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
Still appreciate depth
 Monocular cues help us with depth
 Patient’s often confused about this.
Cues not hard wired
 Learned inferences that the visual system
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
Emmert’s Law
 Afterimage
Problem with using retinal images
We do not perceive real life objects based
on visual angle.
 Familiar objects would be growing or
 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)
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
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
 Result of chromatic aberration
 Longitudinal vs transverse
Depth discrimination threshold
 Hyperacuity similar to vernier acuity – can
get 4 to 5 seconds of arc
Calculating disparity – Howard Dolman
 n = 2a/2.9 x 10-4 x d/d2
= angular stereoscopic disparity in radians
 2a = interpupillary distance
 d = fixation distance
 change in d = linear distance between the two
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
Stereo Targets
Local stereopsis
 Line
 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
 Targets have to be fused for form
perception to occur
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
Takes longer to perceive global stereopsis
 Can have good stereoacuity but poor
global stereopsis.
 Implications
Methods of Presenting Stereo
Linear displacement methods
 Vectographic methods
 Polaroid
 Anaglyph
 Local
and Global stereopsis
 Linear displacement
 Far point measurement
Rods appear equidistant
 No head movement
 Influenced by skews in the horoper
Local test
 Can be used at a variety of distances
 Monocular and stereo cues conflict
 Uses
relative size
8 settings
Verhoof Decimal Stereoacuity
2.0m (200)
1.5m (150)
1.0m (100)
Clinical linear displacement
Frisby-Davis 2
 Distance
stereo test
 Local test
 Uses
plates for near testing
 Global test
No Glasses
Randot E
Use at variety of test distances
Titmus or Stereofly
Common in ophthalmology
Randot Stereo Test
Newer Version
Randot Preschool Stereoacuity
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
Blur and stereopsis
How does bilateral blur impact
 How does unilateral blur impact
Stereopsis and Blur
Monocular blur has greater impact on
 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
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
 Mean stereoacuity in the monovision was
200 sec and bifocal was 50 sec
Does the patient need good
Prevent problem from arising through early
 Occupation demands
Stereo advantage?
Can you do certain tasks better with
normal stereo?
 Do individuals without measurable
stereopsis (fine stereopsis) adapt to this
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
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.
Two types
 Wheatstone
 Brewster
Septum device that uses mirrors
 Usually set for near viewing – typically 33
 Has a scale of convergence and
 Can use +3.00 to simulate distance
Septum device that uses prisms and plus
 Usually +5.00 with a separation of 95 mm
which creates a Base out effect.
 Target distance of 20 cm
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
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
3-D displays
Increasing use in movies and home
 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.
Tests of relative accommodation or
vergence create conflict between the two
 Vectograms also do this
Clinical Concerns
Very little research on why 3-D effect
induces visual discomfort in certain
 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
Depth of focus and distance
Pulfrich Phenomenon
Inducing the effect
 Place neutral density filter over one eye
 Patient perceives an object moving in
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
Optic nerve disease,
 Unilateral glaucoma,
 Retinal pathology
 Systemic disease
 Trauma
 Amblyopia
 Moving
or parked cars appear to curve
 Difficulty of parking
 Difficulty
crossing roads
 Misjudgment
when pouring liquids
 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.

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