A Brock string (named after Frederick W. Brock) is an instrument used in

vision therapy Vision therapy (VT), or behavioral optometry, is an umbrella term for alternative medicine treatments using eye exercises, based around the pseudoscientific claim that vision problems are the true underlying cause of learning disabilities, parti ...

. It consists of a white string of approximately 10 feet in length with three small wooden beads of different colors. The Brock string is commonly employed during treatment of

convergence insufficiency Convergence insufficiency is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence. Symptoms The symptoms and signs associated with con ...

and other anomalies of

binocular vision In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...

. It is used to develop skills of

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...

as well as to disrupt

suppression Suppression may refer to: Laws * Suppression of Communism Act *Suppression order a type of censorship where a court rules that certain information cannot be published * Tohunga Suppression Act 1907, an Act of the Parliament of New Zealand aimed ...

of one of the eyes. During therapy, the one end of the Brock string is held on the tip of the nose while the other is tied to a fixed point. The three beads are spaced out at various distances. The patient is asked to focus on one of the beads, while noting the visual input of each eye and sensation of convergence. The patient can use variable techniques to make easier or more difficult by bringing the beads closer\further to the nose and by employing

lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

and prisms. Although this procedure is usually associated with binocular vision and anti-suppression therapy, it can also be a valuable procedure for developing accurate fixation skills under binocular conditions. Three (to five) colored beads are spaced on a length of string at least 20 inch long. The patient is instructed to alternate fixation and focus from one bead to the next while maintaining awareness of physiological

diplopia Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often v ...

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Set up

# Tie one end of the string to a doorknob. Position one bead at the end of the cord close to the doorknob. This is the far fixation bead. # Place the middle fixation bead about 10 inch from you. # Place the near fixation bead about 4 inch from you. # Stand directly in front of the doorknob facing it. Hold the cord up to the bridge of your nose so that the cord is stretched tight between your nose and the doorknob.

Procedure

# Look at the near fixation bead. Keep this bead single as you look directly at it. If the near bead is double this indicates an eye teaming problem. If this occurs move the near bead closer or further away until you see it as a single bead. The near bead should be moved closer and closer as the task becomes easier. Eventually the near bead should be only one inch from the bridge of your nose. # As you look at the near fixation bead you should see two strings, each of which appears to come from your eyes. If your fixation on the bead is accurate, the two strings should appear to meet exactly at the bead forming an "X". As the bead is moved to one inch from your nose, the two strings should appear to meet exactly at the bead forming a "V". # Shift fixation to the middle bead and then to the far bead and repeat. If your fixation of the far bead is accurate, the two strings should appear to meet exactly at the bead forming a "V". # Change the location of the fixation beads and again repeat. # Try to be aware of other objects in your field of view as you practice. # Slowly turn your head from side to side through an angle of about 45 degrees maintaining your vision of the two strings at all times. (if you turn too far one string will disappear)

References

* Scheiman, Mitchell and Wick, Bruce. ''Clinical Management of Binocular Vision.'' Lippincott, New York. 1994. pgs 188-192. {{ISBN, 0-7817-3275-1 Eye procedures