The corneas are normally moistened by tears, which are spread around by blinking. As the blink reflex decreases, the corneas dry out, and don't reflect infrared light very well. The Eyegaze System needs to see a corneal reflection in order to function. Traumatic and anoxic brain injuries can result in a variety of problems that may have an effect on Eyegaze operation. Cognition, memory, concentration, or even the ability to read may be affected by the injury.
Vision problems are also common.
In general, if the person has a consistent yes-no communication system, we assume there is a level of cognition that is sufficient for some Eyegaze use. Often memory and attention problems become apparent during the process of evaluating the user with Eyegaze. Vision problems such as diplopia, blurred vision, alternating strabismus and homonymous hemianopsia may also impede Eyegaze use. A number of children with cerebral palsy are currently using Eyegaze Systems to participate in school, from kindergarten through college. People with CP who have uncontrollable head motion do not find Eyegaze System operation acceptable, since the fixed camera can only accommodate for head movement across a two-inch range.