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. Continuous, uncontrolled head movement can make Eyegaze operation difficult, since the Eyegaze System must relocate the eye each time the user moves away from the camera's field of view and then returns. Even though the System's eye search is completed in just a second or two, it will be more tiring for a user with constant head movement to operate the System. And Baclofen, a drug commonly used to decrease muscle spasms, can cause dizziness, drowsiness, headache, disorientation, blurred vision and mydriasis.
Mydriasis can be severe enough to block eyetracking.
If the retinal reflection is extremely bright, and the corneal reflection is sitting on top of a big, bright pupil, the corneal reflection may be indistinguishable and therefore unreadable by the computer. In general, a user with average intelligence will best maximize the capabilities of an Eyegaze System. The System is text-based. A young child with average intelligence may not be reading yet, but probably has the capability to learn to read at an average age. He may be able to recognize words, and may be moving his eyes in a left to right pattern in preparation for reading.