A while ago I stumbled upon an interesting video by the SlowMoGuys. For those who have not heard of them, they basically shoot many things at ultra slow-motion 1000 fps; From bullets to balloons, you name it!
In this video (watch below), they shot something very interesting to me: a pupil constricting during bright light exposure. Read on for some scientific observations on what they say in the clip.
The reason our pupils typically constrict in such a condition, a phenomenon called the pupillary light reflex, is to limit the amount of light entering our eye in order to avoid an over-exposure of our photoreceptors, just like the aperture of a camera.
Being controlled by the central nervous system, this reflex has been used as a rapid check for lesions in the oculomotor nerve or the brainstem. [Reference]
Then, at a certain point in the video (minute 2:08 to be exact), Daniel mentioned that it felt like his heart beat was moving his eye. Although it is unlikely that he was feeling his iris moving, veins below the eye can sometimes cause a facial nerve to pulsate, especially with lack of sleep or with stress. However, since our main topic here is the pupil, it has been found that the pupil diameter actually changes in accordance with our heart beat. More specifically, the frequency of pupil diameter fluctuations is highly correlated with the frequency of our heartbeat. [Reference]
In a similar way, pupil diameter fluctuations also follow changes brightness of an attended object by constricting and dilating at the same frequency. Check out this video to see how this effect can be used to type without moving one’s hands (or eyes).
Other factors that affect pupil diameter include cognitive aspects, such as cognitive work load and memory [Reference]. I have also learned recently that pupil diameter changes when we move our focus from a near to a far object and vice versa, which is part of the accommodation reflex of the eye, or the pupillary near reflex.
I hope I didn’t leave anything out, but if there’s something else you’d like to know about, especially if it is related to our visual and cognitive systems, feel free to drop me a line using this form or on social media below. If you are interested in pupil data analysis, make sure to check my other articles on the topic.
P.S. A last remark about the video, which is not pupil related: Right after the first exposure to sudden light [at minute 2:43], Daniel says he sees a “doughnut” even after the light was turned off. This effect is know as the “after-image effect”. It is basically caused by the adaptation of photoreceptors due to over-exposure to a visual stimulus which makes it seem like it is still present even after its disappearance.