The Frisby Stereotest                                                                         View Shopping Cart

By LS Sasieni

The measure of the stereoscopic acuity has considerable significance in the study of binocular vision, and has been used in the diagnosis and assessment of visual dysfunctions other than that of stereopsis. In the subjective appreciation of stereoscopic depth by fusion, there has to be a binocular parallax, which can involve a vergence movement, and this fact can be used in the diagnosis of small heterophorias and small heterotropias. Frisby (et al) have described a study of 27 strabismic patients in whom microtropias and other muscular anomalies were related to the stereo acuity as measured on the Titmus test and on a random dot test, and referred to other investigations which had been carried out earlier.

The Titmus Test

In the Titmus test, devised by Wirt in 1971, the subject wears a polarizing spectacle to separate two superimposed polarized monocular images. The picture for the left eye is polarized at 450 and that for the right eye at 1350, and the viewers correspondingly orientated. The test comprises three subtest. The first is a large picture of a fly, in which parts of the monocular pictures are separated by some 7.5mm, which, held at a viewing distance of 30cm from the eyes, and assuming a PD of 60mm, represents a stereo disparity of about 1016'--which, compared to the average normal stereo parity of 20-50 seconds of arc, demonstrates only the presence of gross stereopsis. This is intended as a demonstration of the test to the patient, and, by its striking nature attracts the attention and interest of children.

The next subtest consists of a series of nine squares in each of which are four circles. In each set of four, one circle is made stereoscopic and the subject has to detect which of the four is seen to 'stand out'. Each successive set of circles represents an increasing degree of stereopsis, from 800 seconds of arc to 40 seconds.

The third of the Titmus subtests is three rows of animal pictures. One animal in each row appears to stand out, and the three rows represent stereo acuities of 400 seconds, 200 seconds and 100 seconds respectively.

The Frisby Test

The Frisby test is designed on a different basis from the Wirt test. Whereas in the latter the stereoscopic effect is artificially produced by superimposing two dissimilar pictures and separating them by viewing them through a polarizing spectacle (Figure 1), the Frisby test presents targets which are actually 'in depth' (Figure 2). The pictures are random dot patterns.

Random Dot Stereograms

Random dot stereograms were introduced about 1959, and in 1969 printed on cards to be viewed in a stereoscope with some separating device. They have the advantage of excluding the possibility of any extraneous clues contributing to the sensation of depth--movement parallax, size, perspective, experience, etc. The principle is the same as in pictorial stereograms except that nothing is recognizable when viewed monocularly. In a small area of each of the dot patterns, the dots are displaced with respect to the surrounding dots, so that when viewed binocularly the separation between the small areas is different to the separation of the remainder of the 'picture'.

The patterns do not necessarily have to be composed of dots. They can be lines, letters, figures or shapes, but the essential feature is the random arrangement over the area. Because of the random arrangement of the characters, no difference is visible when viewed monocularly, but when seen binocularly the central area appears at a different distance from the observer, and therefore stereoscopically. The amount of displacement of the small area, relative to the distance at which it is viewed, and the PD, is the measure of the stereo acuity.

The Frisby test differs again from the printed stereograms in that no spectacles or other separating device need be worn by the patient. The targets are actually 'in depth', being printed on the two sides of transparent plates of different thicknesses. There are three plates, 1mm, 3mm and 6mm thick respectively. Each plate has four squares of random dot patterns, one square having the central portion printed on the opposite side of the plate from the surrounding portion. Viewed against a featureless background, for example a sheet of white paper, and with head and test held reasonably steady, the only clue to depth perception is binocular parallax. In this case it is the difference between the two levels, related to the distance from the observer, and the PD, which gives the measure of the stereo acuity.

Range of Measurement

As there are four patterns on each plate, they can be presented in any one of four positions and with either side facing the patient, so that the small area may appear in front of or behind the level of the surrounding pattern. The possibility of accurate guessing is thus practically eliminated, particularly as each plate maybe presented repeatedly in various positions without the patient being able to learn the correct response.

The test can be held at any of six distances, from 30cm to 80cm, the distance being controlled by the use of a tape attached to the test and held by the patient against the check. The tape is folded and marked in 10cm divisions, facilitating accurate positioning. The six positions, combined with the three thicknesses of plates, provide 18 values of stereo acuity, from 880 seconds of arc to 20 seconds. These could be extended, of course, by increasing or decreasing the viewing distance, but in practice these would amply cover the range needed.

Identification of the Correct Square

Because of the great variety of presentations possible, it is necessary for the tester to be able to know which is the stereoscopic square in order to verify the subject's answers without the necessity for close inspection each time. This is possible by discreetly touching the studs on each corner of the plate, which also act as protective feet. On the stud nearest the correct square is a small flattened area, easily felt, but invisible to the subject. This also enables the test to be administered by persons without binocular vision, or with stereo acuity not good enough to be able to check the answers by observation.

Child Patients

Such a test is extremely useful in the examination of child patients. It is easy to administer and it has been shown2 that this type of test can be successfully used as a screening test for amblyopia, large-angle squints, microtropias, intermittent esotropia, accommodative esotropia and high (5.00D and over) astigmatism, failure to pass the test indicating a high probability of amblyopia, heterophoria, or anisometropia associated with the potential development of amblyopia.

Stereo acuity

The measure of stereo acuity is in terms of the minimum discernible difference of depth between the two levels, or the minimum discernible angle of disparity between the displaced portions in a pair of stereograms. This angle varies with (a) the distance between the dissimilar portions, (b) the distance at which they are viewed, and (c) the PD of the subject.

If we consider the simplest test of stereo acuity, the three needle test, the stereoscopic angle is given by the difference between the angle of convergence when fixing either of the nearer needles and the angle of convergence when fixing the more remote needle. The minimum discernible difference is the measure of the stereo acuity. Figure 3 represents the principle of the three needle test where:

1 is the viewing distance; d1 the difference in the levels of the objects (A and B); RL or p the subject's PD. If the viewing distance is 300mm and the difference in the distance between the nearer and further needles is 6mm, and the PD is 60mm, then

RO = tan RAO and RO = tan RBO

OA OB

30 = 0.100 = tan 5" 42' 38"

300

30 = 0.098 = tan 5" 35' 58"

306

Angle RAL = 2 x 5" 42' 38" = 11" 25' 16"

Angle RBL = 2 x 5" 35' 58" = 11" 11' 56"

 

The stereoscopic angle is the

difference 13' 20"

If we now consider a pair of stereograms printed on flat card, in which the stereoscopic effect is produced by a displacement of part of each picture in relation to the remainder of the picture, and each picture is seen monocularly, and the two are fused binocularly, as in a stereoscope, or if the pictures are both printed in two colors and viewed through red and green glasses, as in an anaglyph, or polarized and viewed through polarizing lenses, then the displaced parts are seen in crossed diplopia, until the convergence is adjusted to fuse them. This is the principle illustrated in Figure 3.

Again, the difference between the angle of convergence necessary to fuse the similar parts of the pictures (the background) (RAL or RBL) and the angle of convergence necessary to fuse the displaced portions (RNL), is the angle of stereopsis (ARB or ALB).

This is the principle of the Titmus test. In the first ("fly") subtest, the viewing distance is 300mm and the greatest separation of the displaced parts of the picture (the edge of the wing) is approximately 7.5mm.

N is the position of the fused image of the points A and B.

 

CN = ON

AC OR

CN = 300- CN

      1. 30

CN = 3.75 (300 - CN)

30

= 3.75 300 - CN

30

30CN = 3.75 (300 - CN)

33.75 CN = 1125

CN = 33.3mm

Which means that the edge of the fly's wing is seen apparently 33.3mm nearer than its feet.

Now applying this to the principle in figure 3,

1 is 300mm; dl is 33.3mm; so OB is 333.3mm

Assuming p to be 60, then

RO = tan RAO

AO

RO = tan RAO

AO

RO = tan RBO

BO

30 = 0.1 = tan 5" 42' 38"

300

30 = 0.09 = tan 5" 8' 34"

333.3

The angles of convergence are, therefore,

RAL = 2 X 5" 42' 38" = 11" 25' 16"

and RBL = 2 X 5" 8' 34" = 10" 17' 8"

and the difference (the stereo acuity): 1" 8' 8"

If we take 1 as OB, the OA is 266.6 and

30 = 0.1125 = tan 6' 25" and the angle

266.6

of stereo acuity is 12" 50' - 11" 25' = 1" 25'

Another method of obtaining the angle of stereoacuity is from the formula (the angles being small):

p x d1 (in radians) (1 radian = 57.29")

1"

where p is the PD; 1 the viewing distance; and d1 the difference in the distances between the two planes.

60 x 33.3 = 0.0222 (radians) = 1" 16' 18"

300"

The Frisby Test in Practice

Applying this formula, the angles of stereo acuity represented by the three plates of the Frisby test, held at the six viewing distances, are given in the following table, which has been calculated assuming a PD of 64mm, and rounded to the nearest five seconds of arc, but a difference in PD would not make any significant difference. A copy of this table is provided with each test.

Table 1

Viewing Distance

CM IN

Plate Thickness

6mm 3mm 1mm

30

40

50

60

70

80

12

16

20

24

28

32

880

500

320

220

160

120

440

250

160

110

80

60

150

80

55

35

30

20

The Frisby test is extremely simple to apply and need take no more than a couple of minutes. Being equally suitable for adults and children, it could well be routinely included in the normal full eye examination.

References

  1. Frisby et al. October 1975. BJO. 59. 10. Pp 545-552.

  2. Frisby. 1975. Develo9pment Medicine and Child Neurology. 17. Pp 802-806.

STEREO ACUITY ASSESSMENT

Test the patient with various plate/distance presentations selected from the table below to determine the lowest disparity value, which the patient can reliably manage. It is suggested that the tester concentrate initially on the usual reading distance of 40cm. Accurate measurements are best made using the tape measure, held by the patient as illustrated in the figure to control eye-to-plate distance, with the test wallet either rested on a table top or on the tester's lap. There is no need however, to bother with the tape until the tester has established the approximate distance at which the observer begins to fail.

Important: observe the precautions mentioned above about holding the test plates squarely, preventing head movements by the patient, using repeated presentations until satisfied that the patient can or cannot make reliable discriminations, avoiding reflections etc.

DISPARITIES (all values to nearest 5 sec arc)

Viewing Distance

cm (in)

Plate Thickness

6mm 3mm 1mm

30

40

50

60

70

80

(12)

(16)

(20)

(24)

(28)

(32)

600

340

215

150

110

85

300

170

110

75

55

40

100

55

35

25

20

15

Record the lowest disparity, which the patient can reliably discriminate. This stereo threshold is a measure of stereo acuity.

The above values are sufficiently accurate for customary clinical practice but it should be realized that (as for other stereo tests) they are only approximations to the exact disparities. Interpupillary distance will vary for different patients, it is difficult to be completely sure about eye-to-test distance (although the Frisby Stereotest does provide a tape measure to help with this problem) and there will inevitably be slight manufacturing variations in the tests themselves. In the case of the Frisby test, slight variations in the thickness of the Plexiglas®TM plastic sheet used for the plates means that any given set of plates is likely to depart in some degree from the nominal real thicknesses of 6mm, 3mm, and 1mm, and hence there will also be slight departures from the expected apparent thicknesses (of 4.027mm, 2.013mm, and 0.67mm respectively - reductions caused by the 1.49 refractive index of the plastic). Consequently, if the Frisby test plates are being used in a context where it is meaningful to know the exact disparities being presented, then these should be worked out from the usual disparity formula, suitably adjusted to cope with the apparent depth reduction effect. This formula is:

Disparity = 206264.81 (p.d1)

      1. (12)

where p is the interpupillary distance, 1 is the viewing distance, and d1 is the plate thickness as measured with a micrometer.

Technical note: the stereo threshold is the smallest angle of disparity, which can be discriminated by the patient. The associated stereo acuity is technically the reciprocal of the stereo threshold.

SCREENING PATIENTS UNABLE TO GRASP VERBAL INSTRUCTIONS

Note. While it is often easy to record a Stereopsis Present result even for very young pre-verbal children by virtue of their consistently correct pointing responses, it is not so easy to be as confident about a Stereopsis Deficient result for such young subjects. For example, a run of incorrect responses may be due to the patient not fully understanding what is required of him, rather than because he lacks stereopsis. This is a problem common to all currently available stereopsis tests but the Frisby Stereotest keeps such "don't know" verdicts to a minimum. This is because it uses a natural depth stimulus (and so avoids the need for often troublesome red/green or Polaroid spectacles) and because it permits repeated presentations without the patient learning the 'correct' response. It thus makes it a feasible proposition to test children considerably younger than has been possible hitherto, sometimes children even less than 1 year old.

Important: Observe the precautions mentioned above about holding the test plates squarely, preventing head movements by the patient, avoiding reflections, etc.

CAUTION

The corner studs help protect the plates when they are laid on table tops, etc. It is desirable, however, to replace the plates in their envelopes in the test wallet when they are not in use.

Frisby Stereotest

 

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