SEEING STARS

OUCH! You bump your head and "see stars". You stand up too quickly, feel light-headed and "see stars". You rub your eyes too hard when you wake up in the morning, and "see stars". Cartoon characters "see stars" circling their heads in all sorts of painful situations. While cartoon characters see star-shaped figures, what we see in the real world is specks of light that resemble stars.

In effect what happens when we see stars is that we are tricking our brains into seeing something that is not actually there.

The human brain is surrounded by cerebrospinal fluid, which acts as a cushion, protecting this important organ from injury. Because there is some space around the brain, it is able to move within the skull. When we turn our heads suddenly or bump them, our brains move with the impact, but are protected from banging against the bony skull too hard. The visual cortex is located at the back of the brain in the occipital lobe. This is where we receive, interpret and make sense of information received by the eye. If we hit our heads with more force, this rapid movement of the skull causes the nerve cells at the back of the head to discharge a "waterfall" of electrical impulses. The electric discharge is interpreted as light in a twinkling disorganised pattern that can be likened to the pattern stars make in a dark sky.

If you stand up too quickly, the process is different but the effect of "seeing stars" is similar. As the blood is coming up from your legs into your heart to then get pumped around the body, the perfusion pressure drops just briefly. Before your heart compensates, there is a momentary reduction in the supply of oxygen and sugar to your retina, causing the retina to send abnormal light signals to the vision centre in the brain. The brain is fooled into thinking you're seeing light when it's not there.

You can artificially create the same experience by rubbing your eyes in the morning when you wake up. By putting abnormal pressure on the optic nerve behind the eye, a confusing jumble of signals needs to be processed by the visual cortex.

The interruption of normal blood flow and pressure generates these false images of "stars", which are essentially the random firing of electrical impulses through your visual nerve centre.

This effect is harmless and doesn't last for too long, and as the tissues return to normal and the pressure is eased from those nerve cells, the stars gradually fade. Seeing flashes when there has been no blow to your head may mean something quite different. Sometimes these images signal a tear in the retina, a condition that requires immediate medical attention. And some sufferers from migraine headaches report star sightings as the headache begins.

Next time you see stars lets hope they are the ones in the sky!