Why do you see stars when you hit your head?

A hard knock briefly jolts the brain inside the skull and slams it against the occipital lobe at the back of the head, where the visual cortex lives. The mechanical stimulation makes neurons in the visual cortex fire as if they were responding to light, even though no light has entered your eyes. The brain interprets that activity as flashes of light — the medical term is phosphenes.

Do you actually see stars when you hit your head?

Yes, in the sense that you really do experience flashes of light — the visual cortex is genuinely active. But there are no real stars and no light is entering your eyes; the perception comes from neurons firing because they have been mechanically jolted, not because they have detected photons.

Why do you see stars when you hit the back of your head specifically?

Because the visual cortex is located in the occipital lobe at the back of your brain. A blow to the back of the head jolts the brain forward and back inside the skull and the occipital lobe takes the brunt of the impact, so the visual cortex is the first part of the brain to misfire.

Does seeing stars mean you have a concussion?

Not necessarily. A brief shower of phosphenes after a minor bump is usually harmless. But if seeing stars is accompanied by symptoms like dizziness, nausea, prolonged headache, blurred or persistent visual disturbance, confusion, or loss of consciousness, that combination is more concerning and warrants medical evaluation for concussion.

Phosphenes are flashes or shapes of light that you perceive without any actual light entering the eye. They can be triggered by mechanical pressure (rubbing your eyes, a blow to the head), electrical stimulation, low blood pressure, or even simply closing your eyes in the dark. The common factor is that the visual system is being activated by something other than photons.

Why Do You See Stars When You Hit The Back Of Your Head?

Table of Contents (click to expand)

A Bump To The Brain
Tricking The Visual Cortex

The “stars” you see after a knock to the head are phosphenes — flashes of light created when your visual system is mechanically stimulated rather than by actual light. A blow snaps your brain inside the skull, briefly jolting the visual cortex at the back of the head (the occipital lobe). The neurons there respond by firing as if light had hit them, so your conscious experience is a brief shower of bright dots and streaks.

The chaotic discharge of energy tricks the brain into thinking that it is seeing a twinkling array of lights without any discernible order or pattern. However, this effect doesn’t last for too long.

If you’ve never cracked your head on something at some point in your life, then you’re very lucky, and should probably stop living in such a safe little bubble. However, for those of us who have ever walked into an open cabinet, taken a bad fall, or been smacked with the backside of a 2×4, we know what it feels like for our noggin to take a beating. Aside from the pain and the inevitable bump on our skull, a peculiar thing happens when we sustain a head injury – we end up “seeing stars”.

If you get hit hard enough, you end up seeing darkness, but somewhere between a bump and a knockout blow, this phenomenon occurs, but why?

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A Bump To The Brain

Humans have evolved over hundreds of thousands of years to protect the most important part of our anatomy – the brain. Our grey matter is surrounded by something called cerebrospinal fluid, which acts as a cushion for our brain. When we turn our heads suddenly, bang our forehead on a table, or rock out to our favorite song, our brains move in accordance with the impact, but doesn’t bang against the sides of our skull too hard.

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However, in more extreme cases, when we hit our heads with more velocity (or when an object hits us with great speed), something different occurs. Let’s take the example of getting punched in the face. When the blow lands, our skull will snap back, at which point it will hit the front part of our brain. When we fall to the ground and bang our head on the ground, the skull will then be shoved forward, hitting the back part of the brain, namely the occipital lobe. This rapid impact can disrupt the flow of blood in that area, and can also affect the huge amount of nerve cells at the back of the head.

This rapid movement of the skull and its impact on the brain is what causes us to “see stars”, because the occipital lobe is where the visual cortex is located.

Tricking The Visual Cortex

When we open our eyes and look around, those nerve impulses are translated into images in the tissues of the visual cortex. Essentially, that is how we see the world around us. However, when those tissues are impacted so violently (by the skull snapping back against the occipital lobe), the nerve cells at the back of the head discharge a waterfall of electrical impulses, which scatter through the tissues.

This chaotic discharge of energy tricks the brain into thinking that it is seeing a twinkling array of lights without any discernible order or pattern. This effect doesn’t last for too long, however, and as the tissues return to normal and the pressure is eased from those nerve cells, the stars will gradually fade. “Seeing stars” is harmless, although whatever caused you to see those stars may do some real damage.

You can artificially create the same experience by gently rubbing your eyes when you wake up in the morning. Pressing on the eyeball mechanically stimulates the photoreceptor cells of the retina (not the optic nerve, contrary to a common myth), which then send light signals to the visual cortex even though no light has entered the eye. The result is the same kind of phosphene — swirling patches of colour, blobs, or shimmering dots — with no head injury required.

While looking up at a star-filled sky can be a transcendent way to view the world, sustaining head injuries to replicate that experience isn’t a very wise choice. Protect your brain at all costs!

References (click to expand)