The research shows that by measuring a person's movements as they use a computer, it is possible to judge their level of interest by monitoring whether they display the tiny movements that people usually constantly exhibit, known as non-instrumental movements.
If someone is absorbed in what they are watching or doing - what Dr Harry Witchel, from the Brighton and Sussex Medical School in UK calls 'rapt engagement' - there is a decrease in these involuntary movements.
"It's the same as when a small child, who is normally constantly on the go, stares gaping at cartoons on the television without moving a muscle," he said.
The discovery could have a significant impact on the development of artificial intelligence, researchers said.
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Future applications could include the creation of online tutoring programmes that adapt to a person's level of interest, in order to re-engage them if they are showing signs of boredom, they said.
It could even help in the development of companion robots, which would be better able to estimate a person's state of mind.
While viewers can be asked subjectively what they liked or disliked, a nonverbal technology would be able to detect emotions or mental states that people either forget or prefer not to mention.
"Being able to 'read' a person's interest in a computer programme could bring real benefits to future digital learning, making it a much more two-way process.
"Further ahead it could help us create more empathetic companion robots, which may sound very sci-fi but are becoming a realistic possibility within our lifetimes," Witchel said.
Their movements were quantified over the three minutes using video motion tracking. In two comparable reading tasks, the more engaging reading resulted in a significant reduction (42 per cent) of non-instrumental movement.