Dilation and constriction of the pupil are not merely reflex responses. Cognitive processes like attention also influence the pupillary light response, recent research shows. Psychologist Sebastiaan Mathôt explores how the pupil can help in optimal perception and received a Veni grant from the Dutch Science Association NWO to continue this line of research at the University of Groningen.
Traditionally scientists viewed the pupil as a more or less passive player in the process of perception, dilating when it gets darker and constricting when there is more light available. But there seems to be more to it.
When the pupil is dilated, a lot of light reaches the photoreceptors in the retina, which would theoretically be beneficial for perception because it means the sensitivity of the resulting image is boosted. So why not always keep the pupil at its most dilated state? Because the lens is imperfect, Mathôt explains. ‘These imperfections become more influential in distorting the image when the light flows through a larger diaphragm into the lens,’ Mathôt explains. ‘So a dilated pupil results in less detail in the projected image.’
Therefore, Mathôt expects the pupil to strike a balance between sensitivity and sharpness of the image. But varying circumstances call for different qualities in perception. Mathôt: ‘As an extreme example, imagine you are at an open battle field. You would not be interested in the exact design of the jacket of your fellow soldiers. Instead, you would want to be as sensitive as possible to visual clues around you. This would call for a more dilated pupil than you would have when you are at ease, on your couch, reading a book.’
Mathôt will test this principle in two lab experiments that are very subdued versions of the battlefield and reading a book scenarios. He will measure how pupil diameter influences performance on these two types of tests.
In the first part of the test, participants stare at a cross in the middle of a white computer screen and are asked to press a button when they see a stimulus appear elsewhere on the screen. This requires high sensitivity and low detail of the image. In the second part of the experiment, people again fix their gaze at a cross in the middle of the screen and are asked to describe stimuli that briefly pop-up. In order to do this well, the subject will need to register a more detailed image of the shown stimulus.
By adjusting the amount of light in the room, giving subjects eye drops that dilate their pupils or having them watch either a white or black screen prior to the test, Mathôt will influence the pupil diameter, so he can analyse whether that tweaks performance in the two perception tasks.
Foremost, Mathôt hopes to simply help the understanding of human perception progress. But he also sees some more practical applications of his pupil research. In a previous study, he showed how attention to either a dark or a light object influences pupil diameter. He now wants to use this principle to see if it could help paralysed people in the final stadium of the neurological disease ALS to communicate with the outside world through pupil size. The system Mathôt is testing for this distinguishes between the letters in the alphabet by giving the unique ‘barcodes’ of alternating light and dark spheres surrounding them. Measuring the changes in pupillary diameter in the patient subsequently indicates which letter they were focusing on. ‘Testing this system with actual patients will probably be the most intense intense part of my research. But I would be really happy if I would be able to help people who are suffering from such a horrible disease.’
Text: Marieke Buijs