The following user characteristics contribute to successful Eyegaze Edge use:

Good control of at least one eye

Adequate vision

Mental abilities that improve the probability for successful Eyegaze use

Good control of at least one eye:

The user must be able to look up, down, left and right. He must be able to fix his gaze on all areas of a 15-inch screen that is about 24 inches in front of his face. He must be able to focus on one spot for at least 1/2 second. Several common eye movement problems may or may not interfere with Eyegaze use. These include:

• Nystagmus (constant, involuntary movement of the eyeball):

  The user may not be able to fix his gaze long enough to make eyegaze selections.  The Eyegaze Edge has an optional calibration that can often accommodate nystagmus.

• Alternating strabismus (eyes cannot be directed to the same object, either one deviates):

  The Eyegaze Edge is constantly tracking the same single eye. If, for example, a user with alternating strabismus is operating the Edge with the right eye, and that eye begins to deviate, the left eye will take over and focus on the screen. The Edge camera, however, will continue to take pictures of the right eye, and the System will not be able to determine where the user's left eye is focused. When the left eye deviates and the right eye is again fixed on the screen the Eyegaze Edge will resume predicting the gazepoint. Putting a partial eye patch over the nasal side of the eye not being observed by the camera often solves this tracking problem. Since only the unpatched eye can see the screen, it will continuously focus on the screen. By applying only a nasal-side patch to the other eye, the user will retain peripheral vision on that side.

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Adequate vision:

Several common vision problems may affect a user's ability to see text clearly on the Eyegaze Edge monitor. These include the following:

• Inadequate Visual acuity:

  The user must be able to see text on the screen clearly. If, prior to his injury or the onset of his illness he wore glasses, he may need to wear glasses to operate the Edge. If he's over 40 years old and has not had his vision checked recently, he might need reading glasses in order to see the screen clearly. In most cases, eyetracking works well with glasses. The calibration procedure accommodates for the refractive properties of most lenses. Hard-line bifocals can be a problem if the lens boundary splits the image of the pupil, making it difficult for the System's image processing software to determine the pupil center accurately. Graduated bifocals, however, typically do not interfere with eyetracking. Soft contact lenses that cover all or most of the cornea generally work well with the Eyegaze Edge. The corneal reflection is obtained from the contact lens surface rather than the cornea itself. Small, hard contacts can interfere, if the lens moves around considerably on the cornea and causes the corneal reflection to move across the discontinuity between the contact lens and the cornea.

• Diplopia (double vision):

  Diplopia may be the result of an injury to the brain, or a side effect of many commonly prescribed medications, and may make it difficult for the user to fix his gaze on a given point. Partially patching the eye not being tracked may alleviate double vision during Eyegaze Edge operation.

• Blurred vision:

  This is another occurrence associated with some brain injuries, as well as a side effect of medications. A user who sees a blurred image the screen may not have accurate eye fixations.

• Cataracts (clouding of the lens of the eye):

  If a cataract has formed on the portion of the lens that covers the pupil, it may prevent light from passing through the pupil to reflect off the retina. Without a good retinal reflection the Eyegaze Edge cannot accurately predict the user's eye fixations. The clouded lens may also make it difficult for a user to see text on the screen clearly. Surgical removal of the cataracts will normally solve the problem and make Eyegaze use possible.

• Homonymous hemianopsia (blindness or defective vision in the right or left halves of the visual fields of both eyes):

  This may make calibration almost impossible if the user cannot see calibration points on one side of the screen.
  

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Mental abilities that improve the probability for successful Eyegaze Edge use:

• Cognition:

  Cognitive level may be difficult to assess in someone who is locked in, especially if a rudimentary communication system has not been established. In general, a user with average intelligence will best maximize the capabilities of an Eyegaze Edge.

• Memory:

  Memory deficits are a particular concern in considering the Eyegaze Edge for someone with a brain injury. A user who can't remember from one day to the next how to operate the system may find it too difficult to use effectively.

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