People naturally move their eyes to take in information from the environment as they perform cognitive tasks (such as comprehending language) and plan their behavior. Where, when, and for how long people look at words in text and/or other types of information in the world can therefore be revealing of the nature of their cognitive processing. To track eye movements during cognitive tasks, we use a remote camera that is focused on the eyes as participants read or look at pictures. The eyetracker locates the center of the pupil and uses infrared light to create what is known as a corneal reflection. The vector between the pupil and the corneal reflection can be used to compute where the participant’s gaze intersects with what they are looking at (generally, a computer screen). Because we can track eye movements, but not head movements, we often ask participants to stabilize their heads by resting their chin on a chin rest. At the end of the experiment, we convert the raw eye position data into measures such as number and length of fixations on areas of interest on the screen, such as a particular word or picture.
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