MECHANISMS AND TERMINOLOGIES IN BINOCULAR VISION.pptx

terminologies in binocular vision

1. MECHANISMS AND TERMINOLOGIES IN BINOCULAR VISION MODERATOR: Mr. Pankaj Ray Adhikari PRESENTANTED BY: Kajal Bhagat 2 nd batch BOVS ,NAMS Biratnagar Eye Hospital

2. CONTENT: Corresponding retinal points Horopter Panum’s fusional area Fixation disparity Associated phorias

3. Corresponding retinal points: INTRODUCTION: Two  points  (or small areas), one in each  retina , which when simultaneously stimulated give rise to the perception of a single object.  These  points  share a common subjective visual direction . Image falling on corresponding points give rise to a single mental impression to visual direction. Example: Fovea of two eyes.

4. Definition:

5. Normal retinal correspondence (NRC) If corresponding retinal areas in the two eyes bear identical relationships to the fovea in each eye. Abnormal retinal correspondence(ARC ) Dissimilar relationships in the two eyes, between corresponding retinal areas and their respective fovea.

6. Corresponding retinal points are principal points of two retina that give rise in binocular vision. Single vision is the hallmark of retinal correspondence.

7. Tests for retinal correspondence: Worth’s four dot test Bagolini striated glasses test After image test Red filter test

8. HOROPTER: The term horopter was introduced by Franciscus Aguilonius in 1613 A.D. Literally means the horizon of vision. Derived from two Greek words horos and opter . horos = limit horopter opter = person who looks

9. CONTINUED: Horopter is defined as the locus of all object point that are imaged on corresponding retinal elements at a given fixing distance. In other words, the horopter can be defined as the sum of total points in physical space that stimulate corresponding retinal elements of the two eyes.

10. CONTINUED: It is an imaginary surface (usually slightly curved either concave or convex towards the observer), centered on the fixation point that moves with eye. Shape of horopter according to fixation distance: For abathic distance- 1m Frontoparallel plane Beyond 1m convex for observer Near than 1m concave for observer

11. CONTINUED: Geometrical /Theoretical Horopter Curve: -Horizontal (Vieth Muller Circle) -Vertical Empirical Horopter: -Horizontal -Vertical

12. Vieth Muller Circle(VMC): A theoretical circle in space in front of an observer containing points that falls on corresponding retinal locations in the two eyes when a point in a circle is fixated.

13. Vertical Geometric Horopter: The straight line on the sagittal plane and passing through the intersection between the sagittal plane and the vieth Muller circle(typically if the observer fixates straight ahead, but not necessarily)

14. Empirical Horopter: Actual experimental determination of the horopter curve. Flatter than the theoretical horopter. Distribution of the corresponding elements are nor the same in the nasal and temporal part of the retina. Shape varies to person to person i.e , each person has his personal horopter.

15. HERING-HILLEBRAND DEVIATION: The Hering-Hillebrand deviation describes the mismatch between the theoretical and empirical horopter. When the horopter is determined experimentally, The Hering-Hillebrand deviation describes the fact that the empirical horopter does not fall on the geometrical horopter.

16. MEASURING HOROPTER:

17. PANUM’S FUSIONAL AREA: Panum , the Danish physiologist first reported this phenomenon. Panum’s area: The retinal area surrounding the corresponding retinal points within which BSV can be maintained. Panum’s space: A narrow band around the horopter within which object gives rise to BSV.

18. Size of panum’s fusional area:

19. Illustration:  In this diagram, both eyes are looking at the red dot, the fixation object. The lines from the dot show the direction the light goes from the object to where it falls on the foveae of both eyes. The fovea is indicated by the small dot in the middle of each retina. The green dot, the disparity object. It does not fall on the foveae of the two eyes. The bright blue arc is the horopter. The dark blue area around the horopter is  Panum's area of fusion . Source: https://isle.hanover.edu

20. When the disparity object is inside of Panum's area of fusion, the image is fused and we have a single percept of the object. When the object is outside of Panum's area of fusion, we can see two images of the object. That is double vision or diplopia.

21. If the fixation object point is near to eyes ,area of panum’s fusion is narrower . If the fixation object point is far from eyes, area of panum’s fusion is wider.

22. If the separation between eyes decreases, the panum’s fusional area becomes wider. If the separation between eyes increases, the panum’s fusional area becomes narrower .

23. FIXATION DISPARITY: Fixation disparity is a small error in the visual system. Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision. The misalignment may be vertical, horizontal or both The misalignment (a few minutes of arc) is much smaller than that of strabismus, which prevents binocular vision, although it may reduce a patient's level of stereopsis. It doesn't give rise to diplopia because the disparity with which the fixation point is imaged on the two retinae is less than the size of panum’s fusional area.

24. Under laboratory conditions, it can be demonstrated that during binocular fixation, the point of fixation is rarely ever imaged exactly on corresponding points of the two fovea but that the primary line of sight of one eye misses the fixation point very slightly, being either under converged or over converged. This phenomenon is called fixation disparity.

25. Definition: Fixation disparity can be defined as the condition in which the image of binocularly fixed object are not imaged on exactly corresponding retinal points but are still within panum’s fusional area. NOTE: A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.

26. History:

27. Types: 1.Exofixation disparity : If the line of sight insert beyond object of regard.

28. 2.Esofixation disparity : If the line of sight intersect before object of regard.

29. 3. Hyperfixation disparity: If the line of sight intersect above object of regard.

30. 4. Hypofixation disparity: If the line of sight intersect beyond object of regard.

31. Fixation disparity curve: A fixation disparity curve is an x, y coordinate plot of the angular amount of FD in minute of arc as a function of the amount of prism through which the patient views. Components of FD curve: the horizontal axis intercept (associated phoria) The vertical axis intercept (FD amount) Slope at the vertical axis intercept The centre of symmetry Shape / type of curve

32. In the commonly used classification system of ogle et al, FDCs are categorized as type I, II, III and IV TYPE I: Present in about 55% of the population. The curve has vertically ascending and descending segments that asymptote on the base-in and base-out sides and a relatively flat central portion. Patients are usually asymptomatic.

33. Type II: Present in about 30% population. A curve is flat on the base out side and ascends in base-in side. Most type II occurs with esophoria.

34. Type III: Present in about 10% of population. The curve is flat on the base-in side and descends on the base-out side. Most often found in high exophoria

35. Type IV: Present in person with unstable binocularity. The curve is flat on the base-in and base-out sides and has a higher slope in the central portion. Seen in poorly developed binocular coordination.

36. Measurement of fixation disparity: There are several methods to quantify fixation disparity . They are: 1.Mallett card 2.The Bernell lantern slide , 3.The Wesson Card 4.The Disparometer

37. Mallet fixation disparity test:

38. Associated Phoria:

39. References : Squint and orthopics -A.K. khurana Anatomy and physiology of eye – A.K. Khurana Clinical procedure in optometry Slideshare internet