Grand Rounds Timetable

Wednesday 14th November

• Please take a note of the time you have been allocated and arrive promptly

• Bring with you the equipment necessary to enable you to perform:

1 A cover test (i.e. an appropriate cover)

2 Ocular Motility (i.e. a pen torch)

• Please come smartly dressed.

• You will be working in pairs as indicated and will spend between 5 – 10 mins examining a subject.

Supervisors will instruct as to what test to perform & what to look out for.

You hopefully will see 3 patients

• Record the patient’s name and the results you obtain on the record sheets provided.

The Clinical Assessment of Stereopsis

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 (α and β).

The resulting value is relatively independent of distance specified in seconds of arc.

β

α

d

l

∆l

Instantaneous Parallax = α - β tan β = d/l + ∆l tan α = d/l for small angles if l is large with respect to d then :

tan α = α (in radians)

then: α - β = d/l - d/l + ∆l

= dl + d∆l – dl l2 + l∆l

If l is large with respect to ∆l, l2 + l∆l tends to l2

∴ α - β = d∆ l l2

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.

a b a bA

B

a b a b

b a b a

L RPatterns 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).

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)

Titmus Fly Test – Polaroid Vectograph

•itmus Fly Test – Polaroid Vectograph

This test uses crossed Polaroid filters to present slightly different aspects of the same object to each eye. The test comprises of three sections:

) The Housefly - which shows large disparities and should be seen in depth by most subjects.

h)ircle Patterns – this section consists of patterns containing four circles. One of the circles in the pattern contains a graded disparity (crossed), so that when it is viewed binocularly it is seen to float in front of the others. The disparities of the circles range from 800 to 40 secs of arc.

c)Animals – there are 3 rows of animals, one animal in each row having a crossed disparity which ranges from 400 – 100 secs of arc.

One disadvantage of this test is that it contains monocular cues.

The Frisby Stereo-test

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.

The TNO Test

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.

The Random Dot Stereogram

The TNO test has 7 plates.

The first four plates are for screening purposes, the disparities are large and ungraded.

Plate I 2 butterflies are present, one can be seen monocularly, the other is only seen in stereopsis.

Plate II 4 discs, 2 are seen monocularly two require stereopsis.

Plate III four hidden shapes (O, � , ∆ , , ) are arranged around a centrally placed cross

Plate IV This is a suppression test. There are 3 discs, one seen by the right eye, one by the left, and one is seen binocularly.

Plate V-VII Here the test items (Pac-man Shapes) are presented at 6 different disparities ranging from 15 – 480 secs of arc.

Plate V Plate VI Plate VII

480 120 30

240 60 15

The Lang Stereo-test

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.