When "The Polar Express" film came out, it was creepy to a lot of people. It was a cartoon with faces modeled after the real actors, but still a digital creation. The same response happens when people are around a robot that veers closer to being human in appearance.

It's called The Uncanny Valley - robots have an upward curve of fascination and then suddenly plummet into a valley of repulsion. But it's getting uncanny as we get more comfortable with robots, according to a new study. So much so that touching a robot's intimate areas elicited physiological arousal in humans.

The researchers conducted an experiment using Aldebaran Robotics' NAO human-shaped robot, programmed to verbally instruct participants to touch 13 parts of its body. Participants were fitted with an Affectiva Q-Sensor on the fingers of their non-dominant hand. This measured skin conductance, a measure of physiological arousal, and reaction time of the participant.

The findings showed that when participants were instructed to touch the robot in areas that people usually do not touch, like the eyes or the buttocks, they were more emotionally aroused when compared to touching more accessible parts like the hands and neck. Participants also were more hesitant to touch these intimate parts based on the response times.

A large body of research in communication shows how touch is used as a social "glue" between people - building relationships and influencing trust. Not as much is known about touch between a person and a robot. Touch has been under-examined compared to other aspects of robots, such as its appearance and shape. 

"Our work shows that robots are a new form of media that is particularly powerful. It shows that people respond to robots in a primitive, social way," said Jamy Li of Stanford University. "Social conventions regarding touching someone else's private parts apply to a robot's body parts as well. This research has implications for both robot design and theory of artificial systems." 

"Touching a Mechanical Body: Tactile Contact With Intimate Parts of a Human-Shaped Robot is Physiologically Arousing," by Jamy Li, Wendy Ju and Bryon Reeves; to be presented at the 66th Annual International Communication Association Conference, Fukuoka, Japan, 9-13 June 2016.