D stimulation - the test is based on real depth - no 2D stimulation

Question 45

what are the two test of no dissociation ?

Answer 45

. Langs 2 pen | . frisby

Question 46

explain Langs 2 pen test?

Answer 46

. 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

Question 47

explain the frisby test ?

Answer 47

. 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)

Question 48

which test would you use for steropsis?

Answer 48

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

Question 49

what questions do you ask with each stereo-test?

Answer 49

. 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'