Brock String – Double Vision

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.
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.

Let’s start this discussion with a simple demonstration: Hold up your index finger on each hand. (You can use pencils or pens if you like.) Position your upheld fingers so that one is about 20cm in front of your nose. Position the other upheld finger about 20 cm behind the first. You should have two fingers pointed upwards, one in front of your nose, the other behind it, but both are about the same level. Now look at the nearest finger. As you are looking at the near finger, become aware of the finger in the background – you will notice it is doubled. Likewise, when you look now at the finger in the background, the foreground finger appears doubled.

This ‘doubling’ of targets we are not looking at is a natural and expected occurrence. Vision scientists call this ‘physiological diplopia’, meaning natural or expected double vision.

Physiological diplopia (‘physdip’) is an important phenomenon clinically for a number of reasons:

  • Double vision like this provides cues to the brain that the eyes must move inwards or outwards in order to both be targeting an object of interests. In other words, the double vision tells us if our eyes are pointing in the correct direction.
  • The awareness of foreground and background diplopia (double vision) helps our brains to keep our eyes fixated on a target of interest. ‘Slip’ happens when our eyes stray off target and the target of interest itself begins to appear doubled.
  • When observing physiologic diplopia, such as in the picture to the right, we can determine if one eye is ‘suppressed’ or not. A suppressing eye will give the appearance of one of the doubled strings being faded compared to the other, or perhaps the doubled line will not be apparent at all and the string will seem like one strand only.


Being aware of natural double vision is an essential part of Brock String activities. This activity helps you to learn the basic starting position for Brock exercises, and will give you an appreciation of what normal double vision looks like.

Learn the Starting Position:

  1. Take one end of your Brock String and line up three beads, equally spaced, starting 15 cm from the end of the string. So, in other words, the string will have one bead at about 15cm from the end, the next at about 30cm, and the third about 45cm.
  2. Grab the near end of the string in your non-dominant hand and hold the tip of the string against your nose, a centimeter or two down from the bridge (if you are right-handed, the non-dominant is your left).
  3. Hold the string just past the third bead with your dominant hand (10cm or so) and stretch it out so that it is sticking straight out from your nose. It’s ok if the string is at a bit of slant away from you so that your dominant hand is slightly lower than your non-dominant (near/nose) hand.
  4. Be careful with steps 2 and 3 as the beads can shift on the string. If you make your own Brock String, you can select beads and string in such a manner that the string is thick enough to hold the bead in place instead of allowing the bead to float freely along the string. If the beads move while you’re getting it into place, try these solutions:
    1. Place the string on a table and reposition the beads. Start over.
    2. Use a small piece of tape to hold beads in place.
    3. Tie a light knot in the string to keep the bead from sliding.
  5. You will be referred to this starting position (steps 1-4) in the other Brock String exercises. Now continue with the remaining steps:
  6. Now observe the outstretched string. Look at the middle bead. What do you observe? Most people will see one bead and the appearance of a double string in front of and behind the middle bead. It appears as though the string makes an ‘x’ that crosses as it passes through the bead.
  7. Continue looking at the middle bead. Become aware of the bead in the background. You will notice it appears blurred and doubled. This is the normal response, what we called ‘physiological diplopia’. Maintain focus on the middle bead.
  8. As you are watching the middle bead, become aware of the near bead. Again, it will appear blurred and doubled.
  9. Shift your attention from the near bead to the far bead and back. Only ever look at the middle bead.
  10. Focus on the middle bead. Does one of the string images appear lighter or darker to you? This is a sign of suppression and should be discussed with your optometrist.
    1. There is a simple rule to determining which eye is suppressing: “Reality is opposite to perception.”
    2. Look at the picture above. The right eye sees the lefthand image, and the left eye sees the righthand image.
    3. So, if the right side string appears brighter than the left, then the left eye is suppressing.

Continue reading the remaining Brock String posts.

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.