stereopsis Flashcards
what is stereopsis?
- depth perception
- when you look at an object you judge the distance of object to you
what are the two forms of depth perception ?
- monocular ( one eye) cues
- binocular ( two eyes) cues
what is the best form of depth perception ?
- binocular form - stereopsis ( BEST FORM)
what do people with monocular vision do to judge distances ?
- they use monocular cues - it’s not as good as stereopsis but useful in helping px to judge distance
what are monocular pictorial cues ?
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
- occlusions ( interposition)
- one object hidden, then the hidden object is considered further away - relative height
- below horizon: objects further away have higher bases
- above horizon: objects further away have lower bases - 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 - atmospheric perspective - due to light scattering by the atmosphere, objects that are a great distance away appear hazy
- linear perspective - with depth the distance between parallel line becomes smaller and they converge in 2D picture - come closer together
- 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.
- texture gradient- fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects
why does in the picture with two monsters, the monster at the back appears larger?
- both monsters are the same size.Due to the background we perceive the upper one to be further away and therefore larger
why does the ames room picture give the impression that the lady is significantly smaller than the man?
. 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
what are the monocular movement produced cues?
- motion parallax
- near objects pass retinal image plane faster than distance objects- such as mountain remains in background but trees just go by - kinetic depth effect
- when a 3 dimensional form of an object can perceived when the object is moving
what is different oculomotor cues?
- they are kinesthetic which means we can feel the sensations of theses changes
what are the oculomotor cues?
1- accommodation (monocular )
2-convergence ( binocular )
explain accommodation(monocular) as an oculomotor cues?
. 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
explain convergence (binocular) as a oculomotor cues?
- 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
how does stereopsis arise?
- arises from disparate objects, slightly different objects being fused together
when would we not be able to see in stereopsis ?
- if information from both eyes was the same we wouldn’t be able to see in stereopsis
what allows us to see in 3D?
- 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
explain binocular disparity cues?
. 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
what are we using when looking at pictures in cinema ?
- 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
when are binocular disparities naturally present?
- binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes
what happens when viewing a 2D ?
when viewing a 2D scenes disparities need to be stimulated artificially by presenting two different images separately
what are the advantages of having stereopsis ?
- you’re binocular summation is better
- better space perception
- better eye hand co-ordination
- better reading
- stereopsis
- seeing objects in camouflage
what jobs would be difficult to carry out without stereopsis?
- cannot be a pilot
- job that you need to judge distances
what happened in delta flight ?
- 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
what is stereo acuity ?
- smallest depth difference we can detect
- the greater the disparity, the greater the depth effect
what are the observed limits of stereopsis ?
- best observes as good as 2 sec of arc
- on average 10 seconds of arc
what are the factors affecting stereo thresholds?
. practice effects . luminance . exposure duration . retinal eccentricity . crowding effects . motion in depth . monocular blur is more detrimental to stereo acuity than binocular blur
what is the development of stereopsis ?
. initially absent
. develops between 3-4 months
. reaches normal level at 6 months ( visual evoked potentials )
what do we use to measure stereopsis ?
- use books ( titmus) or (TNO)
- frisbee
what is normal in stereo-tests?
- depending on test we are using , dependent on what is considered normal
what is considered normal when using a titmus in children?
- children achieve 40 seconds of arc > 9years old
what is considered normal when using a TNO in adults?
- majority of adults achieve 60 seconds of arc (TNO) <70 years old
how can we measure stereopsis ?
- tested clinically
- quantitative - will get a number- lower number- better stereoacuity
- evidence of stereopsis - qualitative
- a measure of stereoacuity
- graded response( yes or no)
. calibrated for a specific viewing distance
. assumes an average p.d.
what is total dissociation ?
- the right and left eye gets totally different images
what is the method for the synoptophore ?
- 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
what happens in partial dissociation ?
- 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
what is partial dissociation based on ?
1- based on polarisation . titmus .randot . mallett unit 2. red/green .TNO 3. optical . Lang
explain Titmus( Wirt)?
- 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
how to reduce monocular cues when carrying Titmus out ?
. monocular cues minimised if you put on glasses first
what to avoid with titmus test?
. 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
explain randot test?
- 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
explain mallet test?
- 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
explain the TNO test?
- 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 )
what are the method of choice subject to in TNO test ?
. age
. understanding
. wearing glasses
explain the Lang test ?
. 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
what is no dissociation ?
- no dissociation
- perception of real depth
- no 2D stimulation
- the test is based on real depth - no 2D stimulation
what are the two test of no dissociation ?
. Langs 2 pen
. frisby
explain Langs 2 pen test?
. 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
explain the frisby test ?
. 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)
which test would you use for steropsis?
- 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 - 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 - you require the most accurate test and they are 8 years old:
TNO - goes down to 15 secs of arc - child refusing to wear glasses:
- Frisbee or Lang 1 or Lang 2 - home visit and you have NO stereo-tests:
- Lang 2 pen
what questions do you ask with each stereo-test?
. 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’
