Fusion, Rivalry, Suppression
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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