Function of Pupillary Dilation in Sympathetic Arousal

A commonly cited response to sympathetic arousal is pupillary dilation: the eye’s pupils open up. Italian women of the Renaissance thought that by applying atropine (belladonna, literally beautiful lady) to their eyes, causing pupillary dilation, they could enhance their beauty, and modern studies by psychologists have shown that they were right: photographs of people with large pupils are judged to be more attractive than photos with small pupils (presumably because when you look at a person with large pupils you are looking at someone who is aroused while looking back at you). Psychologists have attempted to use pupillometry as a measure of emotion.

Dilated pupil (from Nutschig at the English language Wikipedia)

Constricted pupil (public domain)

What is almost always glossed over is the function served by dilated pupils in times of high emotional arousal.  One often sees statements such as “The pupils dilate, letting in more light so you can see better,” but this has never made much sense to me; why would making the scene brighter than normal make vision better?

I think I know what those dilated pupils are doing for us. At this point consider it a strong hunch, guided by years of thinking about both the adaptive value of behavior and about photography.  In photography, the term “depth-of-field” (DoF) refers to the region from the closest to the farthest part of the photo that appears to be in focus.  A large DoF means that objects near and far are in focus; landscape photographers typically want large DoF.  A small DoF means only a narrow region at a particular distance from the camera appears in focus and the rest is blurry; portrait photographers usually want small DoF resulting in a blurry background that does not distract from the model’s face. Enlarging the aperture of the lens decreases the depth-of-field, and reducing the aperture increases the depth-of-field.

Depth of field illustration. larger aperture on the left, small aperture on the right. (photo by Alex1ruff, licensed under the Creative Commons Attribution-Share Alike 4.0 International license)

Why should sympathetic arousal be associated with a large-aperture pupil and thus a small DoF? Wouldn’t you want a large DoF, with as much as possible in the scene in clear focus? I argue that no, in times of “fight or flight” a small DoF is beneficial. Accurate movement is essential, and judging the distance of objects (the branch to which you must jump to escape; the neck of the predator you have to bite to disable it) is critical. One unconscious cue that we use to judge distance is the extent that the lens must be curved or straightened to bring an object into focus — a process called accommodation. With a large DoF (small aperture pupil) there is a lot of leeway or “slop” in how accurately the lens must be curved. As the DoF gets smaller (with a large aperture pupil) lens curvature is more critical to focus as their is less slop. The result of that dilated pupil with its small DoF is more precision in the information that is provided by the lens accommodation; the brain therefore more accurately interprets the distance of the object being viewed. Behavior is then more accurate. Our arboreal primate ancestor escaping from the proto-leopard gauges the distance to the neighboring branch correctly because his pupils were dilated and he survives; his friend lacking the pupillary dilation misses the branch, and his genes are gone from the gene pool. 

To examine human DoF for constricted and dilated pupils, I used a web-based DoF simulator designed for photographers, applying photographic parameters to the human eye.  A web search suggested that a commonly-suggested value for the focal length of the human eye is 22 mm (the DoF simulator needs the entered focal length to be stated as if the camera in use is a 35-mm film camera, with film size of 24 x 36 mm; at least one online site suggests that in these terms the human eye’s focal length should be entered as 32 mm).  Another site suggests that the aperture of the human eye, expressed in the traditional f-stop terminology of photography, ranges from f/8.3 when constricted to f/2.1 when dilated. 

Taken from the online tool DOF Simulator, the grey regions illustrate the distance from the human eye that will appear in focus (depth of field) when an object is 3 m away. On the left – the DoF when the pupil is constricted; on the right – the DoF when the pupil is dilated.

The images (clipped from my session with the online DoF simulator) show the DoF when the pupil is constricted compared to the DoF when the pupil is dilated. Here the model is 3 m from the camera, a reasonable distance for that escape branch. With a constricted pupil (left), the area from ~0.8 m in front of that branch to ~1.6 m beyond the branch appears in sharp focus, a distance of 2.4 m. With a dilated pupil (right), the clear focus ranges from ~0.25 m in front to ~0.25 m beyond the branch, only 0.5 m. Clearly the dilated pupils provide better information about exactly how far to leap.

Please recognize that the values used in the calculations that created these estimates were at best rough approximations of the parameters of the human eye. However, whatever values are used, the dilated pupil will produce a DoF that is much narrower than that afforded by the constricted pupil, so the dilated pupil will always provide better information about the accurate distance to the object. 

[If someone wants to take this idea and develop it more fully or more rigorously please do so – just acknowledge my contribution, please.]

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