The Frisby Stereotest 3 Plate Near Test

An Overview of the Frisby Near Stereotest 3 plates

First launched in 1983, it is used in orthoptic and ophthalmic clinics and by optometrists world-wide.

Tests for stereopsis using natural vision, with no special glasses. It has three transparent plates, which are presented to the patient one at a time against the clear background provided by the fold-down flap of the storage box.
Tests for stereopsis, even if reduced vision or amblyopia is present because of the coarse texture elements included in the pattern.

The Frisby Near Stereotest has 3 plates of different thicknesses, providing stereoacuity measurements in the range 600-5 sec arc. They are stored in a box with a fold-down flap that provides a clear background. A tape measure is provided for controlling eye-to-plate distance. When testing, the plates should be raised a few cm/inches above the background to prevent monocular cues (see photos above and the Frisby Stereotest Near 3 Plates Instructions Booklet, as below).

The plate is shown a few cm or inches above a clear background, such as the fold-down flap on the box as shown here. Note the tape is fixed to the flap for measuring and controlling eye-to-plate distance.

Read a recent review of the Frisby Stereotest in Optician magazine

This article reviews our tests in the light of recommendations in the literature about the criteria for a ‘good’ stereotest, particularly those made by Westheimer who concluded that the Frisby Stereotest best met his criteria. The article also makes recommendations for best practice in the use of our tests. It is suggested that reading this article is a good introduction to our tests, prior to reading the Instructions booklet next. The article is used in the Optician magazine’s Continuing Education & Training (CET) Module C40819. To take this module visit www.opticianonline.net/continuing-education/frisby-stereotest-introduction-review/.

The Frisby Near Stereotest 3 Plates Instruction Booklet

This is the booklet supplied with the 3-Plates test (Frisby Near Stereotest or FNS for short). It has been updated in the light of versions of this test shipped after 1st October 2020 no longer having studs at the corner of the plates. This change is part of our ongoing development of Frisby Stereotest products. Testers can still find the the location of the target by touch. This is because three of the plate corners are rounded, the fourth is square indicating the quadrant with the target. In this way the tester can discover where the target is by touch, whilst concentrating on the patient and where they are looking. This change has been approved by our orthoptists consultant.

How the Test Works

The target is a clus target random pattern ter of randomly arranged arrowheads of differing sizes, as shown alongside.

It is printed on one side of the plate in one of the four quadrants; on the other side of the plate similar pattern elements are printed around the target and in the other three quadrants.
An observer with normal binocular stereovision can readily detect the target because it appears to stand out from the background or to recede from the background, depending which way round the plate is shown.
Such an observer is essentially seeing the thickness of the plate by virtue of the texture elements printed on the two sides.
An observer lacking normal binocular stereovision (or a normal observer viewing with one eye only) fails to detect the target as it can be distinguished only on the basis of binocular disparity cues to depth, as long as the plate is shown correctly (stationary, viewed square-on, and placed about 5-10cm in front of a clear background).

Stereopsis Screening

For stereopsis screening the test objective is to discover whether the patient can reliably discriminate the circle-in-depth using the thickest plate.
Present the 6mm plate with the position of the circle-in-depth chosen randomly. The stud alongside the target has a flat surface, allowing the tester to know discretely by touch where the target is whilst following the observer’s eye movements. Hence the position of the target can be selected discretely by feeling for the stud with the flat top.

Patients with stereopsis usually find the target-in-depth quickly and confidently, perhaps pointing to it if young, or staring at it.
If necessary, use a sequence of presentations varying the position of the target randomly. Varying plate position can be done while the tester holds the plate behind her or his back. Try making a game of the test saying “Can you find the hidden target?”
Use as many trials as needed to be able to make a confident decision. A single very speedy correct response may be sufficient but sometimes 2 or 3 presentations are needed for a cautious patient to decide if stereopsis is or is not demonstrated.
Patients with defective stereopsis usually make slow hesitant responses with a high frequency of errors.

Preferential Looking

As part of our ongoing development of Frisby Stereotest products, since 1st October 2020 there are no longer studs fixed at the corners of the plates but testers can still find the target by touch. Three of the plate corners are rounded and the fourth is square, indicating the quadrant with the target in-depth. In this way the tester can discover the target by touch, whilst concentrating on the patient and where they are looking. This change has been approved by our orthoptist consultant.
This allows for a positive ‘preferential looking’ response, a distinct advantage with young children.
Another advantage of the square corner showing the quadrant with the target, testers without stereopsis can still administer the test.

Checking Test Understanding

Understanding of the test by patients making hesitant responses can be checked by standing the plate on a corner and twisting it slowly to and fro by about 10⁰ either side of square to the patient’s line of sight.
Twisting the plate in this way introduces the monocular depth cue of motion parallax which makes the target-in-depth readily visible, even for patients without stereopsis (testers can check this for themselves by viewing the moving plate with one eye only).
Hence, if a patient can find the target-in-depth when the plate is twisted to and fro but not when it is held still, a confident judgement of defective stereopsis can be recorded because the patient has demonstrated understanding of the test.
Be sure to use a new random position when stereopsis is being assessed.
Another way of checking test understanding is to lay the 6mm plate flat on the box’s fold-down lid. This reveals the target by a shading cue. If the patient can then detect the target they have demonstrated test understanding. BE SURE NOT TO USE THE TEST IN THIS WAY WHEN CHECKING FOR STEREOPSIS – THEN THE PLATE MUST BE HELD A FEW CM ABOUT A PLAIN SURROUND – SEE PHOTOS ABOVE. Beware many YouTube sites that show the test being incorrectly used by holding it flat up against the box. For further details on how to use monocular cues for demonstrating test understanding see the Optician article referred to above.

Testing Precautions

Note carefully that, once the initial phase of explanation is over and the patient is being called upon to demonstrate stereoscopic ability, the patient should view the plate squarely with head and plate held still, and with the plate held about 5-10cm in front of a clear background.
It can be helpful, with the permission of the carer, to steady the head of the young and active patient using one outstretched hand, while holding the plate steady with the other.

Measuring stereoacuity

There are 3 plates of of differing thickness, 6mm, 3mm and 1.5mm, which present different sizes of disparity cues.
The plates can be presented at differing distances to further vary the disparity cues.
The lid of the test box has a table showing disparities for each plate for 30-150cm viewing distances. A measuring tape assists in controlling eye-to-plate distance (see test photo).
The test objective is to record the smallest disparity which the patient can reliably discriminate.
Begin by showing the 6mm plate from the usual reading distance of 40cm.
If the patient makes a confident and correct identification of the target then show the 3mm plate and thereafter the 1.5mm plate. Move quickly to the zone of uncertain judgements to avoid losing the patient’s interest.
Vary plate distance and target position, trying to find the disparity at which they patient becomes hesitant or begins to make errors. Record as the stereoacuity the smallest disparity for which the patient can make 2 or 3 confident and reasonably speedy target identifications.

Test Repetition

The patient can be shown the plates over many presentations, using a different target position each time, without fear of the patient learning a correct response.
Making repeated presentations without patient’s learning the correct response is a unique feature of Frisby Stereotests.

See the other Frisby Stereotest:

Frisby Pocket Stereotest