Frisby Screener Stereotest – now replaced by Pocket Stereotest which has a 6mm plate and the option of an additional 3mm plate
- The Frisby Screener Stereotest comprises one plate of 6mm thickness in a special presentation box that makes it particularly easy to administer quickly. When opening the box the lid folds back so that the top two quadrants of the plate are viewed against a white background.
- The purpose of the test is to demonstrate stereopsis by the subject detecting the target hidden on the plate.
- It is mostly used by optometrists, orthoptists and those running stereovision screening programmes as part of their core stereovision equipment.
- It is an effective test for patients with limited ability to understand instructions or communicate to the tester, such as when screening young children or when there are language barriers present. For these observers spontaneous pointing or preferential looking responses can be sufficient to establish that stereopsis is present.
- It can be used on children as young as 6 months.
- It tests for stereopsis, even if there is reduced vision or amblyopia is present because of the coarse elements included in the texture.
How the Test Works
- The patient looks to detect a target, which is a cluster of randomly arranged arrowheads of differing sizes, as shown here.
- The target is printed on one side of the plate in one quadrant, and on the other side of the plate similar texture 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 test is held still).
- Space is maintained between the plate and folded-back lid of the box (see photo below) to avoid monocular cues.
- If the plate is removed from the box and viewed from 40,50 and 60cm then the disparities of the target are respectively approx. 340,215 and 150 sec arc (to the nearest 5 sec arc).
A Positive Response
- With young children, a positive response may often be demonstrated by spontaneous pointing. Alternatively, preferential looking at the target may allow a positive response to be recorded
- If in doubt, the tester can take the plate out of the box and show it several times with the target in different positions.
- To ensure the patient knows what to look for, the target can be pointed out in a trial run, or made visible using monocular cues by resting the plate on a corner and rotating it slowly to and fro in front of the clear background provided by the box lid. Once the patient has identified the target in this way then stereopsis can be tested in the usual way with the plate held still, over a series of varied presentations as necessary.
- Thus the patient can easily be taught what to search for but cannot learn a correct answer when the target position is varied randomly from trial to trial after training.
- The opportunity to train a patient in this way without compromising subsequent test presentations is a valuable and unique feature of Frisby stereotests.
See the other Frisby Stereotests:
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Se si pensa che in italia l’8% della popolazione totale ha problemi di obesita e che almeno il 40% e in evidente
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