Since its early days, eye tracking has come a long way. With the evolution of computer technology, eye tracking has become less intrusive, more affordable, accessible, and sessions increasingly comfortable and easy to set up. Long gone are the scary Frankenstein head-mounts. Modern eye trackers are hardly any larger than smartphones and provide an extremely natural experience for respondents. Remote, non-intrusive methods now render eye tracking an easy to use tool in human behavior research that allows to objectively measure eye movements in real time.
Beyond doubt, eye tracking enjoys a rapidly growing popularity within a vast variety of academic and commercial research areas. Providing information on how visual attention is distributed and changing for a presented stimulus, eye tracking is widely used to assess human behavior in market research, neuroscience, human-computer interaction, and numerous other scientific domains.
What is eye tracking?
Put most simply, eye tracking refers to the measurement of eye activity. More specifically, eye tracking implies the recording of eye position (point of gaze) and movement on a 2D screen or in 3D environments based on the optical tracking of corneal reflections to assess visual attention.
How does eye tracking work? Most modern eye trackers utilize near-infrared technology along with a high-resolution camera to track gaze positions. The underlying concept is commonly referred to as Pupil Center Corneal Reflection (PCCR): Near-infrared light is directed toward the center of the eye (pupil) causing visible reflections in the cornea (outer-most optical element of the eye), which are tracked by a camera.
There are two types of eye tracker: Remote (also called screen- or desktop-based) and head-mounted (also called mobile). While remote eye trackers are mounted below or placed close to a computer or screen, mobile eye trackers are mounted onto lightweight eyeglass frames (eye tracking glasses) and allow the respondent to walk around freely.
What can eye tracking reveal?
Regardless of which eye tracking system you use, gaze points and fixations are the raw metrics you can derive from eye tracking. Gaze points constitute the basic unit of measure – one gaze point equals one raw sample captured by the eye tracker. Gaze points can be aggregated into fixations, a period in which our eyes are locked toward a specific object. To concentrate your analysis on specific regions of the stimulus, you can define Areas of Interest (AOI). Generating heat maps allows you to visualize fixation positions and temporal changes of fixations as an overlay on a specific stimulus across different respondents.
There are several ways to analyze gaze positions and fixations. For example, you can analyze the fixation sequences and the performance of different regions in an image or a video with respect to the Time to First Fixation (TTFF), the number of respondents looking toward a specified region (respondent count), or AOI revisits after looking away.
While these metrics are commonly used to track eye movements modulated by visual attention and stimulus features, there are a few others that allow to assess states of emotional arousal and cognitive workload that drive eye motion. Those “advanced” metrics include pupil dilation, distance to the screen, ocular vergence, and blinks.
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Why does it make sense to combine eye tracking with other biometric sensors?
Eye movements are tightly linked to visual attention. As a matter of fact, you just can‘t move your eyes without moving attention (if you manage to cheat science, kudos to you! You‘ll probably enter the Guinness Book World of Records). However, you certainly can shift attention without moving your eyes. While eye tracking can tell us what people look at and what they see, it can’t tell us what people perceive.
Eye tracking gives incredible insights into where we direct our eyes at a certain time and how eye movements are modulated by visual attention and stimulus features (size, brightness, color, and location). However, tracking gaze positions alone doesn‘t tell us anything particular about the cognitive processes and the emotional states that guide eye movements. In these cases, eye tracking needs to be complemented by other biometric sensors such as EEG, GSR, EMG, or facial expression analysis to capture the full picture of human behavior in that very moment and gain meaningful insights into the spatio-temporal dynamics of attention, emotion, and motivation.
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