Image caption Automaton Museum: Blank expressions in zombies or automated figures are psychologically unsettling

The horror-film appeal of zombies is rooted in an instinctive disquiet at faces that are almost human, says a psychologist.

A research project by the Open University's Stephanie Lay found a particular fear of "near-human" faces.

It explains why blank eyes, automatons, masks and ventriloquists' dolls are common cinema devices to instil fear.

The psychologist says a study of 3,000 people found widespread "repulsion" at near-human faces.

The finding is part of research into the so-called "uncanny valley" phenomenon in how people react to robots.

'Uncanny valley'

The effect was discovered in the 1970s in research into whether giving robots human features would make them more reassuring to humans working with them or using them in the home.

The research found that people were initially more receptive to robots which were made slightly human looking, but if they began to imitate humans too closely, people were repelled.

Instead of being reassured, too close a resemblance to a human form was seen as sinister.

Mrs Lay's research is examining this "uncanny valley" effect from a psychological perspective.

She describes it as "the sense of unease that accompanies the sight of something almost, but not quite, human".

It is the sudden plunge in reaction at a point just before a face is fully humanised.

Image caption Ventriloquist act from BBC television in the 1930s: Funny or unsettling?

For instance the empty eyes of a zombie figure in an otherwise human face is highly disconcerting. And it explains why the faces of dolls, clowns and dummies are used so regularly to frighten audiences.

"A consistent finding of the survey was people's reactions to images where the face was convincingly human but with lifeless eyes or where eerily human eyes appeared in a non-human face," says Mrs Lay.

"These were perceived to be the most uncanny and disturbing of all the images and explain why characters such as zombies in horror films unsettle people to such great effect."

This is particularly the case with psychological horror films, she says, rather than bloodthirsty shockers.

She says this uneasy reaction could be caused by the sudden disruption in how the human brain processes human faces.

When people begin to interpret something as a human face, they are left with a sense of disquiet when they come against something that does not match their expectations.

Being able to "read" another person's eyes is particularly important in this process of interpreting faces, she says.

"It may be that our normal processing mechanisms are not being engaged for these faces, causing the unsettling effect when people want to like a near-human face, but realise something is wrong."