Eye Tracking
http://en.wikipedia.org/wiki/Eye_tracking

Wikipedia

Abstract:

Eye tracking is a technique used in cognitive science, psychology (notably psycholinguistics), human-computer interaction (HCI), marketing research, medical research, and other areas. The most widely used current designs are video based eye trackers. A camera focuses on one or both eyes and records their movement as the viewer looks at some kind of stimulus. Most modern eye-trackers use contrast to locate the center of the pupil and use infrared and near-infrared non-collumnated light to create a corneal reflection (CR). The vector between these two features can be used to compute gaze intersection with a surface after a simple calibration for an individual.