Video-based eye tracker: A clinical tool in assessing binocular vision?

The electro-oculogram (EOG) historically has been the most common clinically-used objective eye movement recording device. EOG is based upon the corneo-retinal potential difference1 and has many limitations2-4. More recently photoelectric techniques have been used to objectively record eye-movements. These instruments also have a limited linear range2. Eye movements can be recorded accurately using a magnetic search coil5, but this requires that a contact lens annulus containing the coil be placed on the patient's eye. The procedure tends to be time-consuming and is not well tolerated by a significant number of patients. Video technology has lead to a new generation of user and patient-friendly, non-invasive eye tracking devices. One such instrument, the El-Mar 2020 (Downsview, Ontario) eye-tracker, calculates eye rotation from the differences between the corneal reflections and the center of the pupil. This makes head stabilization unnecessary for accurate tracking, as it eliminates artifacts caused by lateral motion of the camera with respect to the head. These translation artifacts can be quite large (1 mm = 5 degrees for pupil tracking only, 1 mm = 10 degrees for corneal reflection tracking only). The system is free from drift, has a maximum resolution of 6 minutes of arc, a 120Hz sampling rate and a linear range of +/-40 and +/-30 degrees in the horizontal and vertical meridia respectively6. The instrument has been compared favorably to the magnetic search coil for oculomotor testing in humans6.