PRIMARY OPTOMETRY LABORATORY 6163, (Wensveen)

PRIMARY OPTOMETRY LABORATORY 6163, (Wensveen)

THE ADVANTAGE OF BINOCULAR VISION

EQUIPMENT

please bring:

calculator

pencil or pen

trial lens kit

supplied:

VHS video player and TVHoward-Dolman apparatus

Pulfrich effect videotape & viewersVerhoeff stereopter

Stereo effect videotapeTitmus, Reindeer & Randot stereotests

TNO stereoscopic test

Plumb bobsRandom-dot E

Table lampsFrisby test

Neutral density filtersLang stereo test

Stereoscopes (Brewster)

Stereograms demonstrating rivalry, luster, and stereopsis

Graph paper

INTRODUCTION

Binocular vision is the use of the two eyes in a coordinated manner so as to produce a single mental impression. Normal binocular vision implies binocular single vision (fusion) and a high level of stereoacuity.

In this laboratory we will examine several phenomena that cannot be appreciated monocularly. Not all are necessarily an advantage of binocular vision as far as visual function is concerned, but all have stimulated thought as to the mechanisms of binocular vision. We will also learn about the threshold for stereopsis and utilize several tests for assessing stereoacuity.

Binocular vision entails the integration of two monocular images into a single binocular percept. The integration occurs at cortical levels and is therefore essentially a perception. The formation of two suitable monocular sensations requires:

1. overlapping of the visual fields of the two eyes so that the same object can be seen by both eyes

2. the simultaneous stimulation of corresponding retinal areas

3. the ability of the extraocular muscles to adjust the visual axes so that the corresponding retinal areas are placed in a position to receive simultaneous stimulation by the same object.

Given that these requirements are met, the uniquely binocular phenomena of physiological diplopia, Pulfrich effect, retinal rivalry and luster, Fechner's paradox and stereopsis can be appreciated.

Physiological Diplopia

When two images of an object fall on corresponding retinal areas, single vision is attained. The extent of corresponding areas allows a certain amount of error; however, if the images fall on widely disparate areas, diplopia results. Physiological diplopia is inevitable and is always present. The double images are not noticed in everyday activities; we apparently see only one view of the external world as if we possessed only one centrally placed eye.

Pulfrich Effect

The Pulfrich effect is the phenomenon whereby a moving object binocularly perceived with unequal retinal illuminances appears to be displaced from it’s objective location. Swinging a pendulum in a frontal plane and viewing it binocularly with a neutral density filter placed before one eye usually demonstrates this effect. The resultant percept is of the pendulum moving about an elliptical path. This effect has also been used to create stereoscopic effects in broadcast television programs (Superbowl half-time show of 1989).

The Pulfrich effect occurs because the transmission time of signals from the retina to the cortex depends inversely on retinal illuminance. At photopic levels, reducing retinal illuminance by one log unit (a factor of ten) increases the transmission time by 15-20 milliseconds. If an image is moving across both retinae, a delay in one eye results in an effective shift of the image in that eye relative to the other. If the motion is horizontal, this effective disparity is perceived as depth when the signals from each eye are combined in the cortex. The amount of depth depends on both the velocity of the moving image and on the amount of delay produced by the filter.

Note that unequal retinal illuminances may also be produced by unilateral mydriasis or by disease processes that effect neural transmission of the visual signal; a person in this condition may experience potentially dangerous stereo percepts while driving.

Figure 1. Pulfrich Effect
Illustration of how a swinging pendulum appears to follow an elliptical path when viewed with a neutral density filter over one eye. The delay in visual processing due to the filter causes the central nervous system to fuse images which originated at different times, resulting in an effective disparity for moving objects. /

Retinal Rivalry

Retinal rivalry or binocular rivalry (and rivalry suppression) occurs when images of dissimilar targets presented to the two eyes cannot be fused. When images cannot be fused, the visual system alternately gates signals from the left and right eyes, so that the two targets appear to alternate in unpredictable ways. The classic demonstration uses targets of equal size consisting of parallel diagonal lines arranged in opposite directions for the two eyes. The direction of tilt seen in each part of the visual field reveals which eye is seeing and which is suppressed at any instant. This suppression can be localized, resulting in a patchwork of left and right eye signals, or it can be total so that only one or the other eye is represented.

/ Rivalrous patterns:
When presented to corresponding areas in each eye, this pair of images produces binocular rivalry.

Luster

Luster is a rivalry-like phenomenon which occurs when corresponding retinal areas have dramatically different color or brightness, for example when viewing a matte white figure outlined in black with one eye and a matte black figure of the same size outlined in white with the other eye. The resultant perception differs from what one would expect if the images were merely combined, i.e., a gray. The figure has a shiny, metallic appearance, which is called luster. Although considered a limiting case of retinal rivalry, this phenomenon has not been explained satisfactorily. Nevertheless, luster effects are useful in vision training of A.R.C. (anomalous retinal correspondence). The strabismic (typically esotropic) patient (with ARC) views a brightly illuminated wall while wearing red/green glasses. The patient should be able to see luster over the entire surface of the wall as the red and green colors undergo rivalry. If, on the other hand the patient sees a split field (i.e., red on one side and green on the other) the prognosis for remediation of the ARC is poor and no further attempt at training is made.

Fechner's Paradox

Fechner's paradox is the phenomenon in which a monocularly viewed white surface appears brighter than when it is viewed binocularly with one eye viewing the surface directly and the other eye viewing it through a neutral density filter. The binocular brightness is less than the brighter of the two monocular brightness’. In the monocular viewing situation, one eye sees zero brightness, so if left and right eye brightness’ are simply added then the image should appear brighter when viewed binocularly, even if one eye’s image is dimmed. The paradox is that adding light to one eye has made the perceived brightness reduce. The explanation for the phenomenon is that brightness averaging occurs when both eyes are viewing, but suppression occurs when one eye is completely occluded.

Stereopsis

Stereoscopic vision may be defined as the ability to see relative distances of objects on the basis of visual information available only to a two-eyed observer. The two retinal images (one in each eye) are slightly different due to the horizontal separation of the eyes, and it is the horizontal retinal image disparities that provide the essential information for stereoscopic depth perception. In the figure below, whether B is seen nearer or farther than A depends only on whether the separation of the images of the two lines for the right eye is smaller or greater than that for the left eye.

A Stereoscope is an instrument that separates the field of view of the two eyes so each eye can view a different image. There are two basic forms of stereoscopes. The Wheatstone stereoscope consists of two plane mirrors joined at one edge to form an angle of 90 degrees. Targets placed to the left and right of the mirrors are viewed by the left and right eyes, respectively. The targets can be moved laterally to induce convergence or divergence. The Brewster stereoscope uses a pair of base out sphero-prisms as eyepieces to separate the right and left eye fields of view. Targets are placed in front of the left and right eyes, and the eyepieces allow the viewer to fuse them as if they were a single object at distance. The amblyoscopes used in the clinic are examples of stereoscopes.

The threshold of stereopsis can be specified in terms of the minimum perceptible binocular disparity. Binocular disparity refers to the difference in the angles subtended at the eyes by two objects, one being the fixation point.

/ Binocular disparity:
Stereoscopic depth is perceived when images have different angular separations on the two retinae. Images of A and B in this figure are closer together in the left eye view than in the right, resulting in a percept of depth. Disparity is defined as the difference in subtended angles (A1 - A2) or the difference in target vergence (B1 - B2).

Binocular disparity = a1-a2 or b1-b2 and is also called the stereopsis angle. Stereoacuity corresponds to the threshold detection of this binocular disparity, usually expressed in seconds of arc. This threshold angle of binocular disparity for a given subject is essentially invariant with viewing distance. This indicates that the visual system utilizes angular information in the disparity of the two retinal images and that the associated information processing is independent of viewing distance.

Stereopsis requires fusion. If your patient has a history of a condition known to disrupt fusion (strabismus or anisometropia) then it is unlikely that he will ‘see the depth’ in screening tests for stereopsis. When children fail screening tests, we suspect that they may not be using their 2 eyes properly together and are alerted to look for the reason.

BEHAVIORAL OBJECTIVES

• Explain how we think these phenomenon occur: physiological diplopia, retinal rivalry, luster, Fechner's Paradox, Pulfrich effect and stereopsis.
• Define the threshold of stereopsis.
• Recognize the differences between the tests of stereopsis: what the name of each is, principal behind the test, how to administer the test properly, it’s advantages and disadvantages, and special uses. Do not memorize the formulas given.
• Know how anisometropia of increasing magnitude affects stereopsis, and how measures of stereopsis with monocular blur compare to measures of stereopsis with the same amount of binocular blur.

READINGS

• Reading RW. Binocular vision. Butterworths, Boston, 1983: 25-26 50-52 151-190.
• Bishop PO. Binocular Vision. In Moses RA. Adler's physiology of the eye. The C.V. Mosby Co St Louis 1988: 619-689.
• Heron G., Dutton GN. The Pulfrich phenomenon and its alleviation with a neutral density filter. British J. Ophthal; 1989;73:1004-1008.
• Griffin JR. Binocular anomalies: Procedures for vision therapy. 1st Edition. Professional Press Chicago 1982: 50-53.
• Rosner J., Rosner J. Pediatric Optometry. 2nd Edition. Butterworths Boston 1990:175-203.
• Brooks, SE. Anisometropia and Binocularity. Ophthalmology; 1996;103:1139-1143.

PROCEDURE

Physiological Diplopia

Hold up a pencil about 25 cm from the eyes and look, not at the pencil, but at the wall beyond. Two images of the pencil will be perceived. Close each eye in turn and note which image disappears.

Now, hold your finger before your eyes and the pencil further away and a bit higher than your finger. Fixing your eyes steadily on your finger, notice that two images of the pencil appear. Bring the pencil in slowly toward your finger. The extent of diplopia decreases. We have demonstrated here the two types of diplopia: crossed and uncrossed. Which one is which?

How might ‘physiological diplopia’ be used in making sure both eyes are seeing during convergence training?

Retinal Rivalry

This demo is set up in the stereoscope. First view the stereoscopic slide, and adjust the viewing distance by sliding the cardholder closer and further from you until you perceive maximum stereopsis; now keep the cardholder in this position. Insert the card with the oblique lines on it. Describe your perception when dissimilar targets, one imaged in each eye are observed. Do you get complete suppression? Rivalry?

Luster

This demo is set up in the stereoscope. Describe your perception of the black and white targets, when an integrated image is perceived.

Pulfrich Effect

Swing the pendulum so that it’s path is parallel to your subject's face plane. Have your subject place the neutral density filter before his right eye. Describe the apparent path the pendulum takes including shape and direction.

Now place the filter over the left eye. What differences did you notice?

View the Pulfrich video - do you see the 3-D effect?

Fechner's Paradox

Take a neutral density filter (0.8 - 1.0 ND). Look at a wall illuminated by the adjustable table lamp, close one eye, and hold the filter before the closed eye. Note the brightness of the wall with the open eye and assign that brightness a value of 10. Now open the other eye behind the filter so as to see the wall binocularly. Does it's brightness increase, stay the same or decrease? Compared to the value of 10 that you assigned the brightness of the wall with just one eye viewing, what value would you assign to your brightness impression now? Try to explain your results.

CLINICAL TESTS OF STEREOPSIS

Titmus StereoFly Test

The Titmus Test is one of the most widely used stereotest for measuring stereoacuity, even though stereoacuity of only up to 40 arcsec can be measured. It is designed for testing young children but is commonly used for adults as well.

For the Titmus test and the other printed tests of stereopsis, direct the overhead reading lamp onto the book. Either you or the patient can hold the book at the regular reading distance (40 cm), but it must be held perpendicular to the line of sight. Jiggling or twisting the book to see the figures in depth is cheating. Have the patient wear the polarizing glasses. If your patient cannot see both the ‘R + L’ on the right-hand side he may be ‘suppressing’ the right eye (R not seen) or left eye (L not seen). If he can see the ‘R + L’ have him view the fly on the right-hand page. Instruct your patient to try to grab the fly by the wings. He should grasp about 4 inches above the surface of the page. This is a test of ‘gross stereopsis’ and shows the patient what to look for in the tests of ‘fine stereopsis’. Now have him view the left-hand page, and identify which animal in each row ‘stands up off the page’.

Cat: 400 arcsec Rabbit: 200 arcsecMonkey: 100 arcsec

Next have him view the panels with the circles. Starting with number 1 have him indicate which circle stands off the page. Don’t let the patient actually touch the surface of the page because finger smudges are not good for the test. The disparity gets smaller as the panels progress and the number of arcsec stereoacuity required to discriminate the depth on each panel is tabulated. The limit of stereoacuity measurable with this test is 40 arcsec.

1bottom800 arcsec6left80 arcsec

2left400 arcsec 7right60 arcsec

3bottom200 arcsec 8left50 arcsec

4top140 arcsec 9right40 arcsec

5top100 arcsec

Randot Stereotest

All of the targets in the Randot stereotest are vectographic, so the patient must wear polarizing filters to separate one eye’s view from the other. The novel feature of the Randot test is that some targets – the shapes –are random-dot stereograms. These have 2 distinct advantages over ‘line stereograms’: there are no monocular cues and instead of discriminating between depth planes, the patient must identify the shape of the test target. The patient can only see the target and identify its shape if he is sensitive to the disparity in the stereogram. A limitation of the Randot test is that random-dot stereograms are used only to screen for global stereopsis.

In the same posture as for the Titmus test, have your patient look at the right-hand page and identify the shapes there. This is the random-dot part of the stereotest. Identifying the figures in the top half requires 500 arcsec, identifying the letters in the bottom half requires 250 arcsec stereoacuity.

TopBottom

CircleStarISquareTriangle

BlankLetter EIPlus signBlank

The cat (400 arcsec), rabbit (200 arcsesc) and monkey (100 arecsec) are scored the same as on the Titmus test. Try the circles next. The smallest disparity tested is 20 arcsec.

1left400 arcsec 6middle50 arcsec

2right200 arcsec7left40 arcsec

3left140 arcsec8right30 arcsec

4 middle100 arcsec9 middle25 arcsec

5 right70 arcsec10right20 arcsec

Reindeer Stereo Test

This test is similar in the method of administration and in principle to the Titmus Test, with just a few differences. The test distance is 14inches = 35 cm and stereoacuity is expressed in stereo percentage. Measure and record your stereothreshold with the Reindeer test. Convert the percent stereopsis you find to stereoacuity in seconds of arc using the table or the formula below.

Table 1. Conversion of Percent Stereopsis to Stereoacuity for the Reindeer Stereotest