The Brock String - a common Vision Therapy Tool


A Brock string (named after Frederick W. Brock) is a tool used in vision therapy. It consists of a white flexible cord or string of approximately 10 - 15 feet in length with a red, yellow and green colored small wooden bead.

Frederick W. Brock (1899–1972), born in Switzerland, was an optometrist, a major contributor to vision therapy, and the inventor of various vision therapy tools. Brock's approach to treating eye disorders was instrumental in paving the way to overcoming a long-standing medical consensus that stereopsis could not be acquired in adulthood but only during a critical period early in life before the age of 6.

Susan R. Barry, a neuroscientist was the first person to have received widespread media attention for having acquired stereo vision in adulthood and attributes Dr. Brock an important aspect of vision therapy and in her recovery of stereopsis. This discovery maintains the scientific conceptions with regard to the neuroplasticity of the visual system

How to Use it

Set Up:

  1. Make a loop at the end of both of the white string.

  2. Affix one end of the string to a doorknob.

  3. Position the far bead near the end of the string closer to the doorknob. This is the far fixation bead.

  4. Place the middle fixation bead about 2 ft -5 ft from you.

  5. Place the near fixation bead about 6 inches from your nose.

  6. Stand directly in front of the doorknob facing it. Put the loop around your index finger and hold the white string under your nose like a mustache,

  7. Hold the cord up to the bridge of your nose so that the cord is stretched tight between your nose and the doorknob.


  1. 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.

  2. Eventually, the near bead should be only one inch from the bridge of your nose.

  3. 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 of the bead is accurate, the two strings should appear to meet exactly at the bead forming an "X". As the bead is moved into one inch from your nose, the two strings should appear to meet exactly at the bead forming a "V".

  4. Shift your eyes to the middle fixation bead and then to the far fixation 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".

  5. Change the location of the fixation beads and again repeat.

Try to be aware of other objects periphery or in your field of view as you practice.

What does it do?

The purpose of the Brock string is to learn to see with both eyes simultaneously. When the subject looks at the first bead, he or she should see one bead with two short strings leading toward it and two longer strings leaving it. The patient should see two strings entering the bead and two strings leaving it, making a large“X”. The beads that are not being looked at will appear in the distance on each of the white strings you see coming out or in from that fixation bead.

Why is it important in vision therapy?

Suppression, convergence excess and convergence insufficiency, among others, may be detected and treated using the [physiological diplopia] (/info/physiological_diplopia )techniques employed by the Brock string.

How do I make one?

The material needed to make a Brock string. A string is 10 -15 ft of white small flexible cord or thick string, 3 different brightly colored beads usually red, yellow and green.


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