Brock String – Bug on String

The Brock String is the basis for some wonderful activities, helpful in developing visual skills. Brock Strings are used to

  1. Create awareness of space and depth.
  2. Increase the range of motion of the eyes.
  3. Increase accuracy and range of targeting.
  4. Improve scanning ability.


Bug On String

Fun and effective, the ‘bug on string’ requires a 50cm length of a Brock String and only one bead. Three hands are required for this activity. The bead is brought progressively closer in towards the nose until the brain can no longer align both eyes on the target. One or both eyes is seen to ‘escape’ outwards. Over time, the client is able to maintain both eyes on target at nearer and nearer distances. It is crucial that the client remain aware of the doubling of the string in front of and behind the bug at all times. See the post on double vision.


  1. Glasses should be worn.
  2. Begin with the Starting Position, described in Physiological Diplopia: When seeing double is correct. Use only one bead positioned at the Harmon distance. Let the remaining beads drop to the far end of the string and keep them out of the way.
  3. The client holds the string in place against the nose with the non-dominant hand and holds a pencil in the dominant hand. The pencil is used not for writing, but for tapping the bead during the exercise. The therapist holds the string approx 50cm from the child’s nose. The bead is positioned here to start.
  4. The general play is to slowly bring the bead in towards the client’s nose along the string. The therapist gently nudges the bead in, or the client uses the pencil to move it inwards while tapping on it.
  5. The bead is steadily brought inward toward the nose until the client can no longer target the bead with both eyes. One or both eyes will be seen to move outwards, away from the bead, and the client will report seeing two beads (or possibly one bead if one eye is suppressing). This is called the break point. Bear this in mind as you proceed: The therapeutic goal is to bring the break point in closer to the nose while maintaining binocular vision (that is, where both eyes are locked in on the target bead). Watch the eyes to ensure they remain on target throughout.
  6. When the break point is reached, bring the bead out away from the nose and start over.
  7. Fast is hard, slow and steady is easier. Vary the speed, but always ensure the client is engaged in the activity, and that you are not going too fast or starting too close for the eyes to even target together to start with.
  8. Continue to move the ‘bug’ inwards to the break point, then start over from the farther point on the string.
  9. Do not patch during this activity.
  10. Try moving the bug outward from the nose to see at what distance the eyes can lock on to the bead. Closer is better.
  11. Allowing the client to tap the bead usually allows for quicker improvement.
  12. Play with speed. A nice steady pace is best. Try to cover the length of the string (50cm or so) in 10 seconds or less. Avoid making it a race and don’t go faster than 5 seconds to cover the length of the bug’s journey.
  13. As the client progresses, you will be able to start closer than 50cm.
  14. Advanced Technique: Have the client do this activity with the express purpose of imagining it later. Then, put the string down and have the client revisualize the activity, complete with the inwardly moving eyes, both locked on the imaginary target.

See Also:

You should study all notes relating to Brock String activities prior to attempting them. Always follow the guidance of your vision care provider. Do not exceed what clients can comfortably tolerate.

Photo shows how when viewing one bead, the other beads should appear to be 'doubled'. This is the normal expected response.

Photo shows how when viewing one bead, the other beads should appear to be ‘doubled’. This is the normal expected response.