2. The Clinical Assessment of Stereopsis

3. Stereopsis is the ability to perceive the relative distances of objects from the observer. In order to quantify this percept one could measure the minimum separation between the objects that can be perceived. However, this approach is not very useful as the value measured will vary depending on the distance from the observer the test is carried out. Therefore the standard method of measuring stereo-acuity is to calculate what is known as the instantaneous parallax of two objects. This is defined as the difference between angles subtended by the objects at the nodal points of the eyes (  and  ). The resulting value is relatively independent of distance specified in seconds of arc.

4. Instantaneous Parallax =  -  tan  = d/l +  l tan  = d/l for small angles if l is large with respect to d then : tan  =  (in radians) then:  -  = d/l - d/l +  l = dl + d  l – dl l 2 + l  l If l is large with respect to  l, l 2 + l  l tends to l 2   -  = d  l l 2   d l  l

5. Maximum values of stereo-acuity appear to vary between different studies and vary between 1.6 – 24 seconds of arc and are largely dependent upon the nature of the task used in measurement. There are a variety of different methods by which stereo-acuity can be measured. Some of these are more likely to be used in basic research rather than in clinical practice. Many of these tests rely on the artificial simulation of depth by the presenting flat patterns to the two eyes, and introducing a small degree of disparity between the two patterns. The patterns must be similar so as not to generate rivalry, and the disparity must be small so that the images are capable of being fused. Monocular cues must also be eliminated.

6. Patterns L & R are presented separately to each eye. The horizontal separation between the vertical lines a & b is different in the two slides, so that the angles subtended at each eye by the two lines, are unequal. Provided that the difference in separation is not too great the images of the lines in each eye will be fused so that only two vertical lines at A & B are perceived (B appearing further away). a b a b A B a b a b b a b a L R

8. The separate presentation of images to each eye can be achieved by the following methods: I Synoptophore II Stereoscope III Polaroid Filters IV Complimentary Coloured Filters (anaglyphs)

9. Titmus Fly Test – Polaroid Vectograph

11. The Frisby Stereo-test

12. This is the only clinical test based on actual depth, where random shapes are printed on three clear plastic plates of different thickness. The test does not require any form of dissociation. Each plate has 4 squares of curved random shapes, and one square contains a hidden circle that is printed on the opposite surface. Disparities range from 600 to 15 secs of arc. Care should be taken that neither the plates nor the Px’s head move significantly during the testing procedure, as this may provide monocular cues. The thickest plate is held in front of a plain white background and the Px is questioned as to the position of the hidden circle. If the first plate is recognised successfully then the thinner plates, which give smaller disparities, are presented in a similar fashion.

13. The TNO Test

14. Each test plate consists of a stereogram in which the images presented to each eye have been superimposed and printed in complimentary colours. The stereograms are viewed through a pair of red and green filters. The most interesting feature of this test is that the stereograms are based upon the random dot principle first described by Bela Julesz in 1964. He devised pairs of stereo patterns composed of randomly arranged dots or squares. On its own, neither one of the stereo pair is visible as a distant pattern or object, but when viewed stereoscopically the observer can see a figure either in front or behind the plane of the background. The random dot stereograms have the advantage that they completely eliminate monocular cues, the patient is required to describe the shape which can only be seen stereoscopically.

15. The Random Dot Stereogram

17. Plate V Plate VI Plate VII 480 120 30 240 60 15

18. The Lang Stereo-test

19. The test consists of vertical sections that are seen alternately by each eye as they are seen through in-built cylindrical lens elements. Displacement of the random dots creates the disparity which ranges from 1200 to 550 secs of arc. The cards are held at the subject’s reading distance and he or she is asked to name or point to the pictures.