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!! What is eye-tracking?
An eye-tracker makes it possible to track gaze-position (i.e., where someone is looking) or someone's pupil size (also called pupillometry). There are several types of eye-tracking systems. The eye-tracker does this by shining (near) infrared light onto the participant's face. This light is not visible to the participant, but it can be picked up by the eye-tracker's camera. The light bounces off the cornea, which the eye-tracking software then uses to determine gaze-position.

Tracking someone's gaze-position can for example be used in reading research (e.g., to see how quickly someone reads low-frequency words). Pupillometry is often used in experiments with a cognitive focus, and it is often taken to be an index of mental effort.

!! Conducting eye-tracking research with older adults
When collecting eye-tracking data with older adults, there are several practical considerations to keep in mind. These include:

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1) It is very common for older adults to wear reading glasses. This makes data collection more challenging, as glasses refract the light, subsequently making it more difficult for the computer to know where someone is looking. To increase your chances of a good signal, it is good practice to use a chin-rest.
2) Other types of prescription glasses (e.g., bifocal or varifocal glasses) do pose a problem, since they caused 'jumps' in the eye-tracking signal
3) Older adults may experience a range of conditions thay can affect eye-sight (glaucoma, cataracts, macular degeneration, to name a few). In theory it should be possible to collect data, provided that the participant's pupil is unobscured and that their cornea is undamaged
4) Older adults may also have a condition called ptosis, meaning that the upper eyelid droops over the participant's eye. If someone's pupil is obscured because of this, it is not possible to collect data. As such, it is advisable to screen for this
5) Regardless of the presence of conditions of the eye, it may be advisable to include a 13-point calibration procedure (as opposed to the less stringent 9-point calibration) to ensure good signal quality
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