Portable stone paintings and engravings may have been the original moving pictures, dating back to Paleolithic times. Archaeologists believe they were positioned around cave hearths so the figures portrayed would seem to come alive in the shifting light from the fires.

Human neurology is particularly attuned to interpreting shifting light and shadow as movement,” Dr Andy Needham of the University of York and co-authors write in PLOS ONE. Our pareidolia, the seeing of faces in non-human forms, goes into overdrive in such conditions.

While this phenomenon presumably evolved to avoid our ancestors being eaten by night-hunting predators, some of those ancestors appear to have put the capacity to use to make their art more powerful. After all, when the safest thing to do at night was to stay in the cave, no one wanted younger members of the tribe getting bored. Art that constantly changed in the shifting firelight may have been the paleolithic equivalent of a Playstation or a movie.

The artwork Needham is discussing is not the fabulous paintings of the cave walls we associate with Paleolithic art. Instead these are what are known as plaquettes, stones small enough to be portable and flat enough for easily visible engravings. Although they have been found across Europe, these plaquettes are most common from Portugal to Germany and nearby islands. They depict animals, humans and abstract symbols, with occasional depictions of what are thought to be rivers in styles matching what is known as Magdalenian.

Plaquettes were probably used in many ways, particularly using their thermal properties. Often, however, archaeologists have not been able to establish their purpose. Needham and co-authors conducted a detailed study of 54 plaquettes collected in the 19th century from Montastruc rock shelter, Southern France and mostly made of limestone.

Most of the plaquettes are engraved with images of bison, ibex, horses and deer. A few have birds, parallel lines or undetermined subjects. Often the lines are adapted to follow natural features of the rock. The largest have faces roughly the size of an A4 sheet of paper, but 3 centimeters (1.2 inches) thick. Most are less than half that size.

Crucially for this work, all the limestones show evidence of heating after they were engraved, with cracking and thermal fractures visible, as well as color changes specific to limestone heated to certain temperatures.

“These plaquettes would have been placed in close proximity to hearth structures in low light levels, perhaps as a means of emphasising the relationships between engraved forms and natural features in the rock, with the dynamic light cast from the hearth bringing the depictions to light,” the authors conclude.

That doesn’t mean this was the only purpose for their placement close to fires. Limestone could also have served a function by radiating heat effectively through the site and storing it for after the fire burned down, which the authors note would have been crucial during a time of bitter cold.

Larger plaquettes could also have served as cooking stones. However, if these were their only purposes, the engravings would have been unnecessary. When the researchers placed replica stones around open fires they found the light enhanced the relationship between the natural shape and the engravings and the moving flames made the carvings appear dynamic. Some of the engravings, such as those portraying herds of horses and ibex, appear to have been specifically designed to capture this effect.

In combination with the recent hypothesis that certain cave paintings were placed in locations where oxygen deprivation would induce hallucinations, we may only have scratched the surface of the ingenuity of cave art.

Worked antlers found at Montastruc have been dated to around 14,000 years ago, towards the end of the last Ice Age, but whether the plaquettes are from the same time is uncertain.