Things we see when our eyes are closed

I would guess most of you don’t spend much time with your eyes closed for the sole purpose of looking at the backs of your eyelids. I don't either. In fact, the only reason this topic is on my mind at all is because recently I have been treating some intermittent eye-itchiness by applying warm compresses to my closed lids. This requires me to spend several minutes with my eyes shut with nothing to do but watch the backs of my eyelids. How pleasantly surprised I was to find that there is ample entertainment back there to keep me from either going crazy or falling asleep.

What I discovered while reclining with a hot towel on my face were phosphenes: brilliant patterns of light, pulsing and evolving with intricate motion and color, intensified by the application of  pressure on my covered lids. You've probably seen phosphenes before, perhaps while rubbing your weary eyes or while dosing off in a pitch dark room, but have you ever purposely tried to evoke them? Or spent anytime observing them as they happen?

Phosphenes were first described (in written form anyway) in the fifth century B.C.E. by Alcmaeon of Croton, the O.G. of medical writers and an early neuroscientist. His theory was that pressure on the eyeball actually produced light within the eyeball that was responsible for the flashes and dashes of light -- eyeball fireworks, if you will. It sounds crazy, and it definitely is hogwash, but remember that Alcmaeon was from a time long before we even knew what the retina was, much less how it worked. So let's not judge him too harshly. 

Fast forward to the twentieth century, where scientists are beginning to tease apart the molecular processes that lead to pressure-induced phosphenes. It is now thought that pressure can actually directly activate some of the neurons in your retina, leading to the sensation that you are seeing visual stimuli. Normally, special cells at the back of your retina called photoreceptors sense light energy and convert it into electrochemical energy. This new form of energy is passed from cell to cell through several layers of neurons in the retina, and eventually to your brain. Pressure doesn't turn on the photoreceptor cells though -- good thing too, because then they wouldn't be just photoreceptors anymore would they? We would have to call them mechanophotodetectors, or phototactilia cells or something ridiculous like that! Anyway, the pressure I've been applying while I have the towel on my eyes directly activates some of the middleman neurons instead, bypassing the need for light to start the neural domino effect. 

 Pressure bypasses photoreceptor cells and activates horizontal cells. Adapted from Kimbrel and Lanza, 2015.

Why pressure would be an effective stimulus for some retinal cells is not clear. Perhaps pressure sensitivity is actually a universal property (or at least more ubiquitous than we tend to think) of neuronal membranes. It would be hard to test this on humans, however, because most of our brain is protected by a thick, rigid bone that prevents access to pokes and prods (thank god!). I think it’s pretty cool that I can exert some control over my visual system with nothing more than a hot compress and some time on my hands. I highly encourage you to explore your own pressure phosphenes*…you might be surprised by what you see!

*Start out gently, and if it hurts – STOP!

Sources and further reading:

Celesia, Gastone G. (2012) Alcmaeon of Croton's Observations on Health, Brain, Mind and Soul.  Journal of the History of the Neurosciences: Basic and Clinical Perspectives, 21(4), 409-426.

Grüsser and Hagner (1990) On the history of deformation phosphenes and the idea of internal light generated in the eye for the purpose of vision. Documenta Ophthalmologica, 74, 57-85. 

Grüsser, Grüsser-Cornehls, Kusel and Przybyszewski (1989) Responses of retinal ganglion cells to eyeball deformation: A neurophysiological basis for "pressure phosphenes". Vision Research, 29(2), 181-194. 

Kimbrel and Lanza (2015) Current status of pluripotent stem cells: moving the first therapies to the clinic. Nature Reviews Drug Discovery, 14, 691-692.

Taylor, Walsh and Eimer (2010) The neural signature of phosphene perception. Human Brain Mapping, 31(9), 1408-1417.