Test Procedure
Frisby Near Stereotest
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Introduction and Test Familiarisation
Correct Presentation of the Test
Screening Patients Able to Understand Simple, Verbal Instructions
Screening Patients Unable to Grasp Verbal Instructions
2 Frisby tests, Each Suitable for Different Clinical Aims
Introduction and Test Familiarisation
Testers should first familiarise themselves with how the Frisby Near Stereotest works by examining the thickest plate (6mm) carefully, initially using both eyes normally and then with one eye covered.
Using two eyes, an observer with normal stereoscopic binocular vision will easily be able to see, in one of the four random-pattern squares, the target as below - a circular patch of elements - lying in depth relative to its surround.
With one eye covered (a viewing condition which simulates a patient lacking binocular stereopsis), the depth effect no longer is visible. The depth effect is due to the target and its surround being printed on opposite sides of the plate.
Testers with poor stereoscopic vision who have difficulty in discriminating the target-in-depth themselves can nevertheless administer the test by discreetly feeling the shapes of the corners on each presentation. Three corners are rounded. The corner that is not rounded but is square, identifies the target.
Correct Presentation of the Test
Correct presentation - side view
Hold the top edge of the plate a few cm/inches above flap, while keeping the plate square to the patient’s line of vision. The top edge is lifted up from background to avoid shading cues.
Correct presentation - front view
Be Careful Not To Present the Test Incorrectly, Which Gives Monocular Clues
Do not hold the plate flat against the box, demonstrated below, as this introduces shading cues, and makes the target visible monocularly. This must be avoided when administering the test and measuring stereoacuity, however this form of presentation is useful when checking for test understanding.
Incorrect presentation during test, above
If the patient initially fails to find the target when the test is presented correctly, then test understanding can be checked by holding the plate flat against the box (or a piece of paper) to reveal the target monocularly. This deliberate introduction of shading cues to show the target can help the patient know what they are looking for.
Stereopsis Screening
For screening, the objective is to discover if the patient can reliably discriminate the target-in-depth using the thickest plate. Present the plate several times with target position varied randomly.
Patients With Stereopsis
Patients with stereopsis usually find the target quickly and confidently.
Patients With Defective Stereopsis
Patients with defective stereopsis usually make hesitant responses, with errors. Test understanding by patients not finding the target can be checked in two ways.
(1) Make the target visible monocularly by resting the plate on the box flap or on a piece of white card. If the patient can then see it / find it, they have demonstrated they know what they are looking for. However, if they cannot then see the target with the plate lifted away from the background, defective stereopsis can be recorded.
(2) Stand the plate on a corner and twist it slowly to and fro. The monocular cue of motion parallax then makes the target visible in depth, even for patients without stereopsis. Testers can check this for themselves by viewing the twisting plate with one eye closed. If a patient can find the target when the plate is twisted to and fro but not when still, defective stereopsis can be recorded because the patient has demonstrated test understanding.
Be sure to use a new random position and to hold the plate and the patient’s head and the plate are still when stereopsis is being tested.
Stereoacuity Assessment
For stereoacuity assessment, the test objective is to find the finest depth discrimination which the patient can reliably manage, using the full range of plate and distance combinations. The thinner the plate and/or the greater the distance, the finer the depth discrimination. Start with the 6mm plate, and then move onto testing with the 3mm plate and then the 1.5mm plate.
Test the patient with various plate/distance presentations selected from the table shown below - also included in the lid of the test box - to determine the lowest disparity value that the patient can reliably manage.
It is suggested:
(1) The tester concentrates initially on the usual reading distance of 40cm.
(2) Accurate measurements are best made using the tape measure, held by the patient to control eye-to-plate distance, with the test box rested either on a table or on the tester’s lap. However, there is no need to use the tape until the tester has established the approximate distance at which the observer begins to fail.
Hook the tape on to the flap. The patient then holds the tape up to their eye to control eye-to-plate distance.
(3) It is sufficient to observe just one correct confident response for a given plate/distance combination until the threshold region is reached as this helps ensure that the threshold region is found speedily, thus avoiding the hazard of loss of attention in young children.
(4) Presenting disparities in decreasing octave steps is recommended by Adams, Leske, Hatt & Holmes (2009, Ophthalmology, 116, 281) as this reflects the degree of accuracy that can be expected from stereotests generally. The table in the test lid suggests suitable plate/distance combinations when using octave steps.
(5) Observe the precautions mentioned above about holding the test plates squarely and still, and using repeated presentations until satisfied that the patient can or cannot make reliable discriminations.
(6) Record the lowest disparity which the patient can reliably discriminate. This stereo threshold is a measure of stereoacuity. The table values are sufficiently accurate for customary clinical practice but, as for other stereotests, they are only approximations to the exact disparities.
Interpupillary distance will vary for different patients (the often-used value of 65mm applies for the disparities shown in the table). Also, slight variations in the thickness of the acrylic used for the plates mean that any given set of plates is likely to depart in some degree from the nominal real thicknesses of 6mm, 3mm and 1.5mm, creating slight departures from the expected apparent thicknesses (of 4.03mm, 2.01mm, and 1.00mm respectively, due to the 1.49 refractive index of the acrylic).
Consequently, if the Frisby Test plates are being used in a context where it is meaningful to know closer approximations to the exact disparities, these can be worked out from the usual disparity formula suitably adjusted to cope with the apparent depth reduction effect.
This formula is:
where I is the interpupillary distance, d is the viewing distance, z is the average plate thickness as measured with a micrometer, and 1.49 is the refractive index of the acrylic from which the plates are made. The number 206,264.81 converts radians to sec arc.
Disparities for Stereoacuity Assessment
The table gives disparities rounded to 5 sec arc. If exact values are needed, these should be calculated from first principles, see formula above. Distances for disparities in octave steps are also given in the lid of the test box. Record the lowest disparity that the patient can reliably discriminate. This stereo threshold is a measure of stereoacuity.
Screening Patients Able to Understand Simple, Verbal Instructions
(1) Begin by establishing test understanding. If the patient does not see the target-in-depth quickly and easily, draw attention to it. Besides pointing to it, it can be helpful to say that the target is “sticking out” in front of its surround. Alternatively, if the plate is held the other way round, one can say that the target forms a “hole” in the pattern. The target printed on the back of the Instructions booklet can be helpful in explaining the nature of the target-in-depth. And finally, the techniques described above in the Screening section for making the target visible monocularly while checking test understanding. •
(2) Next hold the thickest plate steadily a few cm/ inches in front of any convenient bright plain background, the drop down flap of the test box is designed for this. Do not hold the plate directly against a background after the initial test understanding phase, as that can introduce monocularly visible cues to the target. Avoid reflections and shadows caused by light sources behind or over the patient.
(3) If the patient claims to be able to see the target-in-depth, take the plate away, turn it unobtrusively (e.g. behind your back) to some new random position, and then show it afresh, this time asking the patient to point to the square containing the target.
(4) Give as many such presentations as you need to establish whether the patient can reliably discriminate the target. Feel for the square plate corner (or flat stud as in some Frisby Stereotests) to know where the target is while concentrating on where the patient looks. The interest of the young patient can be held by challenges to “Find the hidden target”, or “Find the hidden hole” and usually the test is seen as fun and enjoyable.
(5) It is important that the patient should view the plate squarely with their head and the plate held still once the initial phase of explanation is over and the patient is being called upon to demonstrate stereoscopic capability.
Record Stereopsis Demonstrated if 2 or 3 confident and speedy correct responses are made. Try a few more if in doubt because the patient is slow and hesitant. The practitioner should be aware that it has been reported that some subjects can perform some stereotests monocularly. It is therefore always wise, for this test as for others, to suspect very slow but correct responses. Only record Stereopsis Demonstrated if reliable discrimination is established.
Record Stereopsis Not Demonstrated if the patient fails to pick out the square with the target reliably and confidently over repeated presentations and/or if the patient reports being unable to see the target-in-depth even when it is pointed out. Be sure to try the plate both ways round before recording this result, and consider checking the patient’s understanding using the ‘twisting presentations’ and other techniques described above.
Screening Patients Unable to Grasp Verbal Instructions
(1) Present the thickest plate as previously but without comment. Infants will often spontaneously touch the target-in-depth. If this does not happen, draw attention to the target-in-depth by pointing one of the patient’s fingers towards it.
(2) Take the plate away, turn it around unobtrusively (e.g. behind your back) to a new random position, and then present it afresh, again if necessary pointing one of the patient’s fingers to the target-in-depth. Repeat this procedure until you think the patient has grasped the idea that the plate contains a target-to-be-identified.
(3) Present the plate again in a new position, but this time encourage the patient to point a finger to the target unaided. If this is done reliably over several presentations record Stereopsis Demonstrated.
(4) Should the patient not volunteer clear pointing responses, a positive result may still be recorded if scanning eye movements stop consistently at the correct square on repeated testing. The tester can concentrate on the patient’s eyes and still know the position of the target by discreetly feeling the corner shapes: the corner that is square indicates the quadrant with the target. The other three corners are rounded. On earlier Frisby Stereotests, a flat stud denotes the quadrant with the target.
(5) If the observer consistently makes incorrect pointing responses despite every effort to direct attention to the correct square initially, record Stereopsis Not Demonstrated.
Note: It is often easy to record a Stereopsis Demonstrated result even for very young pre-verbal children by virtue of their consistently correct pointing responses. It is not so easy to be as confident about a Stereopsis Not Demonstrated result for such young subjects. For example, a run of incorrect responses may be due to the patient not fully understanding what is required.
Beneficial Feature
The Frisby Near Stereotest keeps such ‘don’t know’ verdicts to a minimum because:
- it uses a natural depth stimulus and so avoids the need for often troublesome red/green or polaroid spectacles
- it permits repeated training presentations without the patient learning the ‘correct’ response. Moreover, even for very young subjects it is sometimes possible to check understanding using the techniques described above of showing ‘twisting’ to and fro presentations and/or placing a piece of white paper up against the plate, both of which make the target visible monocularly.
In these ways, the Frisby Near Stereotest thus makes it feasible to test children even under one year old.
Remember: After training and when stereoscopic ability is being demonstrated, observe the precaution to hold the test plates squarely and still, preventing head movements by the patient, and choosing new random positions over a series of presentations.
2 Frisby tests, each suitable for different clinical aims
1. The Frisby Near Stereotest - 3 Plates
- Used world-wide in Orthoptic and Ophthalmology Clinics, and by optometrists. Established since 1975.
- An easy-to-use test for stereopsis using natural vision with no special glasses needed, designed for early detection of problems with sterescopic vision in children.
- 3 plates of different thicknesses - 6mm, 3mm and 1.5mm, presenting differing sizes of disparity cues and allowing stereoacuity to be measured.
2. The Frisby Pocket Stereotest
- Available with one 6mm plate or with an additional 3mm plate, extending the range of stereo disparities.
- Ideal for simple and quick screening, including very young children.
- Small, portable – a lightweight test, ideal for screening programmes such as at schools and early years' health visitor clinics.
