5 minutes of neuroscience
What is “pupillometry”?
Pupillometry observes the change in the pupil’s diameter. There are plenty of different mental processes or neurophysical reactions that can contribute to the change of the pupil’s size. In the following article we are going to introduce 2 of the major ones.
Harsh brightness
Probably everybody noticed that our pupil contracts to harsh brightness. This so-called pupil reflex regulates the amount of light that will arrive to our retina, to keep the brightness of what we see more or less the same. Of course, we can still perceive a difference in brightness (e.g. in the morning compared to at sunset), but it is less extreme. Think of a picture you can take with your camera. If the level brightness in some areas is not balanced, it can result in fully black or white pixels in your photos. In photography these kinds of pixels are deemed „meaningless”, as they do not carry too much information about what is in the picture, only the level of brightness in that area.
Mental effort
The pupil’s diameter was also found to show a correlation with mental effort: it dilates if a situation requires more effort. This is a result of the body’s sympathetic response. The autonomic nervous system is the primary mediator of physiological responses to internal and external stimuli, and it has 2 parts: the sympathetic and the parasympathetic nervous system. Their roles are easily separable: the sympathetic system is responsible for a fight-or-flight response, meaning that the body prepares to allocate energy for a possible attack or run away; meanwhile the parasympathetic system includes every process that is important for rest and digest. So when we experience a harder task, the body gets ready to allocate more energy, and this results in a dilation of the pupil.
How can we detect pupil size?
The short answer is: with the help of eye trackers. There are of course multiple types of eye trackers, but the most common one is the remote eye tracker. This device is placed right below the screen, what the participant is watching. It basically is an infrared light source and sensor in one. The pupil absorbs light, so eye trackers with this method easily detect the pupil as the area which has zero reflected infrared light. With the pupil fully detected, we are only a step away from calculating its dilation, which is usually a built-in function of an eye tracker.
written by Nóra Paller
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