By Mark Ward
Technology correspondent, BBC News website, San Diego
Every video gamer knows that it does not take much to destroy the sense of immersion evoked by the best games.
Making convincing artificial humans is a tough business
A clunky interaction with a computer-controlled character or even bodily movements that somehow just don't seem right can throw a player right out of the game world.
For those who do not work in the animation or video games this may seem to be only a temporary problem that steady improvements in graphics will overcome.
The bad news is that the more realistic a human shown on screen or in a game becomes, the more jarring are the moments when what they do fails to match what we expect.
In a classic paper printed in 1970, Japanese roboticist Masahiro Mori coined the term "uncanny valley" for the region of human experience that robots and animations fall into as they look ever more life-like.
Dr Mori argued that this problem would only become more acute as animators and robot-makers found ways for their creations to look more real.
One of the panel sessions at the Siggraph show in San Diego discussed insights from psychology and neuroscience that could help understand and confront this pressing problem.
To begin with, said Professor Jessica Hodgins, from the computer science and robotics lab at Carnegie Mellon University, it was clear that notions of what counts as "human" involved much more than just life-like looks.
Brain scans are helping shed light on how humans spot a fake
Humanness has many dimensions, she said, and includes such things as facial expressions, skin transparency, motion of the hair and the unity of motion and emotion seen in people when they gesture as they talk.
As a result, she added, making convincing artificial humans had to involve progress on all these fronts.
The reality of the uncanny valley has also been exposed by work in cognitive neuroscience that uses brain scanning techniques to spot which areas of the brain are in use when people look at images of fake and real people.
This had shown that what triggered the sense of unease is the way that the movements of androids, robots and animated characters diverge from what we expected, said Dr Thierry Chaminade of the Wellcome Trust Centre for Neuroimaging at University College London.
"When you see activity in an android your brain reacts a lot to make sense of it," he said. "For a human it does not need to get to this level of activity."
The really bad news, said Karl MacDorman, associate professor in the school of informatics at Indiana University, was that thousands of years of evolution could be responsible for helping us pick out fake folks with such ease.
Broadly, he said, evolutionary pressures had taught humans how to spot people that were healthy and fertile.
"Human beings have evolved to perceive those who are more fertile as more beautiful," he said.
Anything that does not conform to the range of characteristics we look for, or undermines their usefulness, was likely to be distrusted and disliked.
We had also become very adept at spotting the symptoms of sickness and illness and, he suggested, were uneasy around unconvincing non-human characters because of our suspicion that they were sick in some way.
"They evoke a fear of death, dying and mortality," he said.
In a very similar fashion, said Professor MacDorman, leprosy aroused similar feelings because it too conjured such profound notions of disease and illness.
But, he added, the good news was that it might just be a question of time before we get used to human-like characters and adapt to what they look like.
In the meantime animators have to do a lot of work to make their creations convincing.
Oscar winning animator Joe Letteri from the Weta Digital special effects studio, who brought Gollum to life for Peter Jackson's Lord of the Rings, explained how they could do that.
Increasingly film makers and animators were relying on motion capture to ensure that gross bodily movements and fine facial expression were realistic.
But this should not be viewed as an easy escape and the uncanny valley still creeps in, he said.
"You cannot rely on it 100% given the current state of the art," he said. "Often you find yourself asking 'why does it not look as good as seeing the actor perform?'."
Overcoming the uncanny valley meant huge amounts of work and real attention to detail by animators, he added.
He explained that when animating Gollum the artists realised that the movements of corners of his mouth seemed to play a big part in convincing someone that the creature was saying the words in the script. Other tiny movements of eyes and eyelids were also key in making Gollum convincing.
But, Mr Letteri said, getting it right was not easy.
"Every facet has to fall into place for an audience member to believe what they are seeing is real."