Stereopsis

Question 1

How do we percieve depth perception?

Answer 1

Using Binocular and monocular cues.

Question 2

What is the best form of depth perception?

Answer 2

Stereopsis which comes from both eyes working together.

Question 3

If the best depth perception comes from both eyes working together does that mean people who do not have two eyes for whatever reason, cannot judge depth?

Answer 3

They can still judge depth to some extent using monocular cues (both still and moving), however, it will obviously not be as good as binocularly.

Question 4

What are monocular cues for pictoral (in pictures ) depth perception?

Answer 4

Relative size - relatively things closer to you tend to be bigger (thus your fingernail held in front of your eyes will seem bigger than the tree at a distance)

Occlusion (interposition) - basically things closer to you will cover things further away (e.g. the fact that my thumb can cover the tree in the distance) thus with one eye you could determine what is closer and what is further away from you.

Relative height - things above the horizon look further away whereas things below the horizon look closer (think of the sea above the water/horizon line the sea looks far and the sand looks close).

Shadows

Atmospheric perspective - basically that due to light scattering by the atmosphere, objects that are a great distance away appear hazy

Linear perspective - so basically think about looking at a straight path into the distance, the further down the path you look the narrower the path gets until eventually it meets at a point.

Familiar size - naturally we know a human isn’t as big as the leaning tower of piza to push it.

Textural gradient -Fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects.

Question 5

What are monocular movement cues?

Answer 5

Motion parallax- e.g. if you are on a train the objects close to you move really fast but the objects further away move slower.

Kinetic ​depth effect- When still objects look like they have depth to them when you move. (Basically those art pieces where its a box sticking out and then when you walk around it and it looks like a box sticking in (not out)).

Question 6

What doe oculomotor refer to?

Answer 6

relating to the motion of the eye.

Question 7

What oculomotor cues help us determine depth and size?

Answer 7

• Accommodation (Monocular)
• Kinesthetic sensations from ciliary muscles are sent to the visual cortex where it is used for interpreting distance/depth
• Convergence (Binocular)
• Kinesthetic sensations from these extraocular muscles also help in depth/distance perception.

[Basically moving your muscles to focus on something either in the distance or near results in kinesthetic sensations which are sent ot the visual cortex and allow context to be added to the image you see - hence you have a working understanding of depth]

Question 8

What are binocular disparity cues that allow for depth perceptions?

Answer 8

—Eyes located at different positions on the head

—Binocular vision results in two slightly different (disparate) images being projected to the retinas

—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 9

How do you make a movie 3D?

Answer 9

—Binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes

—When viewing a 2 D scene disparities need to be simulated artificially by presenting two different images separately to each eye thus mimicking the perception of depth - for this we use the 3D glasses - which will often two different colour filters allocated to each eye - e.g. red filter infront of the left eye and a green one infront of the right eye.

Question 10

What are the advanatges of binocular single vision (BSV)?

Answer 10

—Stereopsis

—Binocular Summation

—Better space perception

—Eye Hand Co-ordination

—Better reading

—Seeing Objects in camouflage

Question 11

What job can you legally not do without stereopsis?

Answer 11

Be a pilot - regulations state you must have all three of worth’s grades.

Question 12

What is depth perception and what does it depend on?

What is the observed limits of stereopsis?

Answer 12

—Smallest depth difference we can detect

—Greater the disparity the greater the depth effect

—Observed limits of Stereopsis:

◦Best observers as good as 2 sec of arc

◦On average 10 seconds of arc

Question 13

What factors affect your stereo (stereopsis) thresholds?

Answer 13

—Practice effects

—Luminance - lighting levels

—Exposure duration

—Retinal eccentricity

—Crowding effects

—Motion in depth

—Monocular blur is more detrimental to stereo acuity than binocular blur - i.e. if you don’t correct anisometropia the px has bad stereopsis.

Question 14

Are you born with stereopsis?

Answer 14

No - It is:

—Initially absent

—Develops between 3-4 months

—Reaches normal levels at 6 months (Visual Evoked Potentials)

—Research carried out using real stereo-tests shows stereopsis:

—Varies with age

—Is highly dependent on the test (making sure you have eliminated factors that affect it (a.k.a the factors that limit stereo thresholds).

—Children achieve 40 seconds of arc (Titmus) > 9years old

—Majority of adults achieve 60 seconds of arc (TNO) < 70years old

Question 15

What is the refrence/gold standard test for stereopsis?

Answer 15

Psycho-physics Tests - but these take hours ( around 2) to complete.

Question 16

What quick tests for Stereopsis do we tend to carry out?

Answer 16

Titmus Test or TNO test

(Important to note these give different values and so have different ‘normal’ values).

Question 17

How can we test for stereopsis (3)?

Answer 17

Via Total dissociation (show each eye a totally different image) or Partial dissociation (show each eye slightly different images).

Or a test which involves no dissociation between the eyes.

Question 18

What machine do we use to test stereopsis via total dissociation?

Answer 18

A Synoptophore

[Each eye gets presented with two different images then they are later asked about the direction of the image whether it was coming out towards them or not]

Question 19

What are the different ways we can carry out a partial dissociation to test for stereopsis , and which tests use each method?

Answer 19

—Using Polarisation filters on each eye to view the image through. The tests that use this are:

◦Titmus (Wirt)

◦Randot

◦Mallett unit

—Using Red / green filters. The test that uses this method is:

◦TNO

— Using Optical method of dissociation. The test that uses this method is:

◦Lang

Question 20

Describe the Titmus test for stereopsis and how it is carried out?

Answer 20

It relies on using partial dissociation via the use of polarised 3D glasses.

You must put the glasses on the child straight away (as the child will just pick the image they saw looking most blurry before). {That blurry image is technically therefore a monocular cue however it is gotten rid of by putting the glasses on first]

It is a qualitative test - results are either that the child shows presence f good stereopsis or not.

People that do not have good stereopsis like to naturally assume they do so if you ask them do any pictures stick out towards you they will always say yes. The way to check if they actually are percieving depth is to rotate the image by 90 degrees and 180 degrees. At 90 degrees px should not be able to percieve depth but at 180 they should be able to percieve depthbut this time instead of the image sticking out it will stick in.

Viewing distance is about 40 cm away.

The test tests for 40 seconds of arc max.

The gross image will be the fly if the px cannot see the fly then they don’t have stereopsis.

Question 21

Describe the Randot test and how it is carried out?

Does the Randot test have monocular cues?

Answer 21

It relies on partial dissociation via the use of polarised 3D glasses.

It is a quantitative test. Px can be given a score from 500” to 20”.

Partially - on the right hand side where the images are of differing contour this can be a monocular cue however on the left side (in the random dot principle images) there aren’t any monocular cues. [Due to the right side where there are Part contour stimuli there can be false positives].

Viewing distance is about 40cm (16 inches).

Question 22

What is a vectograph?

Answer 22

A vectograph is a type of stereoscopic print or transparency viewed by using the polarized 3D glasses most commonly associated with projected 3D motion pictures.

Question 23

Describe the Mallet unit test and how it is carried out?

Answer 23

The test uses partial dissociation via polarising glasses- You put the mallet unit on the polarising filters.

There are two rows of symbols but the pateint fuses them together and therefore sees the symbols sticking out.

The symbols vary in disparity.

It tests from –10’ (600”) to 30” of arc.

The viewing distance is 36 cm.

[Calibration given- will change with viewing distance]

Question 24

What is a benefit of the TNO test for stereopsis?

Answer 24

There are no monocular cues.

Question 25

Describe the TNO test and how it is carried out?

Answer 25

Test relies Partial dissociation via Red / green glasses (red before LE and green before RE)

— Images have a Random dot principle

—There are No monocular cues

◦The test conists of 3 screening plates and 3 quantative plates.

–Screening plates are 1980 sec of arc

◦The 3 quantitative plates have 4 presentations on each plate -2 presentations at each level

–The quantative testing ranges from 480 to 15 sec of arc

◦ The Viewing distance is 40 cm

—Often the method of choice

subject to:

◦Age

◦Understanding

◦Wearing glasses

Question 26

Which type of px is the Lang test best for?

Answer 26

Children ( twinkle twinkle little star is one of the first rhymes they learn and also a shape used in the test).

With small children you can see them try to reach for the image.

Question 27

Describe the Lang test for stereopsis and how it is carried out?

Answer 27

It relies on panography dissociation via the use of high powered cylinders ( in each eye) and seperate images on strips.

It relies on random dots ( thus no monocular cues).

There are two types of tests - Lang 1 and Lang 2.

—Lang 1 (involves an image of a cat, a car and a star)

◦It is done ar At 40 cm and tests from 1200” to 550”’

—Lang 2 (involves an image of an elephant, a car, a star and a moon)

◦At 40 cm: 600” – 200”.

In both tests the star acts as the control - the control can be seen with one eye and so is purely there so that optom can know if px understands what they are being asked.

No glasses are requires - which is great for fussy children.

Good screening test.

Question 28

Which tests for stereopsis rely on having no dissociation between the eyes?

Answer 28

Free space tests

Langs 2 pen test

Frsiby Test

Question 29

Describe Langs 2 pen test and how it is carried out.

Answer 29

—Patient and examiner each hold a pen vertically

—Patient brings points (tip of the pens) into contact

—Cover one of patients eyes

◦Repeat test

—If accuracy poorer when using only one eye - this indicates presence of stereopsis under binocularly conditions

—Equal monocular and binocular response (as in the patient is equally as good at getting the tips of the pen to line up) -Indicates absence of stereopsis

—Useful clinical tool

◦It is a Qualitative test

◦ It tests low grade BSV

◦GROSS (Coarse) stereopsis test

Question 30

Describe the Frisby test for stereopsis and how it is carried out.

Answer 30

There are four plates and one of them has a circle in it. You ask the px which one it is , if they get it right you move to the next set of plates (which is thinner) and so on.

—3 thicknesses of sheet:

◦6mm, 3mm and 1mm

The disparity in viewing distance images means it can test for 875” to 20” of arc.

Viewing distance can be varied however this would change the minutes of arc results of the test.

This test doesn’t require glasses.