About Frisby Stereotests
A Random Dot Test
The Frisby Stereotest is a random dot test. This means the test figure has the same random dot pattern as the background. Random dot stereotests are often used to test precise stereopsis, which is particularly needed for fine motor activities.
It has a number of unique features among other stereovision tests.
Viewed with Natural Vision
The Frisby Stereotest presents ‘real depth’ objects, viewed naturally with the eyes. This means they do not use stereograms to create depth effects, and they do not require wearing special glasses.
This avoids the disadvantages of stereograms for some patient groups, for example babies and young children, who may refuse to wear red/green or polaroid spectacles, or become distracted by them and not concentrate on the test. In addition, some orthoptists believe that such spectacles can be dissociating if the patient has fragile binocular single vision, and thus they can give a misleading result.
Easy to Use
Frisby Stereotests have been designed to be very easy to administer. They are made of simple materials and involve simple presentations. Most patients find it easy to quickly understand the task required of them, and the tester has little or no difficulty in learning how to present the tests.
'Testing' Test Understanding
All Frisby Stereotests allow the tester to check the patient’s understanding of the task in preliminary ‘training’ or ‘demonstration’ trials. This is a further advantage that flows from the unique feature that repeated testing is possible, without the patient being able to learn how to pass.
Repeat Testing is Possible without the Patient Learning How to Pass for the Wrong Reasons or by Memory
All Frisby Stereotests permit repeat testing without the patient learning how to pass for the wrong reasons or by remembering where the target is. This is a unique feature among other stereotests. It is particularly advantageous if a patient is re-tested over a course of investigations or treatments, for instance in the management of a microstrabismus (also known as microtropia or monofixation syndrome). This is one of the reasons the Frisby is typically the preferred stereotest used by UK orthoptics and opthamologists.
When it comes to repeat testing for stereovision, not all stereotests offer the same level of reliability. While others claim they provide reliable results with repeat testing - because they state guessing the figures is not possible - there are important considerations to keep in mind.
The stereogram stereotests use random dot stereograms with embedded shapes that are only visible when binocular vision is functioning correctly. While this does reduce the likelihood of purely guessing the shapes, it does not fully eliminate the issue of memory recall. Once a patient has seen and identified the shapes during an initial test, they may remember them in future sessions. This creates the risk that they could identify the shapes based on familiarity rather than actual stereopsis, which can undermine the reliability of repeat testing.
In contrast, the Frisby Stereotest offers a distinct advantage when it comes to repeatability. The Frisby Stereotest uses real, 3D depth cues created by different thicknesses of plates rather than static visual patterns. This design allows for variations in testing distance and plate orientation, making it nearly impossible for patients to rely on memory or familiarity to succeed in the test. The test remains fresh and objective because the physical properties of the task change, providing an accurate measure of stereovision with each assessment.
For situations where multiple assessments are required — whether for vision therapy, tracking visual development, or evaluating stereovision over time — the Frisby Stereotest provides a far more reliable and repeatable solution. Its unique approach ensures that results reflect true binocular depth perception rather than prior exposure to test materials. This makes the Frisby Stereotest particularly well-suited for both children and adults, where consistent and accurate measurements are essential.]]
Built in Precautions to Avoid Monocular Clues
The Frisby Stereotest has precautions built in to minimise the chances of correct responding on the basis of monocular cues. Such cues are a hazard in all stereotests.
It is necessary for the Frisby Stereotests to be presented correctly to avoid any monocular cues. The tests must be viewed squarely and stationary, with the patient's head still.
The plates must be shown lifted above a clear background by about 5-10cm (the stereotest box or pocket is design for the plate to be held against it. If a plate is laid flat on a surface, the target can become visible from shading cues.
A benefit of the Frisby is that it allows for an initial explanatory or ‘training’ phase, without compromising the test. During this period, monocular cues can be exploited to ensure that the patient grasps what is required of them. This can be done by placing the plate face down onto the background as described above, or by resting the plate on one corner edge, and twirling it from side to side, allowing the test pattern to be perceived.
These monocular clues disappear completely and cannot be replicated, once the test is presented correctly. When testing babies and young children, it may be necessary to gently rest a hand on their head to keep it still.
In a research study, the Frisby Stereotest was judged to have the least susceptibility to monocular cues when compared with other commonly used near stereotests (Holmes, J. M. & Leske, D.A. Monocular clues in tests of stereoeacuity. Transactions of Orthoptic Congress, Stockholm, Sweden, 1999).
