Wooosh…blam! A rocket arcs up into the sky, exploding and shooting lines of shimmering sparks in all directions. It’s the Fourth of July, and fireworks are lighting up the sky — and the neurons of everyone watching.

When a firework explodes, light travels hundreds of feet in an instant to trace abstract, moving shapes on the retinas of everyone watching.

This is a visual and neuroscientific treat. The shapes, traced on the retina, cause individual neurons to light up along the optic nerve. Those neurons map to the shapes they represent. Trace a triangle in the sky with a shaped firework, and a triangle of nerves will become active behind the retinas, sending signals back toward the visual cortex in the back of the brain.

Midway through transmission, the neurons tangle up, and that triangle gets reduced a to a gobbledygook of shuffled dots along the nerve. Freeze it there, and it would be unreadable. But by the time the signal races out the end of the long nerve and into the cortex, it’s been unscrambled, the neurons rearranged to their original position on the back of the retina.

The region of the brain the signal lands on forms a pretty accurate map of the back of the eyes from whence it came. Neuroscientists know this thanks to a different kind of explosion — gunfire — and the work of Japanese opthamologist Tatsuji Inouye.

In 1904, as Sam Kean describes in his book The Tale of the Dueling Neurosurgeons, Russia invaded Japan. Russian soldiers were armed with Mosin-Nagant Model 91 rifles, which launched tiny bullets at high velocity toward the bodies of Japanese soldiers. Those bullets moved so fast that they would often enter the skull of a Japanese rifleman, carve a narrow tunnel through his brain, and exit without killing him or destroying the surrounding tissue. When they would pierce the visual cortex, they would leave the soldiers with small holes in their vision.

Inouye was charged with figuring out how large the gap in a given soldier’s visual field was so the army could determine his military pension. And he figured out that those gaps mapped exactly onto the location of the small holes in the soldiers’ brains, which in turn mapped onto the retina.

So we know that in the safer explosive context of a Fourth of July fireworks show, a giant glowing orb in the sky lights up an orb of neurons at the back of the skull of a first-time watcher.

But vision doesn’t end there; the signal still has to be interpreted. Later experiments, conducted by inserting electrodes into the brains of cats, monkeys, and other unfortunate lab animals showed that the deeper cortex breaks down the received image into its component parts. That bright green horizontal line leaping to the left away from the rocket lights up one set of nerves for horizontal lines, leftward motion, and the color green. Another marking an upward stroke activates different nerves for vertical lines. If the firework forms some recognizable shape, like a triangle, deeper regions of the cortex interpret it to the viewer, who might see fireworks as a flower, a face, a star, or something else entirely.

Then all that information gets passed forward again in the brain, where different regions — those involved in memory, perhaps, or the social instincts and motion necessary to generate the ooohs and aaaahs reverberating through the crowd — get involved.

And then the moment ends; the next one begins. Another firework goes off, light splashes again against the crowd’s individual’s retinas, and the whole process begins all over again.

Photos via Flickr / redagainPatti