Photo of a woman's eye
Have you observed bright colored lights when rubbing your eyes? Have you seen transparent stringy particles floating midair when looking at the sky? Have you wondered if they are actually there? Or, are your eyes are fooling you? The answer is no; they aren’t there, but — your eyes aren’t fooling you either. These visual effects are called entoptic phenomena, derived from Greek, ento (within), and optic (eye). Therefore, entoptic means occurring within or inside one’s eye.
The renowned German scientist Hermann von Helmholtz once said, “under suitable light conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical.” Interestingly, this phenomenon is purely subjective. They cannot be observed by an eye doctor using an instrument and cannot be photographed. Sometimes the phenomenon can be used to monitor eye diseases, but most occurrences are unconcerning. In this post, we discuss three commonly observed phenomena and how to differentiate these occurrences from the abnormal ones. Pressure phosphenes
Rub your eyes by applying mild pressure using your index finger while keeping them closed. Do you see stars or circular shaped patterns moving opposite the direction of the pressure surrounded by bright multicolored lights? These patterns are called pressure phosphenes. We encounter them when rubbing our eyes upon waking up.
The word phosphene is derived from two Greek words; phos (light) and phainein (to show). This is the only phenomenon that occurs in the absence of light entering our eyes. We usually see things because light reflected off of surfaces enter our retinas, the backscreen in our eyes, and stimulate retinal ganglion cells that carry information to our brain to process what we see. So, how do we see light when there is no light entering our eye? Vision science researchers believe the mechanical stimulation caused by applying pressure on our eyes stimulates those same retinal ganglion cells. The cells think they perceive light, and we see several multicolored lights and shapes. While seeing pressure phosphenes is normal, they should not be confused with flashes of light or aura seen in certain types of migraines and other conditions such as a posterior vitreous detachment or retinal detachment, where certain layers of deeper retina are peeling away. Phosphenes or star-shaped patterns can also be seen after a hard sneeze, a deep cough, a blow to the head, or low blood pressure as there might be mechanical or metabolic (low glucose or oxygen) stimulation of the visual nerve cells. These can also be perceived by meditators and by those who ingest psychedelic drugs. 
Several phosphene patterns outlined by scientist, Max Knoll during one of his experiments
Blue sheer phenomenon
Have you noticed a small number of circular or squiggly transparent shapes when gazing at the blue sky or on a uniformly bright background like a computer screen or a mobile phone? What do you think caused you to see them? Blue light from the sky enters our eyes and is blocked by red blood cells as they absorb all colored lights and allow only red light to pass. However, since white blood cells are transparent, they allow blue light to pass through them. This light further excites the retinal cells. So, the small transparent shapes we see are actually our white blood cells moving along the thin retinal blood vessels. As red blood cells are not transparent, we sometimes see dark patterns floating next to the transparent shape when observed carefully against a uniformly bright pattern.
Blue field or Sheerer phenomenon is observed only during daylight with open eyes and does not impair vision. However, this should not be confused with visual snow, where small white, black, or multicolored spots are seen in a television static fashion across the entire visual area for long periods. Visual snow usually presents with migraines, can impair vision, and is perceivable even when dark. While the exact cause is unknown, it is believed that visual snow is caused by excessive excitation of neurons, the nerve cells, in our brain and requires immediate medical treatment.
Floaters
Floaters are tiny worm-shaped or transparent blobs that appear when you gaze at the sky or a uniformly bright background. Our eyes are made up of a transparent jelly-like component called vitreous humor that helps maintain the eye's shape and structure and helps keep the retina layers intact. With age, the vitreous humor gradually starts losing its transparency and viscosity. Due to this, the cells, proteins, and other components in the vitreous start forming clumps. When light passes through them, they cast tiny shadows on our retinas, called floaters. Seeing floaters in small numbers is normal, but it is alarming when you see large numbers of them constantly with a sudden onset. They could be due to a tear, detachment, or hemorrhaging of our retina or the posterior detachment of vitreous humor and require immediate medical treatment. Floaters should not be confused with blue field or Sheerer phenomenon as they are slightly longer in size and drift away with our eyes' rapid movements.
Image of floaters over a blue background
Entoptic phenomenon reminds us that what we see depends on the image created by our eye's physiology, i.e. the shape and structure of the eye and what we perceive in our environment. Hence, this should not be confused with optical illusions, which are purely caused by visual structures and are perceived differently from reality. So, next time you see some of these shapes floating, don't rush to rinse your eyes. Enjoy your observation with this new understanding.
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