2. Binocular Instability (Eye Tracking)

Signs and symptoms

  • Missing small words when reading a line and failing to read the last letter in a short word.
  • Difficulty in using a dictionary quickly or being heavily reliant on using a finger or ruler to stay on the line.
  • The ‘crowding effect’: much easier to read a single word than reading  the same word together on a single line.
  • Missing out words and even entire lines of text.


Problem with the way the eyes co-ordinate with each other creating a ‘fixation disparity’ or imbalance between the eyes. This effect can be much worse when there are also issues regarding the ‘cross dominance’ of the eyes or even with the dominance of the hands. Difficulty can also be present when the eyes normally turn in to read (convergence). Eye tracking difficulties are also often associated with headaches, fidgeting, focussing problems, sore, tired eyes with excess rubbing and this makes searching for or finding words in text difficult and can also result in children missing out full stops and commas whilst reading.


A specially trained optometrist often referred to as ‘behavioural optometrist’ can correct this problem normally with the prescribing of glasses that have a special prismatic correction in them. They also may prescribe additional special exercises that improve a person’s eye co-ordination and others that improve their hand-eye–coordination and the ability of the eyes to turn in more easily and focus better when reading.

Continue to Visual Stress (Irlen Syndrome)