Easy and natural stereopsis vision assessment

Pocket Test

tilly pocket Click on this picture to see the Frisby Pocket Stereotest being used

The Frisby Pocket Test – NEWFPS On White 27jul13FPS in Shirt Pocket 27jul13

The Frisby Pocket Stereotest is a new addition to the Frisby Stereotest range.  It comprises either a 6mm plate or a 6mm and a 3mm plate in a pocket-sized case, convenient and easily transportable.

It is ideal for stereopsis screening, in optometrist practices, doctors surgeries or in school screening programmes.  It is very easy to use and can be administered by non-professionals.FPS Jacki Cropped 27jul13

The plates have two quadrants, and are presented to the subject using the white background of the case (but should not be presented flat against the case, see diagrams).  The purpose of the test is to demonstrate stereopsis by the subject detecting in which quadrant the target is hidden.

It tests for stereopsis even if reduced vision or amblyopia is present because of the coarse elements included the texture – this is a valuable feature of Frisby Stereotests.

How the Test Works

  • The target is a cluster of randomly arranged arrowheads of differing sizes, as shown. target random pattern
  • 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 be able to detect the target as it can be distinguished only on the basis of binocular disparity cues to depth.

Presentation

Show the plate using the white case as a convenient background. Avoid monocular cues, that can arise if the plate is laid flat on a surface, by ensuring that a space of about 5cm is maintained between the plate and background, as in the diagram.
If the plate is 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). This variation can be useful but if a full stereoacuity assessment is required then consider using the Frisby Near Stereotest (3 plates).

A Positive Response

tilly pocket Click on this picture to see the child testing her doll using the Pocket test

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  show the plate  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 twisting it slowly to and fro in front of the clear background provided by the case. 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: