Easy and natural stereopsis vision assessment

Near 3 Plates Test

The Frisby Near Stereotest – 3 plates

FNS  Lid Open

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.

  • 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.
FNS 2 photos Website low res300  21jan14

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.

How the Test Works

  • The target is a clustarget random patternter 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.

 FSST Hilda BOS Pic

Preferential Looking

  • Each plate has 4 studs in each corner. The function of these is two-fold. First, they protect the plates if they are laid on table tops etc.  Second, as noted above, the stud alongside the target has a flat surface, allowing the tester to know by touch where the target is whilst following the observer’s eye movements. The photo shows the tester carefully focusing on the eyes of the child when using the Frisby Screening Stereotest  but the same principle applies for all Frisby Stereotests.
  • This allows for a positive ‘preferential looking’ response, a distinct advantage with young children.
  • Another advantage of the stud with a flat surface is that 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.

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.
  • 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 Stereotests:

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