Your retina is a tissue consisting of two types of cells, rods and cones. Cones distinguish between wavelengths, giving you color, while rods sense the intensity of light. Your rod cells have a receptor called rhodopsin, which harbors a special protein called retinal. As a photon of light collides with retinal, it undergoes a conformational shape change, activating the rhodopsin receptor. This in turn activates a glycoprotein, which activates a cyclic Guanine mono phosphate phoshodiesterase. This enzyme hydrolyzes cyclic Guanine mono phosphate (cGMP).

Now the interesting part is that before the photon interacted with this system, cGMP was holding a ion channel in the cell membrane open and letting sodium ions in to the cell. This was causing the cell to secrete a neurotransmitter to the brain. When the cGMP gets hydrolyzed by the enzyme, it shuts the gate, therefore shutting off the neurotransmitter secretion.

In other words your brain SEES the dark, and only knows light by the absence of the dark stimulus.

So in the human perception, light is the absence of darkness, instead of the widely believed counterstatement.

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