ANCIENT Aztec 'death whistles' have a strange 'uncanny valley'-style effect on the human brain, new research has found.

These clay instruments were often shaped like human skulls, and are known for their eerie, blood-curdling sounds.

Each blow of the whistle mimics a raspy human scream.

Modern listeners, and likely ancient ones too, have trouble classifying what they are hearing.

The sounds appear both natural and artificial which helps instill fear, the research, published in Nature, suggests.

It is first study to investigate the psychological impacts of these sounds.

Around 70 European participants, with no forewarning about the 'death whistle' cries, were exposed to the sounds while their brain activity was monitored.

Participants described the sounds as "scary," and "aversive", and associated the noise with alarm sounds such as sirens, firearms, alongside human sounds of fear and pain.

"Skull whistle sounds attract mental attention by affectively mimicking other aversive and startling sounds produced by nature and technology," researchers wrote.

The sounds were classified in the brain, according to the researchers, as a "hybrid mix of being voice- and scream-like but also originating from technical mechanisms".

Participants showed heightened activity in the part of the brain that processes sound, indicating an immediate state of high alert.

According to the researchers, these responses stem from the brain's difficulty in classifying the sound.

It is similar to the 'uncanny valley' effect that human-like robots or even lifelike dolls appear to trigger in humans.

It occurs when our brains can't clearly define what they're seeing as natural or artificial.

"Skull whistle sounds are... rather ambiguous in the determination of their sound origin, which intensifies higher-order brain processing," the researchers added.

The study, led by cognitive neuroscientists from the University of Zurich, suggested the sounds could symbolise aspects of Aztec mythology.

For example, the shrill, high-pitched sounds could have represented the sharp winds of Mictlan, the underworld where sacrificial souls were believed to go.

Others suggest they echoed the presence of Ehecatl, the Aztec God of Wind, who ancient Aztecs believes created humanity from the bones of the deceased.

Some believe Mesoamerican communities used the whistles to strike fear into their foes during warfare.

But that has been disputed as no whistles have been found at battle sites or in warrior graves.