Scheiman pp. 176-181; 204-217

Press pp. 234-242

I. Introduction: Instrument vergence therapy, like the free space therapy regimens discussed last week, is utilized to train positive and negative fusional vergence. Techniques in free space are used more frequently by most practitioners. Stereoscopes produce a less natural visual environment for learning vergence skills. Hence, it has been proposed that fusional ranges trained in instrument were more difficult to apply in a regular working environment. Instrument therapy, however, is appropriate under many conditions, including high phorias with intermittent strabismus or when significant suppression is present. Therefore, methods of instrument visual therapy can be used in addition to free space techniques to improve a patient's binocular skills.

II. The Brewster stereoscope: The myth...the instrument.

A. Most stereoscopes of this type utilize plus lenses (+5.00 lenses mounted base out is the standard) to compress optical infinity into arms reach (...almost poetic). For all you optics buffs out there, this means that optical infinity has been placed at 20 cm. in front of the patient.

1. The stereoscope imitates real fusional convergence and accommodative demands that occur under free space conditions.

a. many argue that, although the optical device can be theoretically set to match "natural " distance viewing conditions, it cannot predict how the patient will respond to the lenses or to the targets that are physically very close: humans usually have a hard time imitating an optical bench

b. it is believed that instrument convergence may result in a shift of about 4 prism diopters BO for "distance" targets viewed through the stereoscope.

2. It is important to note that because the Brewster stereoscope has a septum along the midline of the instrument; the right eye can only view the right target and the left eye can only view the left target. In this set-up, then, only orthoptic fusion is possible.

3. Fusional vergence demand can be changed by several means in these instruments.

a. vary the target separation printed on a stereogram (the cards you will see scattered around the room).

-most stereograms have two perspectives of a scene (half fields) printed on a card

-the separation between corresponding points of the two half fields plays a significant role in the overall fusional vergence demand on the patient

- there are plenty of suppression checks on the stereogram target

b. vary the separation between the optical centers of the lenses of the stereoscope

c. vary the distance between the stereogram and the lenses of the stereoscope: this is possible because most Brewster stereoscopes are equipped with a movable stereogram carrier. This enables movement of the stereogram towards or away from the patient

B. In a Brewster stereoscope, the relationship between lens power, optical center separation, target distance and target separation create a specific fusional vergence and accommodative demand. This relationship is expressed in the Principle Equation for the Stereoscope (Flax, 1976). It is presented here, along with some handy-dandy examples.

1. The Principle Equation for the Stereoscope:

Where:

LP: lens power for the stereoscope (most are +5.00 D)

LS: distance between the optical centers of the stereoscope lenses in centimeters

TD: target distance IN DIOPTERS

TS: target separation in centimeters

C: total convergence demand

positive number: net convergence demand

negative number: net divergence demand

2. LENS POWER (LP): most all Brewster type stereoscopes have +5.00 lenses mounted base out. This means that optical infinity is at 20 cm.

a. the LP term, then is always +5.00 D

b. for the most part, this term is a constant (for a standard Brewster stereoscope)

3. DISTANCE BETWEEN THE OPTICAL CENTERS (LS): most stereoscopes have an optical center separation of 85 mm. Therefore, the LS term is usually 8.5 (cm). This, however , is not true for all stereoscopes.

a. therefore,different vergence demands in stereoscopes may be created if the distance between the two optical centers is different from the "standard" 8.5 cm.

b. the changes are usually not drastic, as the principle equation points out (a 10 mm change in optical center separation results in a 5Æ change in fusional vergence demand at a 20 cm viewing distance (optical infinity)

c. in fact, the Keystone Telebinocular has a lens center separation of 95 mm which translates to a 5Æ increase in BO demand at a 20 cm viewing distance

4. TARGET DISTANCE (TD): assuming the Brewster stereoscope has +5.00 lenses, a distance target (at optical infinity) should be placed at 20 cm. At that distance, the dioptric value is 5.00 D.

a. some stereogram series are designed for near point work as well. The same principle equation can be used here. Specifically, 2.5 diopters of accommodative demand is produced in a stereoscope at a working distance of 13.3 cm. In this case, the TD term changes to 7.5D

b. many practitioners will make use of the movable carrier on Brewster stereoscopes to change the accommodative and vergence demand of a particular stereogram. These changes are predicted by the Principle Equation for the Stereoscope

i. as the carrier is moved closer to the patient: TD values increase

- accommodative demand increases

- convergence demand decreases OR divergence demand increases (this is predicted by the formula....why this happens will be discussed in the next section)

ii. as the carrier is moved away from the patient: TD values decrease

- accommodative demand decreases

- convergence demand increases OR divergence demand decreases (this is predicted by the formula....why this happens will be discussed in the next section)

5. TARGET SEPARATION: vergence demand can be changed by altering the target separation between the two half fields of a stereogram. In the case of the stereoscope, all fusion is uncrossed (orthoptic). The amount of convergence or divergence demand for a particular stereocard is dictated by target separation and may be calculated using the Principle Equation for the Stereoscope.

a. the Principle Equation for the Stereoscope may be used to calculate the ortho vergence demand for a particular stereogram.

At optical infinity:

(5.00 D X 8.5 cm) - (5.00 D X TS) = 0 demand

TS = 5.00 D X 8.5 cm/5.00 D

TS = 8.5 cm

At nearpoint (13.3 cm):

(5.00 D X 8.5 cm) - (7.50 D X TS) = 0 demand

TS = 5.00 D X 8.5 cm/7.50 D

TS = 5.67 cm

b. larger target separation at any distance results in an increase in negative fusional vergence (or a decrease in the amount of positive fusional vergence)

c. a smaller target separation at any distance results in an increase in positive fusional vergence (or a decrease in the amount of negative fusional vergence)

d. it then is possible to understand that as a stereogram is brought closer to the patient (as described above) to increase the accommodative demand, it also increases the effective separation between the two half fields. This is because of the retinal image size changes that occur as these two half fields approach.

i. the patient must used orthoptic (uncrossed) fusion

ii. in order to maintain orthoptic fusion as the stereogram is moved closer, the patient must diverge more OR converge less

iii. it is a similar situation to the lifesaver cards introduced last week

III. The "mirror stereoscopes": These instruments are particularly useful in severe binocular dysfunction, where fusion is very weak and suppression is prevalent.

A. The Wheatstone stereoscope: an economical and useful apparatus in remediating severe fusional deficits. It is a folded sheet of plastic in the shape of a "W" (see Activity E-7 for a complete description). The outside "flaps" of the Wheatstone stereoscope are held at 60 degree angles with a wire bracket. The interior fold is variable and has two mirror attached on either side of the fold. Most techniques utilizing the Wheatstone stereoscope are considered to be at the transitional skill level.

1. A set of paired stereograms (separate cards depicting different perspectives of a particular target) are placed into holders. Each card is reflected off the mirrors.

2. As the patient places his/her nose against the interior fold of the stereoscope, the reflected images of the paired stereograms reach the patient's eyes.

a. positive vergence demands are increased as the angle between the two mirrors is decreased

b. negative vergence demands are increased as the angle between the two mirrors is increased

3. suppression checks are, as always, present.

a. Wirt circles

b. different "control marks" on each card of the paired stereogram set

B. Single oblique mirror stereoscope (SOMS): another mirror stereoscope that utilizes paired stereograms. In this case, the patient looks through a face piece with no lenses. Auxiliary +1.50 lenses are available to imitate nearpoint conditions. The right eye looks directly at one of the stereocards located on the base of the apparatus while the left eye sees a reflection of the other stereocard off a single oblique mirror. The mirror is mounted on an plastic septum. It is considered to be at the transitional skill level.

1. A set of paired stereograms are once again used.

a. the right eye card is situated on the base of the stereoscope. The position of this stereocard determines the fusional demand. The base is conveniently marked off in prism diopters, so you can "read off" the fusional demand.

b. the left stereocard is attached to the side wall of the apparatus, situated so that it will reflect off the oblique mirror and into the left eye

2. As the patient looks through the eye pieces of the stereoscope, the reflected images off the left stereocard reach the patient's left eye and the right eye is allowed a direct view of the right stereocard.

a. positive vergence demands are increased as the right stereocard is shifted to the left

b. negative vergence demands are increased as the right stereocard is shifted to the right

3. suppression checks are, as always, present.

a. Wirt circles

b. different "control marks" on each card of the paired stereogram set

IV. Aperture rule: an excellent therapy technique for all vergence dysfunctions (...I like this one). It is an advanced binocular skill level activity, so it is not for the beginning patient!

1. This simple device consists of a plastic shaft that is attached to a base (see Activity E-10 for a complete description of this device). This device has two types of apertures which slide on the shaft and direct the patient to orthoptically or chiascopically fuse the stereogram targets.

a. the single aperture develops positive fusional vergence ranges via chiascopic fusion

b. the double aperture develops negative fusional vergence ranges via orthoptic fusion

2. The aperture rule card series is in a spiral binding.

a. each successive card increases fusional vergence demand no matter which aperture is used

b. on each card is marked the desired position of the aperture for that particular task. The proper position is etched into the shaft of the apparatus

c. each card has plenty 'o' suppression checks to monitor that binocular system

V. A Brief Description Of Each Stereogram Series:

A. Brewster type stereoscopes are the most common type of stereoscope. They may be different shapes, but each works on the same principles. The most important aspect of this method of training is the type of stereograms used. Most series of stereograms are graded programs that are easy to follow (the directions are on the back of the card, usually). In addition, it provides a fairly objective measurement of progress in visual therapy. Many series of stereograms are still in use . Some of them are reviewed below.

1. The Basic Unit (BU) series of stereograms is a series of 38 stereograms designed to help those patients with very poor fusional skills. It provides anti- suppression activities and basic stereoacuity training activities. It is designed for those patients who have significant suppression and/or poor sensory fusion skills. This series is best to be used at the distance setting (20 cm). Vergence demands printed on the back of the card are based on a 95 mm LS. This card series may be placed into the transitional skills level for vergences.

2. The Ann Nichols (AN) stereogram series was designed for a higher functioning binocular system than the BU series. Here a patient had many tasks to complete as he/she progressed through the 86 cards. In fact nine individual units were developed to improve basic fusion and eye-hand coordination, central stereopsis and jump fusional vergence skills. All but units VII and VIII were designed to be used at the distance setting (20 cm). Unit VII was designed to train intermediate point stereopsis (16 cm) and unit VIII was designed to train nearpoint stereopsis (13 cm). Vergence demands printed on the back of the card are based on a 95 mm LS. This card series may be placed into the binocular skills level for vergences.

3.The Home Training Series (EC) is designed for home use (as the name implies). This series is nice, because it is designed to be used at distance (20 cm.) and nearpoint (13.34 cm.). These cards are divided up into nine units (again!) with half of the units designed to help EXO postures (unit Alpha, Delta, Gamma and Eta) while half of the units are designed to help ESO postures (unit Beta, Gamma, Epsilon, Zeta and Theta). Many of the cards in the Eta and Zeta units are "repeats" of stereograms found in the AN series. Vergence demands printed on the back of the card are based on a 85 mm LS. This card series may be placed into the binocular skills level for vergences.

4. Bioptograms (Bernell) (E-13, 14)

a. BO-1 unit : a series of cards that are designed help with BO therapy. In the first few stereograms (1-4) a scene is presented which contains a picture in the background and numbers and letters which sequentially increase in BO demand. In more advanced stereograms (5-7), jump vergence skills are enhanced.This card series may be placed into the binocular skills level for vergences.

b. BI-1 unit: a series of cards that are designed help with BI therapy. In the first few stereograms (1-5) a scene is presented which contains a picture in the background and numbers and letters which sequentially increase in BI demand. In more advanced stereograms (6-7), jump vergence skills are enhanced.This card series may be placed into the binocular skills level for vergences.

5. "Storybook" Base In/Base Out Slides:

a. BO unit: a series of 8 jump stereograms. These consist of line drawings that depict various scenes of our best loved fairy tales. The fusional demand of each target (at 20 and 13.34 cm) are listed at the beginning of the series. For this series the stereograms train BO jump vergences in a range of 3Æ - 27Æ BO

b. BI unit: a series of 8 jump stereograms. These consist of line drawings that depict various scenes of our best loved fairy tales. The fusional demand of each target (at 20 and 13.34 cm) are listed at the beginning of the series. For this series the stereograms train BI jump vergences in a range of 0Æ - 12Æ BI

6. There are several stereogram series that present continuous text as opposed to pictures. Two such series are made by Keystone.

a. Adult Eye Comfort Series: a series of 24 cards with gradual increase in BO demand with each card. Specifically designed for pre-presbyopic pseudo CI cases

b. Children's Stories Series: two series of 24 cards designed for children that gradually build BI or BO ranges. Total range of vergence demand is 21Æ BO to 35Æ BI

7. Various series of stereograms are "split "(the two half fields are on separate pieces of cardboard). One series of split vectograms is the Stepping Stones Series by Keystone. With this series, the doctor is able to change the target separation (and therefore the vergence demands) rapidly. They are designed to be used with a special stereoscope called a cheiroscope (a Brewster stereoscope mounted to a lightbox). Markings on the stereogram and on the cheiroscope indicate fusional vergence demand.

1. What is the prismatic effect of moving the cardholder of a Brewster stereoscope towards the patient?

2. What is the prismatic effect of moving the cardholder of a Brewster stereoscope away from the patient?

3. What is the prismatic effect of increasing the target separation of the stereogram? How about when this separation is decreased?

4. If the separation between the optical centers of the lenses (+5.00) of a Brewster stereoscope is 85 mm and the target separation of the stereogram is 47 mm, what is the fusional vergence demand on a patient at

a. a 20 cm working distance

b. a 13 cm working distance

5. Now I place a -1.50 lens in front of the same patient . The target separation on the stereogram is 98 mm and the distance between the optical centers is 85 mm.

a. What distance must the stereogram be to present NO vergence demand?

b. What would the accommodative demand be at that point?

c. What distance must the stereogram be to present NO accommodative demand?

d. At that point (in part c), what is the vergence demand?