2. To be discussed
BSV and its stages
Monocular depth clues
Binocular disparity or stereopsis
Stereo aquity
How to test stereopsis
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3. Binocular single vision
It is the coordinated use of both eyes to produce a single mental
impression
or
The simultaneous use of both eyes to produce a single mental
impression is called binocular single vision
It has three stages
Simultaneous perception
Fusion
Stereopsis
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4. Simultaneous perception
When signals transmitted from both eyes to visual cortex are
perceived at the same time .
Fusion
It implies the ability of two eyes to form one image .
Stereopsis
Three diamentional image perception during binocular vision is called
stereopsis .
Depth perception
Depth can be perceived by two ways
Monocular depth clues
Binocular disparities or stereopsis
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5. Monocular depth clues
Monocular depth clues are also important for depth and distance
perception of objects .
It includes
Motion parallax
Light and shade
Linear perspectives
Object overlap
Relative size of objects
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6. Motion parallix
Stationery objects that are closer to the moving observer appears to
be moving faster .
Linear perspectives
The farther away the object is the smaller it appears to be .
Object overlap
Object slightly covered by another object appears to be farther away
Light and shade
Objects further off appears less saturated and less sharp than those of
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8. Binocular disparity or stereopsis
Stereopsis is the visual perception of three diamentions during
binocular vision
It results when horizontally disparate retinal elements are stimulated
simultaneously.
Vertical disparaties produce No stereoscopic effect
For example a solid object placed in the medium plan of the head
produces unequal images in the two eyes due to horizontal separation
of the two eyes .
The sensory fusion of the two unequal images results in a three
diamensional percept .
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9. Types of stereopsis
Coarse stereopsis
it is that type of stereopsis which provide sense of being immersed in
ones soroundings
Also called gross stereopsis
It is important for orientation in space while moving
Fine stereopsis
This is very fine amount of depth perception between two objects
It is important for fine motor tasks such is threading a needle
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10. Stereoaquity
Stereo aquity is the angular measurement of minimal resolvable
binocular disparity which is necessary for perception of three
diamentional images .
It is measured in seconds of arc
The normal level of stereo aquity is being 40 sec of arc
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11. Test for stereopsis
Lang two pencil test
Frisby stereo aquity test
Random dot stereogram
Lang stereo test
TNO test
Lang II test
Titmus fly test
Randot stereotest
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12. Lang two pencil test
This test is performed for gross stereopsis
The patient attempts to place a pencil tip on top of the one held by
the examiner .
Patients who have normal stereopsis will find it easier to perform the
test binocularly than monocular .
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13. It consists of three plastic plates of different thickness 6mm,3mm,and
1mm.
Each plate consists of four squres formed by random shapes formed
on one side of it .
Corresponding to one of the squares a disparity is produced .
This disparity creates a 3D appearance of a circle .
The plates are held at 40 cm normally .
Frisby stereo aquity test
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15. Random dot stereogram test
It comprises images formed of dots which are displaced in relation to
each others .
Both eyes will see different dots
The space between images is filled randomly by dots .
Following are two types of random dot stereogram test
Lang stereo test
TNO test
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16. Lang stereo aquity test
Developed by swiss ophthalmologist joseph lang
Available in two versions as lang I and lang II which only differs in the
type of stereoscopic objects to be recognized .
The lang II test additionally contain a picture that can be recognized
with one eye as a star .
Procedure
Show the test plate exactly at right angle at a distance of 40 cm to
the patient .
Ask the patient if he/she could something on the plate and watch the
searching movement of the eye .
When a 3D object is detected ask the patient to look for additional
objects and to describe them
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17. Disparities in lang I
Car ; 550 sec of arc
Star ; 600 sec of arc
Cat ; 1200 sec of arc
Lang II
Moon ; 200”
Truck ; 400”
Elephant ; 600”
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18. TNO test
This test was primarily designed for screening of pre school children
for binocular vision defects .
Procedure
It consists of seven plates to be viewed with red green spectacles that
carry figure which are only seen when both eyes coordinate to give
stereoscopic vision .
Test should be carried out at a distance of 40cm .
The plates should not be rotated during test procedure.
Spectacles should not be removed
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19. The 1st three plates enable the examiner to quickly asses whether the
stereopsis is present or absent .
The next four plates are used for exact determination of stereoscopic
vision.
Plate V measure stereoscopic aquity of 480-240 sec of arc .
Plate VI measure upto 120-60 sec of arc
Plate VII measures stereoaquity of 30-15 sec of arc .
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20. Plates composition
Plate I is composed of two butterflies one of them is hidden and can
only be seen when both eyes are used .
Plate II contains 4 discs that differs in size .
Two of them are larger and other two are smaller.
The smaller plates can only be seen when there is adequate
stereopsis .
Plate III have four hidden structures like square ,triangle or discs
arranged around a central cross.
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21. Plate IV:
this is a suppression test .
It consists of a small disc and two large dics .
If a patient see only two dics one small and one large . it indicates
suprresion.
The large disc is seen through dominant eye .
Plate V-VII; contain discs that are presented to patient at six different
disparity levels ranging from 15 to 480 sec of arc
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22. Titmus fly test
It is based on principle of linear polarization .
Polarized filters are worn and vertical image of a light source is seen
through one filter and horizontal image is seen through other eye .
The target are presented as vectograph .
The disparity produced by either eye results in stereopsis .
One plate contains the image of a fly .
The 2nd plate consists of nine boxes each containing 4 circles and
three rows of animal.
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23. Procedure
The patient wear polarized filters over his/her correction.
Patient is 1st shown the plate that contain fly .
The patient is instructed to catch the fly by its wings .
If the fingers reach above the plate the patient has stereopsis .
If the fingers touches the plate then there is No stereopsis
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24. Randot stereo aquity test
This test is similar to titmus fly test .
One plate is consists of two groups of four squares three of those
squares contain circle ,square and star .
The other plate consists of animals and circle .
The perception of random shapes give stereoaquity of 500-250 sec of
arc.
The perception of animal shapes give stereoaquity of 400-20 sec of
arc.
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