lecture 5 Flashcards
define depth perception
being able to judge where things are in 3D
how many forms of cues are there for depth perception
2:
- monocular and binocular
binocular cues give you…
stereopsis
list the monocular pictorial cues
relative size occlusion (interposition) relative height shadows atmospheric perspective linear perspective familiar size texture gradient
list monocular movement produced cues
motion parallax
kinetic depth effect
explain motion parallax
near objects pass retinal image plane faster than distance objects
explain kinetic depth effects
when a 3D form of an object can be perceived when the object is moving
how do we perceive depth and size?
oculomotor cues
what is the monocular oculomotor cue?
accommodation
what is the binocular oculomotor cue?
convergence
explain how stereopsis is created
- Eyes located at different positions on the head
- 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
are binocular disparities always present?
when looking at 3D objects - yes
when looking at 2D objects- need to be simulated by presenting 2 different images to each eye
advantages of BSV
- stereopsis
- binocular summation
- better space perception
- hand eye coordination
- better reading
- seeing objects in camouflage
what jobs would be difficult without stereopsis
- cannot be pilot by law
- dentistry
- sports
- seamstress
- jeweller
etc
define stereo acuity
smallest depth difference we can detect
what effect does retinal disparity have on depth effect?
Greater the disparity the greater the depth effect
what is the best stereopsis possible
2 secs of arc
what is the average stereopsis
10 secs of arc
what factors affect stereo thresholds (7)
- practice effects
- luminance
- exposure duration
- retinal eccentricity
- crowding effects
- motion in depth
- monocular blur
what is more detrimental to stereoacuity? mono blur or binocular blur?
monocular blur
why is it important to ensure anisemotropia is fully corrected?
monocular blur is more detrimental to stereoacuity than binocular blur
when does stereopsis develop?
between 3-4 months
when does stereopsis reach normal levels?
6 months
at 9 years old, how many seconds of arc of stereopsis on Titmus?
40
what is a qualitative method of measuring stereopsis? (total dissociation)
using the stereoscope or synoptophore
how does the synoptophore work?
each eye has a different viewing tube
two images presented one to each eye
ask px if the arrows are pointing more towards the clinician or the Px
if px says the arrows are pointing more towards themselves- no stereopsis
towards clinicians - stereopsis?
what are the methods of measuring stereopsis? (partial dissociation)
polarisation
- Titmus (wire)
- randot
- mallet unit
Red/green
- TNO
Optical
-Lang
titmus test uses polarised glasses, why?
to reduce monocular cues
is the titmus test qualitative or quantitative?
can be both (fly is qualitative, animals and circles are quantitative)
what distance is the titmus test carried out at
40 cm or 16”
how many secs of arc is the fly in titmus?
3000
how many secs of arcs are the animals in titmus?
100-500
how many secs of arc are the circles in titmus?
40-500
if you rotate the titmus by 90, will your px have depth perception?
no- the wings should be flat - use this as a test to see if they are lying
does the randot test have monocular cues?
some due to part contour stimuli
does the part random dot principle part of the randot test provide monocular cues?
no
is randot test qualitative or quantitative?
quantitative
how many secs of arc does the randot test measure ?
500- 20
what viewing distance do you carry out the randot test at?
16” or 40 cm
explain the mallet unit test?
you have 2 rows of symbols,
try to fuse the two rows using 3 base out prism
when fused the shapes will stick out
how many secs of arc does the mallet unit test up to ?
10- 30
viewing distance of mallet unit ?
36 cm
why is it important to do the Mallett unit at 36 cm?
calibration will change with viewing distance
how do you dissociate the 2 eyes in the TNO test
use red/green glasses
how many screening plates in the TNO test
3
how many secs of arc do the screening plates in TNO test up to?
1980
how many quantitative plates are there in TNO test?
3
4 presentations on each plate
2 presentations at each level
how many secs of arc do the quantitative plates test up to?
480-15
viewing distance of TNO test
40 cm
who is the TNO test good for?
children
people with special needs
wearing glasses
advantage of the TNO test
it tests up to 15 secs of arc with no monocular cues so good to use to test how good their stereopsis is
who is lang test good for?
toddlers and children
is lang a screening test?
yes
which shape is a control shop ein lang test?
star
how do you present lang plates
ask them what they can see
viewing distance for lang 1 and 2
40 cm
how many secs of arc does lang 1 test up to?
1200-550
how many secs of arc does lang 2 test up to?
600-200
which test have no dissociation
free space tests
langs 2 pen
frisby
what do free space test entail?
perception of real depth
no 2D simulation
what does the lang 2 pen test entail?
Patient and examiner each hold a pen vertically
Patient brings points of pens into contact
Cover one of patients eyes
◦ Repeat test
If accuracy poorer when using only one eye
indicates presence of stereopsis under binocular conditions
Equal monocular and binocular response indicates absence of stereopsis
why is langs 2 pen test a useful clinical tool
test low grade BSV or gross/coarse stereopsis
is langs 2 pen quantitative or qualitative?
qualitative
what does the frisby test entail?
Random array of shapes 4 squares of shapes per sheet One square contains a central circle Observe behaviour or ask question about circles Turn around sheet ask again Responses seen in infants as young as 6 months
what thicknesses of sheets does the frisby test come in?
6mm, 3mm and 1 mm
which fribky sheet tests better level of stereoacuity
1mm
what is the disparity of a frisby sheet?
875-20”
how can you change the level of stereoacuity measurable by the frisby sheets?
- increase viewing distance from 30-80 cm
- change the sheet thickness
which test would you use for an under 2 year old?
land 1/2 or frisby
which test would you use for someone who is about to decompensate?
frisby or lang 1/2
which test would you use when you require the most accurate test and they are 8 years old?
TNO
which test would you use for a child refusing to wear glasses?
lang 1/2 or frisby
which test would you use for a home visit and you have no stereo-tests
lang 2 pen
