Monocular Cues to Depth
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Transcript Monocular Cues to Depth
Test 1
Covers material through fixation disparity
20 multiple choice questions
Study guide questions and problem set
Do not need to know formula for
calculating the amount of fixation disparity.
Cues to Depth
What happens when you close one
eye?
Still appreciate depth
Monocular cues help us with depth
perception.
Patient’s often confused about this.
Characteristics
Cues not hard wired
Learned inferences that the visual system
makes.
Monocular Cues to Depth in a
deprived environment
Retinal image size-this cue works when
other cues are absent
Emmert’s law – the perceived size of the
object producing a retinal image of given
fixed size is proportional to its perceived
distance.
Emmert’s Law
Example
Afterimage
Problem with using retinal images
We do not perceive real life objects based
on visual angle.
Familiar objects would be growing or
shrinking
Need an invariant or constant to
compensate for retinal image size
Holway/Boring Experiment
Size Constancy
Perception needs to account for both
distance and retinal size
S=K(RxD)
S
is perceived size
K is a constant
R is retinal image size
D is perceived distance
Monocular Cues in Natural Setting
See website this is different than in lecture
notes.
http://sites.sinauer.com/wolfe3e/chap6/star
tF.htm
Stereopsis
This is true depth perception.
Preattentive
Absolute Depth
Distance from egocentric position
More dependant on monocular cues
Relative Depth
Objects in relation to each other
Stereopsis ideal for detecting this
Types of Disparity
Horizontal vs. vertical disparity
Other types of disparity
Disparity
gradients give rise to tilted surfaces
Orientation disparity
Types of Disparity
Horizontal vs. vertical disparity
Other types of disparity
Disparity
gradients give rise to tilted surfaces
Orientation disparity
Special Cases
Chromostereopsis
Result of chromatic aberration
Longitudinal vs transverse
Stereoacuity
Depth discrimination threshold
Hyperacuity similar to vernier acuity – can
get 4 to 5 seconds of arc
Howard-Dolman
Calculating disparity – Howard Dolman
Apparatus
n = 2a/2.9 x 10-4 x d/d2
n
= angular stereoscopic disparity in radians
2a = interpupillary distance
d = fixation distance
change in d = linear distance between the two
rods
Characteristics
Effect of exposure time
Retinal eccentricity
Background illumination
Measuring Stereoacuity
Howard Dolman apparatus
Method of adjustment
Method of limits
Stereopsis upper limit
Patent or quantitative stereopsis
Latent or qualitative stereopsis
Purpose of coarse stereo
Processing of depth outside the horopter
Cue for the vergence system
Back up stereo system
May
be primary stereo signal for small angle
strabismus.
Stereo Targets
Local stereopsis
Line
targets
Provides monocular cues
Best used for Stereoacuity measures
Global Stereopsis
Random dot stereogram – provides no
monocular cues to depth
May play a role in detecting camouflaged
object
Targets have to be fused for form
perception to occur
Characteristics
Stereo correspondence problem or how
does the visual system decide which dots
to fuse.
The visual system yields the best global
interpretation of depth
Depth averaging
Characteristics
Takes longer to perceive global stereopsis
Can have good stereoacuity but poor
global stereopsis.
Implications
Methods of Presenting Stereo
Targets
Linear displacement methods
Vectographic methods
Polaroid
Anaglyph
Local
and Global stereopsis
Howard-Dolman
Local
Linear displacement
Far point measurement
Rods appear equidistant
No head movement
Influenced by skews in the horoper
Verheoff
Local test
Can be used at a variety of distances
Monocular and stereo cues conflict
Uses
relative size
8 settings
Verhoof Decimal Stereoacuity
Distance
Decimal
Stereoacuity
Binocular
Disparity
2.0m (200)
2.0
8.25
1.5m (150)
1.5
14.67
1.0m (100)
1.0
33
Clinical linear displacement
Frisby-Davis 2
Distance
stereo test
Local test
Frisby
Uses
plates for near testing
Global test
Frisby
No Glasses
Stereograms
Randot E
Use at variety of test distances
Titmus or Stereofly
Common in ophthalmology
Randot Stereo Test
Newer Version
Randot Preschool Stereoacuity
Test
Distance Randot Stereotest
TNO test
Red/Green anaglyph
Global test
Magic Eye
Wallpaper stereogram
See website chapter 6
Eye Alignment and Stereopsis
How accurate does the vergence need to
be for optimal stereoacuity?
Can a strabismic patient see stereo?
Stereopsis and Phoria
Exophoria up to 7 prism diopters had little
effect on stereoacuity
Esophoria beyond 1 diopter associated
with an increase in stereoacuity with each
diopter increase in phoria.
Exophoria has less impact on fixation
disparity than esophoria
FD and Phoria-Distance
Stereopsis in Strabismus
Leske & Holmes 2004
Measured
stereopsis using three clinical tests
Frisby (global)
Pre-school stereo test (global)
Titmus (local)
Stereopsis in Strabismus
Leske & Holmes 2004
Titmus
tests has monocular cues leading to
false positives
No true stereo responses in strabismus
greater than 4 PD. Positive stereo response
on all 3 tests.
Measuring Stereoacuity
Most patients with normal binocular vision
will get all targets.
Most patients with constant strabismus
(Greater than 4 PD) will get no stereo or
false positives.
Measuring Stereoacuity
Fawcett study (2004 JAAPOS)
Measured stereoacuity in treated patients
with abnormal binocular vision using
Preschool Randot, Titmus Circles, and
Randot Circles. This was compared to a
normal binocular vision group.
Measuring stereoacuity
Fine < 60 seconds
Moderate 70 to 200
Coarse 400 to 800
Nil-not measurable
Measuring Stereoacuity
Fawcett study (2005 JAAPOS)
All normals able to correctly identify all
levels of stereo presented
Better stereoacuity with local stereopsis
tests.
Blur and stereopsis
How does bilateral blur impact
stereoacuity?
How does unilateral blur impact
stereoacuity?
Stereopsis and Blur
Monocular blur has greater impact on
stereoacuity
Can be a more than double decrease at
higher add powers
McKee and Westheimer
Experimental measure with highly trained
subjects. Sample subject started at 6 seconds of
arc
Binocular blur
+1.50
- 10 sec
+2.50 – 37 sec
Monocular blur
+1.50
- 13 sec
+2.50 – 77 sec
Kirschen et al, 1999
Measures stereoacuity in 19 successful
monovision patients. Mean age of 52.
Compared stereoacuity in habitual
monovision versus Acuvue bifocal contact
lens.
Mean stereoacuity in the monovision was
200 sec and bifocal was 50 sec
Does the patient need good
stereopsis?
Prevent problem from arising through early
intervention
Occupation demands
Stereo advantage?
Can you do certain tasks better with
normal stereo?
Do individuals without measurable
stereopsis (fine stereopsis) adapt to this
condition?
Functional impact of stereopsis
O’Connor et al
Purpose of study: functional impact of stereoacuity
deficits in eye hand coordination tests.
Methods: compared motor ability in normal stereo and
stereo deficit (strabismus) on three eye hand
coordination tasks.
Pegboard test
Bead task (small and large)
Water pouring task
Functional impact of stereopsis
Results: Pegboard and bead task
significantly worse in the no stereo group.
The normal stereo group had a significant
performance drop off when performing the
task without stereo (monocular condition).
Conclusion: recommend early treatment to
recover stereopsis or prevent loss of
stereopsis.
Sheedy et al study
18 presbyopes with a mean age of 52
New cases fit with monovision
Measured performance at dispensing, 2
weeks, and 8 weeks.
Tasks done with habitual monovision and
with binocular distance vision correction
with reading glasses.
Sheedy et al study
Compared performance in 3 tasks; pointer
and straw, card filing, and editing
All tasks showed a small but statistically
significant reduction in performance that
persisted after 8 weeks of CL wear.
Patient still preferred monovision and did
not notice performance reduction.
Stereoscopes
Two types
Wheatstone
Brewster
Wheatstone
Septum device that uses mirrors
Usually set for near viewing – typically 33
cm
Has a scale of convergence and
divergence
Can use +3.00 to simulate distance
Brewster
Septum device that uses prisms and plus
lenses
Usually +5.00 with a separation of 95 mm
which creates a Base out effect.
Target distance of 20 cm
Brewster
Vergence Demand
Target
separation: h = S(in millimeters) x u (in
meters) x dioptic lens power
At 20 cm 2 mm equals 1 prism diopter
Effect of instrument on the vergence system
Brewster
Vergence Demand
Target
separation: h = S(in millimeters) x u (in
meters) x dioptic lens power
At 20 cm 2 mm equals 1 prism diopter
Effect of instrument on the vergence system
Vectograms and Stereopsis
Polarized targets used in VT
Introduce disparity
Size Changes
Depth Changes
Cue Conflict
SILO vs SOLI
3-D displays
Increasing use in movies and home
television
Monocular cues to depth were primary
depth cues
History- popular in the 1950’s using redgreen glasses but audiences got lots of
discomfort and poor stereo effect.
Types of displays
Passive Polarization
The Real D
Passive wavelength multiplexing
Active shutter systems
Autostereogram
Clinical Concern
Visual discomfort during viewing
Estimates of 5 to 20 percent of viewers
5% stereoblind
Simulated environment creates potential
conflicts between accommodation and
vergence. This is similar to relative vergence
measures where accommodation is at the target
plane but disparity vergence is changing.
Implications
Tests of relative accommodation or
vergence create conflict between the two
systems.
Vectograms also do this
Clinical Concerns
Very little research on why 3-D effect
induces visual discomfort in certain
individual.
Could become more common clinical
question as more individuals interact with
3-D displays
Possible factors impacting comfort
Amount of disparity in the image
Accommodation vergence mismatch
Zone of comfortable viewing
Depth
of focus
Limits in the vergence system
vergence ranges or panums area
Distortions
Accommodation/Vergence
Depth of focus and distance
Pulfrich Phenomenon
Inducing the effect
Place neutral density filter over one eye
Patient perceives an object moving in
depth.
Pulfrich Phenomenon
Increases the time gap between stimulus
onset and perception
Temporal disparity that creates the
perception of depth.
Filter over left eye creates a clockwise
motion
Etiologies
Optic nerve disease,
Unilateral glaucoma,
Retinal pathology
Systemic disease
Trauma
Amblyopia
Symptoms
Driving
Moving
or parked cars appear to curve
Difficulty of parking
Walking
Difficulty
crossing roads
Home
Misjudgment
when pouring liquids
Symptoms
Sports
Difficulty
with ball sports
Spatial judgments
Moving
objects appear to swerve
Sense of imbalance
Motion sickness
Case Report
42 year old female who had difficulty with
driving (needed to swerve car because
other cars were getting to close) also had
difficulties with doing photography.
Displayed a positive Pulfrich phenomenon
using swinging ball.
Case Report
The pulfrich phenomenon was neutralized
by either a 0.6, 0.8, 1.0 neutral density
filter over the right eye.
Patient was prescribed asymmetrical tints
in each and this resulted in a reduction of
symptoms, especially when driving.
Stereopsis and Phoria
Stereoacuity starts to decrease with
increasing amounts of fixation disparity
May not see this clinically because most
clinically based stereo tests only measures
down to 20 seconds of arc.