STARS IN OUR EYES!

Not unlike cartoon characters who have stars circling their heads when they are hurt, we "see stars" when we bump our heads, stand up too quickly or rub our eyes too vigorously. What causes this to happen? The retina at the back of the eye converts light entering the eye into electrical signals which are transmitted to the visual area of the brain where they are perceived as images. Normally the cells in the retina respond to light but sometimes an electrical impulse is triggered in the brain in other ways, and the brain interprets this as specks of light. In effect, the brain is tricked into seeing light that is not present. The phenomenon of experiencing light without light actually entering the eye is known as a phosphene and may come from a variety of sources.
 
Why does it happen when we bump our heads? The human brain is surrounded by cerebrospinal fluid. Because there is some space around the brain, it is able to move within the skull but is cushioned and protected from injury by this fluid. If, however, we bump our heads with force, the brain may hit the area of the skull where the visual cortex, which processes visual information, is situated. This irritation triggers the nerve cells to discharge electrical impulses which are perceived as spots of light or "stars".
 
Seeing stars when standing up too quickly is a response which occurs from inside the eye itself rather than from an external source, like a bump on the head which stimulates the visual cortex directly. When we change positions quickly, we may have a postural drop in blood pressure, causing a brief delay in blood being pumped back towards the heart from the legs. There is a momentary reduction of blood flowing to the retina, slightly reducing its supply of oxygen. Abnormal signals are transmitted to the brain which perceives light although there is no external light source. This response is sometimes accompanied by buzzing in the ears. Both the stars and the buzzing usually disappear on their own after a few seconds or if the head is lowered.
 
The most common phosphenes are pressure phosphenes which are caused by mechanical stimulation. Rubbing the eyes briefly interrupts normal blood flow to the retina and applies abnormal pressure on the optic nerve which sends false signals to the visual cortex for processing. The "stars" we see are essentially the random firing of electrical impulses to the brain. These images can persist for a while when the rubbing stops and the eyes are opened, but once the tissues return to normal, the stars fade with no lasting effects. Other mechanical causes of phosphenes sometimes include sneezing, laughing, coughing and blowing the nose.
 
Phosphenes have been reported by astronauts exposed to radiation in space, as well as by people who have not been exposed to light or have been deprived of visual stimulation for long periods of time, for example prisoners in isolation. They may also be one of the side effects of certain medications. Some migraine sufferers experience "seeing stars" before the onset of the headache.
 
While phosphenes are generally no cause for concern, seeing sudden or frequent flashes of light in one or both eyes for no apparent reason could indicate a potentially dangerous eye condition, particularly if they are accompanied by pain or vision disturbances. These would be regarded as a medical emergency requiring immediate medical intervention.