New research reveals that one of the Aztecs’ most chilling artefacts, clay death whistles, which resemble a human skull and produce a scream-like sound, not only frightened listeners in ancient times, but also had a profound effect on the human brain’s ability to increase states of alarm and fear.

For the first time, scientists investigated the effects of these disturbing whistles on the brains of modern European volunteers, conducting two separate experiments with different participant samples.

Aztec death whistles usually skull-shaped were designed to produce a high-pitched, penetrating sound similar to a scream, resulting from the collision of different air currents. Numerous examples have been found in graves dated between 1250 and 1521 CE.