Eye Tracking FAQ
How does eye tracking work?
Our eye tracker has two key elements to it: an infrared camera, and an infrared light source. Infrared light is invisible to the human eye, but is present in sunlight and other light sources. In the picture of the eye-tracker to the right, the camera and light source are contained in the box on top of the chin rest.
The infrared light bounces off of the cornea of the eye, creating a bright spot that is visible to the infrared camera. In the image below, this corneal reflection is light blue. By comparing the position of this bright spot with the position of the pupil, the darkest region in the image (dark blue), the computer can detect any movement of the eyes and can calculate where on the display screen a participant is looking.
Because eye tracking technology only involves cameras and light, it is extremely safe and also easy to use.
How accurate is the eye tracker?
Our primary eye tracker is an Eyelink 100 Plus system from SR Research. It has a sampling rate of 1000 Hz, meaning that it measures the position of the eyes 1000 times a second. It also has high spatial precision; the computer's estimates of where a participant is looking are usually accurate to within 2-5 milimeters (a single letter in a word).
Can the eye tracker work with glasses or contacts? Or are there other things that make the eyes hard to track?
The eye tracker can usually work even if the participant is wearing glasses, although very strong prescriptions, glasses with strong anti-glare coating, and bi-focals present problems. Contacts also do not usually present a problem, unless they are hard contacts or the eyes are dry, in which cases the lenses move around on the eye and make the corneal reflection hard to find. Other factors that can make the eyes hard to track include lots of eye make-up and drowsy, droopy eyelids.
Can the eye tracker operate without a chin rest to keep the participant's head still?
For most experiments we want high-precision eye tracking, so we usually use the chin and head rests to keep the head stable. Most adults do not find these rests to be uncomfortable. However, it is possible to reconfigure the eye tracker for use without these - the particpant wears a small sticker on the forehead to help the camera find the eyes, and then is free to move. We use this configuration mostly for children. We also have access to an eye-tracker (a Tobii Pro Spectrum) that does not require a chin rest and so is especially suited for children.
What is eye tracking used for?
Eye tracking can be used to answer two types of questions. First, where the eyes are looking is a strong indicator of what a person is thinking about. This is, eye movements reflect attention. This is because we only get good visual information from the location we are looking directly at; our peripheral vision is blurry, but our central vision is crisp and detailed. So, if we want more information, we look directly at something. Knowing where someone is looking tells us a lot about what they are paying attention to.
Second, the eyes spend more time looking at things (objects, faces, words, sentences) that are more surprising or more difficult to recognize or understand. This means that how long we look at something reflects mental effort. If two people look at the same word, for example, but one person spends longer looking at it, that tells us that recognizing the word was more difficult for that person. Increased looking time can also reflect people's preferences, because we choose to exert more of our mental effort on things that are interesting or appealing.
These two facts make eye tracking a useful tool for investigating the mind. With eye tracking, we can have a good idea about what a person is thinking about (attention) and how difficult or interesting that thing is (mental effort). Eye tracking is used to study how our vision works, how we understand written or spoken language, how we learn and solve problems, how our emotions work, how we recognize faces, how advertising influences us, how we navigate web pages, how we make judgments and form opinions, and a host of other questions about the human mind. It has also been used to investigate how disorders, such as schizophrenia or autism, affect the way the mind works. Eye trackers have been used to explore everyday activites such as how we drive, how we read, how we play sports, and even how we make peanut butter sandwiches. Click here for more information on how we are using eye tracking in the lab.