Four eye-trackers are available for shared use at the lab. Three Eyelink 1000 trackers (SR Research, Mississauga, Ontario, Canada) are located in separate sound-attenuated testing spaces. These systems use an infrared video-based tracking system combined with hyperacuity image processing with average gaze position error of <0.5o. Resolution is 0.01o with a sampling rate of up to 1000 Hz, which is adequate for accurate measures of fixations on individual words during reading. These systems have the possibility of a head-free mount, affording comfort for our participants since they would not need to wear helmet-mounted cameras. One of the eye trackers, situated within the Haskins Child Studies Lab, is primarily used for studies with infants and children. This is an ASL Model 504 pan/tilt remote tracking system, which is a remote video-based single eye tracker that uses bright pupil, coaxial illumination to track both pupil and corneal reflections at 120 Hz. To optimize the accuracy of the pupil coordinates obtained by the optical camera, this model has a magnetic head tracking unit that tracks the position of a small magnetic sensor attached to the head of the participant, above their left eye.