What’s more, a flickering flame in the cave may have conjured impressions of motion like a strobe light in a dark club. In low light, human vision degrades, and that can lead to the perception of movement even when all is still, says Susana Martinez-Conde, the director of the Laboratory of Visual Neuroscience at the Barrow Neurological Institute in Phoenix, Ariz. The trick may occur at two levels; one when the eye processes a dimly lit scene, and the second when the brain makes sense of that limited, flickering information.
Physiologically, our eyes undergo a switch when we slip into darkness. In bright light, eyes primarily rely on the color-sensitive cells in our retinas called cones, but in low light the cones don’t have enough photons to work with and cells that sense black and white gradients, called rods, take over. That’s why in low light, colors fade, shadows become harder to distinguish from actual objects, and the soft boundaries between things disappear. Images straight ahead of us look out of focus, as if they were seen in our peripheral vision. The end result for early humans who viewed cave paintings by firelight might have been that a deer with multiple heads, for example, resembled a single, animated beast. A few rather sophisticated artistic techniques enhance that impression. One is found beyond the Hall of Bulls, where the cave narrows into a long passage called the Nave.
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