How does it work and which are the types of eye tracking?

Thursday, 07 de November de 2019

Do You Know What's Eye Tracking?

Eye tracking is an easy-to-use and non-invasive technique where you can collect data in empirical and real contexts about human perception, cognition and behavior. As you may have already noticed, the name itself already speaks of what it means: eye tracking determines aspects directly related to visual perception, so it analyzes the places of visual focus that are relevant to constructing what we see as a visual scene.

The eye tracking is so utilized, especially in humans, because it allows to empirically verify which pattern of visual exploration of each individual in different situations of life. We humans are not able to have multiple foci in the visual field, as you can see in the figure below (Fig. 1).

Figure 1 - Illustrations showing different visual foci. Choose a spot of light below the yellow arrow in figure 1A, note that now you can't count how many spotlights are on the other side of the image (you only see a blur while the spot of light you're looking at is fully focused and with good resolution). Now try the opposite: fix your gaze on the light below the yellow arrow in figure 1B. You have clearly noted that even though you know that on the other side there are three lights on a pole, you cannot make a clear distinction.

The most curious thing about this is that the focal points of the visual scene are precisely the same points on the retina that render the most focused portion or not. In the retina, there is a point known as the fovea, which processes exactly where we are focusing. As you can deduce, when we move the eye to understand the world, this is the same as speaking that we move the eye to direct the fovea toward various points in space. In the image below you can observe this phenomenon (Figure 2).

Figure 2 - Illustrative image of the congruence between the visual scene and the area where the image touches the retina.

However, the way I explore the world (moving my focal center) is completely different from yours. Just as this pattern is differentiated in people with certain pathologies or emotional conditions. This is the main applicability of eye tracking: identifying eye movement patterns in different environmental and / or emotional contexts, so that, from this, it is possible to predict behavior and cognition. But how does the system that captures eye movements work and what data does this system return to us?

The vast majority of current eye trackers measure eye tracking from the reflection of infrared light in the cornea from a nearby infrared light emitting device (Figure 3A). Deflection, or eye movement, redirects the cornea and alters the reflection pattern of near infrared light, allowing identification of attachment points [1] (Figure 3B).

Figure 3 - Illustration of the basic operation of an eye tracker (A) and the patterns of infrared light reflection in the cornea in different eye directions (B).

The main numerical data that an Eye Tracking system returns to us are the following: balconies, fixation points, fixation time and micro balconies. The balconies are the movements that the eyes make between two focal points, while the fixation points are precisely these focal points. The fixation time is the amount of time (usually in seconds) that the individual has kept his gaze focused and focused on a focal point, and the microsaccades are small movements within a fixation point [2], as can be observed in figure 4.

Figure 4 - Base chart of eye tracking. The lines refer to the balconies, while the green dots are the attachment points. Clamping time is shown by the size of the clamping points.

There are currently several major types of Eye Trackers for a wide range of neuroscientific applications. Scientific applications can be with more robust laboratory eye measurement devices in a controlled environment (Figure 5, ABD) as well as fully portable systems with the possibility of applications in naturalistic and real environments, as shown in Figure 5C. If you are interested in Eye Tracking, Neuroscience and Behavior, please contact us.

Figure 5 - Different Eye Tracking Systems for different applications.

References

[1] - Duchowski, A. T. (2007). Eye tracking methodology. Theory and practice, 328, 614.

[2] - Holmqvist, K., Nyström, M., Andersson, R., Dewhurst, R., Jarodzka, H., & Van de Weijer, J. (2011). Eye tracking: A comprehensive guide to methods and measures. OUP Oxford.

The content published here is the exclusive responsibility of the authors.

Autor:

Rodrigo Oliveira

#neuroscience #eyetracking #eegnirseyetracking #neuroscience #eyetracking #eegnirseyetracking