A brock string is a 10-15 feet string or cord with three small wooden beads of different colors that can move along the line at various positions. It is a tool used for vision therapy, and it obtains its name from a Swedish optometrist Fredrick W. Brock (1899-1972), who also invented various vision therapy tools.


The test mainly addresses eye vergence disorders such as amblyopia, convergence insufficiency and strabismus. It aims at aiding patients to see simultaneously with both eyes by:

  • Increasing eye range and motion
  • Creating awareness of depth and space
  • Improving eye’s scanning ability
  • Increasing accuracy and range of targeting


The brock string also helps to identify problems with convergence or suppression. It is crucial to identify such issues since they can cause headaches, blurry vision, and balance problems.

Convergence is where both eyes look forward and then begin to cross as an object moves nearer. On the other hand, suppression is when the brain ignores signals from one of the eyes.

The procedure is usually linked to binocular vision and anti-suppression therapy and can also be used to develop accurate fixation skills under binocular conditions.

Before the Procedure

Before the test begins, the patient should be in a relaxed or balanced sitting or standing posture. 

The eye specialist will also need to set up the brock string. S/he will begin by making a loop at both ends of the string and then tying one end tightly to a stationary object. The object should be at a height slightly below eye level, such as a door handle.

Moving the bead on the farthest end of the cord close to the door handle follows. At about ten inches from the patient, the doctor positions the middle fixation bead.

The last bead, the near fixation, is positioned about four to six inches from the patient’s nose. The patient should then stand facing the door handle directly. He/she will place the other looped end around their first finger and hold the string beneath the nose.


The doctor first directs the patient to look at the near fixation bead with both eyes open. Patients should see this as only one bead as they look at it directly. If the patient does not see only one bead, there might be an eye teaming problem.

Some of the things that may help to break this are; 

  • Blinking several times
  • Checking posture and peripheral awareness
  • Jiggling or vibrating the string
  • Moving the bead closer or further from the eyes
  • Using red-green glasses.


The doctor will move the near bead closer until it is only one inch from the patient’s nose. The patient then shifts focus to the bead at the center and then to the bead on the far end. The doctor repeats the procedure for both beads.

The goal is to see an X at each bead as soon as the patient looks at the bead. After some minutes, the examiner adjusts the beads to different positions until the eyes get used to making an X at any bead placed on the string.

The activity is then repeated with the patient’s head turned not more than 45 degrees in different positions as the string remains straight. Alternatively, the doctor may also change the string’s position while the patient’s head remains stationary or s/he may move both the string and the patient’s head.

While doing the test, doctors advise patients to be conscious of other objects in their visual field.


When looking at the bead, the patient should see one bead and two strings forming an X. The formation of the X indicates that both eyes are performing as desired.

As the bead moves an inch away from the nose, the string should appear to meet precisely at the bead forming a V shape.

When the patient closes one eye, one of the strings disappears. The same thing should happen when s/he closes the other eye. If the patient sees only one string and one bead when both eyes are open, it means that one of the eyes is suppressed.

If the patient sees two beads and two strings when both eyes are open, then his/her eyes have a convergence problem. The eyes fail to converge at the bead.