May 17, 2023

The pupillary light reflex explained

An article explaining how the iris reacts to light and its relevance to our work with Ocula.

The pupillary light reflex explained

We’re going to take you through some simple anatomy of the eye to help explain what goes on in the background when light hits our eyes.

This is an introductory article which will give you a better understanding of the work we’re doing in AI with our app, Ocula, and how it can accurately detect concussion and make other impairment assessments based on a quick scan of the eyes.

Firstly, what is the role of the pupil?

The pupil is the small dark hole in the coloured part of the eye, known as the iris.  The main job of the iris is to control how much light enters the eye and works with the cornea and lens to make sure it’s appropriately in focus.

To achieve this, the lens plays a vital role in helping the pupil by bending and focusing incoming light directly on the retina, which is how the clearest possible image is produced.

What is the retina?

The retina is a thin sheet of brain tissue, lining the back of the eye that contains millions of light-sensitive cells that receive and organise visual information. The retina sends this information to the brain through the optic nerve, enabling you to see.

As the retina is developed from the brain, it shares many of its properties. Because of this, the health of the retina gives you some idea of what might be happening in the brain.

Image source: Wikipedia

The autonomic nervous system

The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, respiratory rate, urination, arousal, digestion and pupillary response.

The parasympathetic and sympathetic nervous systems are a part of the autonomic nervous system, and they play a significant role in the pupillary light reflex response.

The pupillary light reflex

Under normal conditions, when the body is sedentary or resting, the parasympathetic nervous system is most active. It handles the bodily functions that we don’t need to consciously think about to control like breathing, heart rate, digestion and sweating.

The parasympathetic nervous system also relaxes your body after periods of stress or danger. It is responsible for the constriction response within the eye, making the iris smaller in diameter to let less light in.

Image source: Wikimedia Commons

When the body is experiencing any form of stress that is different to its normal state, for example you become suddenly scared or aroused, or begin some form of exercise, our sympathetic nervous system becomes active. It carries signals to the brain that put your body’s systems on alert, ie. when the body is experiencing ‘fight or flight’ mode.

The sympathetic nervous system is responsible for the dilation response within the eye, making the iris larger in diameter to let more light in.

The take away point here is that these two systems interacting to define the diameter of the pupil at any particular moment, it is crucial that the retina receives just the right amount of light, too much or too little can lead to blindness.

If activity in the sympathetic and parasympathetic nervous system gets out of balance, as happens after a concussion, the iris won’t react appropriately.

Ocula was designed to help detect brain dysfunction by measuring how the iris reacts to light.