1. Fusion, Rivalry, Suppression

2. Fusion Worth Classification
First degree fusion or simultaneous perception
Second degree fusion or flat fusion
Third degree fusion or stereopsis

6. Theories of Fusion
Alternation or suppression theory:
Fusion theory

7. Limits of fusion Panum’s fusional area Fixation disparity
Panum’s limiting case

10. 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

11. 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

12. Binocular Rivalry First degree fusion Allows us to monitor suppression
Used in tests of correspondence

14. Characteristics Greatest with separation 90 degrees apart
Equal stimulus values

16. 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.

17. Stimulus Strength-Reading
Brighter
Luminance
Darker
Higher
Contrast
Lower
Clear
Focus
Blurred
Foveal
Retinal locus
Peripheral
Moving
Movement
Stationary

19. Characteristics Local phenomenon
Reduced sensitivity to area suppressed
DaVinci Stereopsis

24. 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

26. 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

27. Suppression
We do not notice physiological diplopia so we must be suppressing most of time.
Most strabismic patients do not see double, why?

28. 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

29. Adaptations
In normal binocular vision the suppression of physiological diplopia is called physiological suppression or suspension.

30. 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.

31. Adaptations
Pathological diplopia occurs when the object of regard is imagined on non-corresponding points. This can be eliminated by regional suppression.

32. Characteristics Effect of orientation and spatial frequency
Schor (1977) presented sinusoidal gratings at various orientations and spatial frequencies to normal and strabismic subjects.

33. 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.

34. Suppression in Strabismus
Amblyopia
Reduction in acuity under binocular condition

35. 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 naso-temporal decussation line.

37. Suppression in Strabismus
Exotropia
Usually occurs across the entire temporal hemiretina

38. 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.

39. Classification of suppression
Central < 5 degrees
Foveal < 1 degree
Parafoveal < 3 degrees (but > 1 degree)
Paramacular < 5 degrees (but > 3 degree)
Peripheral > 5 degrees

40. What about monovision?
Clear vision to one eye and blurred to the other
Creating binocular rivalry
Clear eye becomes dominate at each distance.

41. Classification of suppression
Shallow
Most similar to regular viewing conditions
Deep
Abnormal viewing conditions

42. Red Lens Test Put red filter of fixing eye.
Can use neutral density filters to measure the depth of the suppression.

43. 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.

44. Tests of Suppression
Worth 4 dot

46. Tests of Suppression
AO vecto slide

47. 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

48. 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

49. Vision Therapy First degree targets can take advantage of rivalry
Change target parameters to alter suppression patterns
Use physiological diplopia to create awareness.

50. 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.

51. 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.

52. 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.

53. Do we get a true average of the two eyes?
What actually happens varies by individual and was researched by Levelt

54. 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

55. 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.

57. Summation
Are two eyes better at detecting targets?

58. Summation
Binocular summation is the additivity of the information from each eye to yield binocular visual performance that exceeds monocular performance

59. Complete Summation (a=b=0.25) B’ = a(R) + b(L) = 0.25 (1) + 0.25 (1)
= 0.50

60. Partial Summation (0.50 > a > 0.25); (0.50 > b > 0.25)
B’ = a(R) + b(L)
= 0.35 (1) (1)
= 0.70

61. No Summation
(a=b=0.5)
B’ = a(R) + b(L)
= 0.50 (1) (1)
= 1.00

63. Temporal Functions

66. Independence Theory
The advantage occurs because you have two sources of information

67. Probability Summation
Pou = (Pod + Pos) – ((Pod)(Pos))

68. Neural Summation Improves detection under binocular conditions
Signal to noise ratio

69. 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

71. Aftereffects
Aftereffects are visual illusions that result from the fatiguing of tuned visual neurons. The fatigue biases our responses and creates the illusion.

72. Interocular Transfer
Motion after effect

73. Interocular Transfer
Tilt after effect

75. 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.

76. Neurological Correlates of BV
Visual Pathway
Partial decussation
Optic Chiasm

78. Corpus Collosum
Fibers in that interconnect the two hemispheres.

79. Lesion at Optic Chiasm What happens to stereopsis
Loss of information from nasal retina and temporal field.

81. Lesion at Corpus Collosum
Loss of stereopsis along the midline

83. Detection of Disparity
Four types of cells
Tuned excitatory
Tuned inhibitory
Near Cells
Far cells

84. Role of detectors
Tuned excitatory and inhibitory are good detection of fine stereopsis.
Near and Far cells good for coarse stereopsis.

86. Development of Binocular Vision
Critical periods
Different vision skills develop at different times
When does the disruption take place

87. Development of Binocular Vision
When do children start to appreciate depth?
How do we measure this?
Visual cliff experiment (Gibson and Walk, 1960)
Monocular Cues

88. 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

89. Teller Acuity Cards

90. When does stereopsis develop?
Stereoacuity develops rapidly
Crossed disparity develops slightly faster than uncrossed disparity.

92. 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

93. What causes amblyopia?
Abnormal visual experience during the critical period of development
The abnormal visual experience disrupts spatial vision and binocularity.

94. Amblyogenic factors Strabismus Anisometropia Refractive
Stimulus deprivation amblyopia
Meridional amblyopia

95. Severity of amblyopia
Disruptions to the binocular system cause greater reduction in acuity.
Bilateral high refractive vs anisometropia

96. Problems in binocular vision
Limited stereopsis
Suppression

97. 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.

100. 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.

102. Importance of early intervention
Need to remove any factor that causes disruption to the binocular system.
Infant see