Transcript Fusion, Rivalry, Suppression

Fusion, Rivalry,
Suppression
Fusion Worth Classification
First degree fusion or simultaneous
perception
 Second degree fusion or flat fusion
 Third degree fusion or stereopsis

Theories of Fusion
Alternation or suppression theory:
 Fusion theory

Limits of fusion
Panum’s fusional area
 Fixation disparity
 Panum’s limiting case

Panum’s area
In the fovea Panum’s area measures 5 to
20 minutes of arc.
 Panum’s area is larger in the periphery.
 Factors influencing Panum’s area

 Vergence
eye movements
 Spatial frequency
 Temporal frequency
Binocular Rivalry
What happens when dissimilar images are
presented to each eye?
 Confusion
 Alternating intermittent suppression occurs
when dissimilar images are presented to
each eye

Binocular Rivalry
First degree fusion
 Allows us to monitor suppression
 Used in tests of correspondence

Characteristics
Greatest with separation 90 degrees apart
 Equal stimulus values

Characteristics

Stimulus strength determines the ability to
induce contralateral suppression in the
other eye. The strength of the stimulus is
related to the amount of contour per area
in the pattern and the contrast of the
contours.
Stimulus Strength-Reading
Brighter
Luminance
Darker
Higher
Contrast
Lower
Clear
Focus
Blurred
Foveal
Retinal locus
Peripheral
Moving
Movement
Stationary
Characteristics
Local phenomenon
 Reduced sensitivity to area suppressed
 DaVinci Stereopsis

Color Fusion and Luster
Different luminance values to each eye but
contours are the same.
 Get a glossy appearance
 Colors can fuse under some conditions

Clinical Applications

Confusion in strabismus can be thought of as
rivalry
 Similar
images fall on non-corresponding points on
the retina-leads to diplopia
 Dissimilar images fall on corresponding points-leads
to confusion
 Confusion can lead to rivalry where foveating eye
becomes dominate
 Development of constant suppression and possible
amblyopia
Suppression
We do not notice physiological diplopia so
we must be suppressing most of time.
 Most strabismic patients do not see
double, why?

Two processes reduce diplopia
Binocular sensory fusion which operates
within Panum’s area
 Suppression which is an interocular
inhibitory process that reduces visual
information from the suppressed eye
below the threshold for conscious
perception

Adaptations

In normal binocular vision the suppression
of physiological diplopia is called
physiological suppression or suspension.
Adaptations

When dissimilar images are presented to
corresponding retinal points confusion
results. Alternating suppression from each
results and is called binocular rivalry (see
above). Confusion can be eliminated by
regional suppression.
Adaptations

Pathological diplopia occurs when the
object of regard is imagined on noncorresponding points. This can be
eliminated by regional suppression.
Characteristics
Effect of orientation and spatial frequency
 Schor (1977) presented sinusoidal
gratings at various orientations and spatial
frequencies to normal and strabismic
subjects.

Characteristics


Strabismic subjects showed normal binocular
rivalry when targets of different orientation. Size
and spatial frequency did not alter result
When orientation difference reduced to less than
22 degrees there was constant suppression of
the deviating eye. In normals a depth effect of
was observed resulting from horizontal disparity
created by orientation difference.
Suppression in Strabismus

Amblyopia
 Reduction
in acuity under binocular condition
Suppression in Strabismus

Esotropia
 Forms
“D” pattern between the fovea and
zero measure point
Usually confined to one hemiretina and
does not extend beyond the nasotemporal decussation line.
Suppression in Strabismus

Exotropia
 Usually
occurs across the entire temporal
hemiretina
Characteristics





Suppression is not uniform across the
suppression zone
Most intense at fovea and zero measure point
Can get an inverse suppression when
stimulation to the deviating results in
suppression of the fixing eye.
Actual suppression areas can vary depending
on which area of the fixing eye is stimulated.
Latency of 75 to 125 msec in normal and longer
in some cases of strabismus.
Classification of suppression

Central < 5 degrees
 Foveal
< 1 degree
 Parafoveal < 3 degrees (but > 1 degree)
 Paramacular < 5 degrees (but > 3 degree)

Peripheral > 5 degrees
What about monovision?
Clear vision to one eye and blurred to the
other
 Creating binocular rivalry
 Clear eye becomes dominate at each
distance.

Classification of suppression

Shallow
 Most

similar to regular viewing conditions
Deep
 Abnormal
viewing conditions
Red Lens Test
Put red filter of fixing eye.
 Can use neutral density filters to measure
the depth of the suppression.

Worth 4 Dot
Similar to red lens test
 1 red, 2 green, and 1 white light
 Wear red-green glasses
 Peripheral target at near and central target
at distance.

Tests of Suppression

Worth 4 dot
Tests of Suppression

AO vecto slide
Tests of Suppression
4 base out test
 Put 4 base prism in front of one eye
 Displaces image
 Eyes should make a version and then
vergence eye movement to follow the
target.
 No eye movement indicates suppresion

Vergence Ranges
Positive Fusional vergence
 Introduce prism in front of each eye
 What happens if only one eye sees the
target.
 Central vs. peripheral
 Shallow vs. deep

Vision Therapy
First degree targets can take advantage of
rivalry
 Change target parameters to alter
suppression patterns
 Use physiological diplopia to create
awareness.

Fechner’s Paradox

This occurs when placing a neutral density
filter over one eye. When you close the
eye with the filter the object looks brighter.
The visual system does not add the
brightness from the 2 eyes.
Fechner’s Paradox
If summation occurs then the binocular
perception should be greater than the
monocular perception.
 Instead of summation the brightness levels
are averaged.

Fechner’s Paradox

For example, if the brightness in the right
eye is 4 units and the left eye is two units
then with both eyes we get 4+2/2 = 3
units. However, the right eye only would
see 4 units and left eye only would see 2
units.
Do we get a true average of the
two eyes?

What actually happens varies by individual
and was researched by Levelt
Law of Complementary Shares
Formula: Eb = wlE + wrtE
 t is the transmission of the filter
 w is the weight of each eye (the dominate
eye receives more weight)
 E is the apparent brightness

Law of Complementary Shares
If no eye dominance then Wl = Wr = 0.5
 If right eye dominant the Wl < 0.5 and if
left eye dominant the Wr < 0.5.

Summation

Are two eyes better at detecting targets?
Summation

Binocular summation is the additivity of the
information from each eye to yield
binocular visual performance that exceeds
monocular performance
Complete Summation
(a=b=0.25)

B’ = a(R) + b(L)

= 0.25 (1) + 0.25 (1)

= 0.50

Partial Summation

(0.50 > a > 0.25); (0.50 > b > 0.25)




B’ = a(R) + b(L)
= 0.35 (1) + 0.35 (1)
= 0.70
No Summation
(a=b=0.5)

B’ = a(R) + b(L)

= 0.50 (1) + 0.50 (1)

= 1.00

Temporal Functions
Independence Theory

The advantage occurs because you have
two sources of information
Probability Summation

Pou = (Pod + Pos) – ((Pod)(Pos))
Neural Summation
Improves detection under binocular
conditions
 Signal to noise ratio

Testing the two theories
Stimulus can be separated by time or
space
 Independence theory would not show any
difference among conditions
 Neural theory would show a difference

Aftereffects
Aftereffects are visual illusions that result
from the fatiguing of tuned visual neurons.
The fatigue biases our responses and
creates the illusion.
 http://www.michaelbach.de/ot/mot_adapt/i
ndex.html

Interocular Transfer

Motion after effect
Interocular Transfer

Tilt after effect
Clinical Implications

Can use summation and interocular
transfer as a measure of binocularity.
 Summation
reduced in early onset strabismus
especially for high spatial frequency targets
 Interocular transfer reduced in early onset
strabismus especially for high spatial
frequency targets.
Neurological Correlates of BV
Visual Pathway
 Partial decussation
 Optic Chiasm

Corpus Collosum

Fibers in that interconnect the two
hemispheres.
Lesion at Optic Chiasm
What happens to stereopsis
 Loss of information from nasal retina and
temporal field.

Lesion at Corpus Collosum

Loss of stereopsis along the midline
Detection of Disparity

Four types of cells
 Tuned
excitatory
 Tuned inhibitory
 Near Cells
 Far cells
Role of detectors
Tuned excitatory and inhibitory are good
detection of fine stereopsis.
 Near and Far cells good for coarse
stereopsis.

Development of Binocular Vision
Critical periods
 Different vision skills develop at different
times
 When does the disruption take place

Development of Binocular Vision
When do children start to appreciate
depth?
 http://vimeo.com/77934

 How
do we measure this?
 Visual cliff experiment (Gibson and Walk,
1960)
 Monocular Cues
When does stereopsis develop?
Use dynamic random dot targets
 Watch to see if infant tracked the motion
 Occurs at 3.5 months
 Correlated with accuracy in the
accommodation and vergence motor
systems

Teller Acuity Cards
When does stereopsis develop?
Stereoacuity develops rapidly
 Crossed disparity develops slightly faster
than uncrossed disparity.

Abnormalities in Binocular Vision

Amblyopia is defined as the condition of
reduced visual acuity not correctable by
refractive means and not attributable to
ophthalmoscopically apparent structural or
pathologic anomalies or proven afferent
pathway disorders
What causes amblyopia?
Abnormal visual experience during the
critical period of development
 The abnormal visual experience disrupts
spatial vision and binocularity.

Amblyogenic factors
Strabismus
 Anisometropia
 Refractive
 Stimulus deprivation amblyopia
 Meridional amblyopia

Severity of amblyopia

Disruptions to the binocular system cause
greater reduction in acuity.
 Bilateral
high refractive vs anisometropia
Problems in binocular vision
Limited stereopsis
 Suppression

Animal models
Induce amblyopia in animals by disrupting
visual input during a critical period.
 What is the effect on binocular
development?

 Reduces
responses to cells in the striate that
respond to binocular input.
Binocular competition
The inputs from the two eyes compete for
synapses.
 If both inputs are strong and equal then
you get the binocular cell.
 In asymmetrical inputs the weaker input
can lose its connection.

Importance of early intervention
Need to remove any factor that causes
disruption to the binocular system.
 Infant see
