# 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.

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 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
 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

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 &amp; Holmes 2004
 Measured
stereopsis using three clinical tests
 Frisby (global)
 Pre-school stereo test (global)
 Titmus (local)
Stereopsis in Strabismus

Leske &amp; 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 &lt; 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

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

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
Pegboard test
Functional impact of stereopsis
significantly worse in the no stereo group.
The normal stereo group had a significant
performance drop off when performing the
 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

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

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.

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