Stereopsis Flashcards
what is the best form of depth perception
stereopsis
what is depth perception
how far or how near something is away from you
& allows you to see the world in 3D
what are the two forms of depth perception
- monocular (one eye) cues
- binocular (two eyes) cues
which type of people do not have stereopsis but can perceive some sort of depth perception which is not as good as a person with normal BSV
- person with one eye
- person with a large strabismus
list and the things which represent monocular pictorial cues
- relative size
- occlusion (interposition)
- relative height
- shadows
- atmospheric perspective
- linear perspective
- familiar size
- texture gradient
explain relative size in relation to monocular pictorial cues
two objects equal in size, the one far away will take up less field of view
explain occlusion (interposition) in relation to monocular pictorial cues
one object is hidden by another object, and that hidden object is considered further away
explain relative height in relation to monocular pictorial cues
Two types which describe the object as being further away:
- below horizon = objects further away have higher bases
- above horizon = objects further away have lower bases
explain shadows in relation to monocular pictorial cues
shadows removes the contradictions of an object which occludes another yet has a higher base and is claimed to be further away, as the shadow shows that the object is in front and elevated
explain atmospheric perspective in relation to monocular pictorial cues
furtherest object is shown by light scatter and haziness in the atmosphere which makes it look further away
explain linear perspective in relation to monocular pictorial cues
with depth the distance between parallel line becomes smaller in 2D picture e.g. train tracks converge as they go further away
explain familiar size in relation to monocular pictorial cues
previous knowledge of the object’s size is used to determine the absolute depth of the object e.g the man is closer to the camera and the taj mahal is further away yet we know the taj mahal is larger than the person
explain texture gradient in relation to monocular pictorial cues
objects are closer as can see more fine detail and objects further away as they are less defined and more blurry
what is terror subterra (shepard)
- due to the background, we perceive the upper object to be further away & therefore the larger one, but both objects (monsters) are actually the same size, but linear perspective and convergent lines makes the back monster bigger
another example - the ames room, where all men are the same size, but the room has been constructed so that the left hand corner is twice as far away from the observer as the right hand corner
name the two types of monocular movement produced cues
- monocular parallax
2. kinetic depth effect
what is monocular parallax
when near objects pass retinal image plane faster than distance objects e.g. japan bullet train
what is kinetic depth
when a three dimension form of an object can be perceived when the object if moving e.g. the moon (which we know is spherical and not flat due to its kinetic effect)
how do we perceive depth and size
oculomotor cues
what are the two types of oculomotor cues which allows us to perceive depth and size
- accommodation (monocular)
- convergence (binocular)
explain how accommodation allows us to perceive depth and size
kinesthetic sensations from the ciliary muscles are sent to the visual cortex where it is used for interpreting distance/depth
explain how convergence allows us to perceive depth and size
kinesthetic sensations from the extra ocular muscles also help in depth/distance perception e.g. the medial rectus sends signals to the brain when it contracts so we know when an object is close, so that we can converge
name a type of binocular disparity (cues)
stereopsis
what is stereopsis known for
the best form of depth perception
explain how stereopsis occurs
- eyes located at different positions on the head which allows us to see two separate images and
- binocular vision results in two slightly different (disparate) images are 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
- binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes
- when viewing a 2D scene, disparities need to be simulated artificially by presenting two different images separately to each eye
what does binocular vision result in
two slightly different (disparate) images are projected to the retinas
where are the disparities processed
in the visual cortex to yield depth perception (stereopsis)
how else is the perception of depth and 3-dimensional structure possible
with information visible from one eye alone however not as vivid as stereopsis
when are binocular disparities naturally present
when viewing a real 3 dimensional scene with two eyes
when viewing a 2D scene, how do disparities need to simulated
artificially by presenting two different images separately to each eye
list the advantages of binocular single vision
- stereopsis
- binocular summation
- better space perception
- eye hand co-ordination
- better reading
- seeing objects in camouflage
what job will be difficult to carry out without stereopsis
delta flight
what can cause bad stereopsis
monovision contact lenses
what is stereo acuity
the smallest depth difference we can detect
if the disparity is greater, what also is greater
the greater the dept effect
what is the stereo acuity of the best observer
as good as 20 sec of arc
what is the average stereo acuity
10 secs of arc
what factors effect stereo thresholds
- practice effects
- luminance (too light or too dark gives different results)
- exposure duration (do it too quick)
- retinal eccentricity
- crowding effects
- motion in depth
- monocular blur is more detrimental to stereo acuity than binocular blur (e.g. must correct both eyes at best especially if -6.00, better to have correction in both eyes rather than just one eye)
when is stereopsis absent
initially when we’re born
when does stereopsis develop
between 3-4 months
when does stereopsis reach normal levels
at 6 months (visual evoked potentials)
at which age will someone reach normal adult level stereopsis
9 years old (achieve 40 secs of arc on titmus
how is research of stereopsis carried out
using real stereo-tests
what do majority of adults achieve on the TNO test
60 secs of arc
at what age does stereopsis start to decline
> 70 years old
where is stereopsis tested
clinically
what two types of data can be collected from stereopsis tests
- qualitative
- quantitative
what does qualitative data show from a stereo test
evidence of stereopsis i.e. it just shows that the px has stereopsis, not how good it is, so a yes/no answer
what does quantitative data show from a stereo test
- a measure of stereo acuity e.g. how many mins/secs of arc
- graded response - calibrated for a specific viewing distance
- assumes an average p.d.
what measures the total dissociation
- stereoscope (not used)
- synoptophore
explain how the synoptophore works
- each eye has a different viewing tube where two images are presented, one to each eye
- major amblyoscope
- ask px is arrows are coming towards or away, if they are doing either, then they have stereopsis
- is a qualitative test
what does a px see when measuring partial dissociation
both eyes see the same picture but each eye sees different parts of the image
which stereo tests use polarisation during the test
- titmus (wirt)
- randot
- mallet unit
which stereo test uses red/green filters
TNO
which stereo test is optical
lang
how is the titmus (wirt) measured
partial dissociation using polarised 3D glasses
what does the titmus (wirt) measure
relative depth
what results of relative depth does the titmus (wirt) measure
- qualitative assessment fly = 3000 sec of arc - quantitative assessment animals = 500-100 sec of arc circles = 500-40 sec of arc
if the px cannot see 3000 sec of arc during titmus (wirt) test, then what can they see
can’t see anything as this is the grossest test there is
which part of the quantitative assessment of the titmus (wirt) is a normal value for adults
circles = 500-40 sec of arc which just touches the normal value
what is the viewing distance for the titmus (wirt) test
16” or 40cm
what happens when rotate the card by 90 degrees with the titmus (wirt)
no depth/3D effect
what happens when rotate the card by 180 degrees with the titmus (wirt)
inversion of depth (goes in rather than coming out)
what is the disadvantage of the titmus (wirt) test
monocular cue as the 2nd picture on row A, is blurry with both and one eye, so some children get the picture right because they’ve guessed it which is not a good clue of whether they have stereopsis = false positive answer
what is the randot test measured with
partial dissociation using polarised 3D glasses
how many monocular cues does the randot test have
part contour stimuli = left side has monocular cues
part random dot principle = right side does not have monocular cues, but has simple geometric shapes (making it better than the titmus)
which type of assessment is the randot test
quantitative
how many seconds of arc does the randot measure
500-20” of arc
what is 20” of arc better than
adult standard
what viewing distance is the randot carried out at
16” or 40cm
what are the disadvantages of randot test
- monocular cues
- some false positive responses
what is the mallett unit measured with
polaroid dissociation
how is the mallett unit test carried out
- two rows of symbols
- fuse the two rows to give a 3D image
- if struggle to fuse together, use 3 base out prism
- when fuse together, the objects should look like they’re in 3D
how do the symbols vary in disparity in the mallett unit test
10’ (600”) at the top, to 30” or arc at the bottom row
what is the viewing distance of the mallett unit test
36cm
calibration given
will change with viewing distance
what is the TNO test measured with
partial dissociation
red/green glasses (red before the LE)
what principle does the TNO use
random dot principle
does the TNO have monocular cues
no monocular cues so is the safest test and can do on all patients
how many screening plates does the TNO test have
three 1980 sec of arc
how many quantitative plates does the TNO have
three
how may presentations are there of each of the three plates of the TNO test
4 presentations of each plate
how many presentations are there at each level of the TNO test
2 presentations at each level
what sec of arc do the 2 presentations at each level range between with the TNO
480-15 sec of arc
what is the significance of 15 seconds of arc which can be measured in the TNO
it is the lowest seconds of arc that can be measured
what is the viewing distance of the TNO test
40cm
what are the disadvantages of the TNO test
often the method of choice is subject to
- age
- understanding
- wearing glasses
how is the lang test measured with
panography dissociation
- high powered cylinders
- separate image in strips
what principle does the lang use
random dot
are glasses required for lang
no
what pictures are present with lang 1
cat, car, star
what distance is lang 1 carried out at
40cm
what is the range of seconds of arc which the lang 1 measures for
1200” - 550” of arc
what pictures are present with lang 2
elephant, car, star, moon
what distance is the lang 2 carried out at
40cm
what is the range of seconds of arc which the lang 2 measures for
600-200” of arc (so better for qualitative yes/no stereopsis test
what are the advantages of the lang test
- the value of having a control - star is a control, if can see star only, theres no stereopsis as can see the control with one eye
- good screening test as a quantitive measure
name a no dissociation test
free space tests
what do free space tests measure
perception of real depth
what simulation does the free space test NOT have
no 2D simulation
give examples of free space tests
langs 2 pen
frisby
explain how to do the langs 2 pen test
- patient and examiner each hold a pen vertically
- patient brings points of pens into contact
- cover one of patients eyes
- repeat test
if accuracy of the langs 2 test is poorer when using only one eye, what does that indicate
presence of stereopsis under binocularly conditions
what does an equal monocular and binocular response indicate from the langs 2 test
absence of stereopsis
what type of assessment is the langs 2 pen test
qualitative test
what grade of bsv does the langs 2 pens test
low grade bsv
gross (coarse) stereopsis
explain how to do the frisby test
- random array of shapes
- 4 squares of shapes per sheet
- one square contains a central circle which stands out in 3D
- observe behaviour or ask px to point to where the circle is
- turn around the sheet and ask again
does the frisby require glasses
no
as low as what age can there be responses of the frisby test
infants as young a 6 months
how many thicknesses of sheets are there in the frisby test
3
- 6mm
- 3mm
- 1mm
what is the range of disparity of the frisby test in seconds of arc
875” - 20”
what does the disparities depend on
- sheet thickness
- viewing distance
what is the viewing distance of the frisby test
30cm - 80cm
what test would you use on a < 2 year old
- lang
- frisby
- titmus or TNO
what test would you use on a 5 year old
titmus or TNO
what test would you use on a small angle strabismus with only very slightly reduced stereo acuity
TNO
to see how far they can go
what test would you use on a patient about to decompensate
frisby
as when you have glasses you decompensate, and frisby requires n specs
what test would you use when you require the most accurate test and they are 8 years old
TNO
what test would you use on a child refusing to wear glasses
lang 2 pen test, but destine rate the stereo acuity
so frisby is better to rate stereo acuity as it goes down the furthest
what test would you use on a home visit and you have no stereo tests
langs 2 pen
what question would you ask to a 2 year old when carrying out a frisby test
in reference to the circle ‘can you see where the ball is’
what question would you ask to an 8 year old when carrying out a TNO test
‘can you see the pizza slice or cake, and ask where the slice is missing by pointing to it’
