If our eyes are the windows to the soul, then the pupils are at least the gateway to the brain. When we look around at the world, our eyes track different features, and both our corneas and the tissue surrounding the lenses of our eyes adjust to focus the light, making a scene visible. The amount of light can be controlled by the size of our pupil, but that’s not the only thing that the pupil size responds to. Pupil dilation is affected not only by how we see the world, but also by how we experience it.
Pupillometry is defined as a measure of pupil dilation, giving unique insights into how an individual views their environment. While it was originally thought (and often claimed in pop-science) that the pupil would only dilate in response to positive or attractive stimuli, psychologists have uncovered that both positively and negatively valenced information can bring about such an increase.
It has since been established that any emotionally charged stimuli (presented as images, words, sounds, etc) will elicit an expansion of pupil size, providing a measure of emotional response not too dissimilar from galvanic skin response (GSR; in which the intensity of an emotion can be uncovered, but not the direction).
Research has also suggested that pupillometry tests may be a more reliable measure of stress-related responses than GSR, although there are more constraints on how this research is carried out which are discussed below.
More than meets the eye
Further research has also uncovered that cognitive load is linked to the degree of pupil dilation that occurs – with more cognitively demanding tasks linked to increasingly large pupil size. An example of this is shown with the Stroop task – in which participants read incongruently colored words (e.g. the word “blue” that is printed in a red color).
These findings further suggest that an increase in pupil dilation is concordant with an increase in cognitive processing – the harder your brain works to process emotion or information leads to larger pupils.
As the amount of cognitive or emotional processing increases, so too does the pupil dilation.
Through fMRI imaging, it has been found that increases in brain activity within different areas have been directly linked to pupil dilation, making this measure a suitable proxy for understanding cognitive processes at a deep level.
On the clinical / medical side, researchers have found abnormal pupillometry responses to stimuli in individuals with depression, anxiety, Autism spectrum disorder, and even Parkinson’s disease. All of this establishes pupillometry as a critically important measure for not only understanding the normal function of individuals, but also what happens in the brain afflicted by disease.
The measurement of pupil dilation does however offer some tentative clues as to how the lives of people suffering from a neurological disease might be improved. Researchers have noted that psychological treatment success can be predicted at a certain level, dependent on pupillometry results. This could provide a quick and easy way in which to ensure that the best possible treatment is matched to the afflicted individual, which could improve not only recovery rates, but also the efficiency of healthcare.
Further to this, the field of brain-computer-interaction (BCI) could be greatly enhanced by utilizing pupil dilation as an additional signal. BCI is often used by people with physical movement impairments, and eye tracking can be an accessible way for computer interaction to occur. By bringing in a reliable measure of pupil size, another cognitive signal could be added, enriching the BCI process.
Combined use of other biosensors and pupillometry could take BCI even further, as Graur and Siegle note in their article (2013): “In particular, if personal computers could perceive a user’s emotion via the use of physiological measures such as heart rate, galvanic skin response, EEG, pupil diameter etc., then it could intelligently alter a user’s experience”
How to do pupillometry
In order to start making and developing new discoveries with pupillometry, you just need an eye tracker, a stimulus (or stimuli) of choice, and iMotions. As a non-invasive measure, it’s easy to get it up and running. Special pupillometry devices aren’t required, as eye trackers can typically perform this function.
One thing that must be controlled for in any test of pupil dilation, is the amount of light that the participant sees. The stimuli luminescence must be standardized to ensure that any pupil reflex is due to an emotional or cognitive reaction, and not a purely physical reaction to light intensity.
Three sources of stimuli that increase pupil dilation – light, emotional stimuli, and information that increases cognitive load.
Once this factor is controlled for (along with the lighting levels in the room), the experiment can begin. However, to really get the most from pupillometry data, it can be beneficial to bring in other biosensors, to complement the findings.
EEG signals can be calculated into metrics that are representative of an individual’s mental stateand can provide information about the level of cognitive workload that is being experienced (among other metrics). Using an EEG with pupillometry can then help decipher whether or not the increased pupil dilation is due to cognitive load, or other factors.
Further to this, using EEG to calculate frontal asymmetry can inform about whether or not the participant is inclined to approach or avoid a stimulus. This measure is automatically calculated in iMotions, making the steps in understanding the underlying cause of an increased pupil dilation even easier to get to.
Additional combinations with other physiological sensors can enrich the understanding of an individual’s thoughts, feelings, and behavior ever further. Utilizing facial expression analysis can provide information about the emotional valence, while the other measures of eye tracking allow a triangulation between what the individual is looking at, how it makes them feel, and how strongly.
A screenshot of iMotions showing pupillometry results, as well as eye tracking, and including a measurement of the respondent’s distance from the screen.
How to set up a pupillometry experiment – what to consider.
Where the usability areas and general applicability of pupillometry are well defined, the same cannot be said for the object it is measuring, namely the pupils. Pupils are not reactive to the strict set of circumstances that researchers are looking for when doing pupillometry studies. They are subject to change depending on external stimuli, tiredness, lighting, and cognitive idleness – i.e not engaging with a specific task.
Setting up a viable experiment means taking all of these factors into account. Respondents should be well-rested, the study should take place in a room where external stimuli are at a minimum, and most importantly, light sources should be kept at a constant.
Controlling the light from screens showing stimuli can be close to impossible, depending on the material. Most videos or image slide shows will have a change in light, which will make it very difficult to get a usable reading, as pupils invariably will react to changes in the light.
When setting up a pupillometry experiment, changes to the light of the screen can be counteracted by setting up the experiment to feature standard luminescence images. These are images that emit the same level of light, through color, brightness, saturation, etc. Controlling the light emittance requires careful planning, preparation, and execution of the experiment.
Pupillometry experiments with eye tracking glasses.
We generally do not recommend doing pupillometry experiments with eye tracking glasses. That is primarily because every time a participant’s head moves or they look at something different, the light will catch the pupil in a different way, thereby making both the reading and data unreliable.
As a measure of cognitive and emotional processing, pupillometry is a suitable measure for understanding the intensity of the stimuli – relevant for furthering an understanding of anything from ad testing to user experience, to psychological testing and beyond.
While there are experimental constraints that must be considered before embarking on a pupillometry experiment, there are also plentiful discoveries to be uncovered with pupil dilation data, and it will continue to be a useful tool in the human behavior researcher’s toolbox. So keep your eyes open.
I hope you’ve enjoyed reading about how pupillometry can inform a greater understanding of human reactions to stimuli. To learn more about pupil dilation, and how eye tracking can be used within research, download our free guide to eye tracking below.