Summary: A new study reports researchers have created a new optical illusion which helps to reveal how our brains determine the material properties of an object.
Source: University of New South Wales.
Researchers have created a new optical illusion which helps reveal how our brains determine the material properties of objects – such as whether they are transparent, shiny, matte or translucent – just from looking at them.
UNSW scientist Dr Juno Kim created a computer image of an object with the refractive index of glass. People perceived it as transparent when they viewed it in one orientation. But when it was flipped upside down, it then looked opaque and glossy to them.
“It’s a brand new optical illusion,” says Dr Kim, of the UNSW School of Optometry and Vision Science.
“It demonstrates the visual system has learnt to rely on the orientation of the horizon, and whether bright highlights are at the top or the bottom of an object, to work out whether objects are transparent or opaque. It shows that our brain is asking: which way is up?”
The study by Dr Kim and Dr Phillip Marlow of the University of Sydney is published in the journal i-Perception.
Crystal balls and other transparent round objects refract light so the sky appears at the bottom and the ground or ocean at the top.
A popular sculpture that demonstrates this inversion effect is Lucy Humphrey’s Horizon, a large transparent acrylic sphere filled with water which featured in the Sculpture by the Sea exhibition in Sydney in 2013, and which was on show again in August 2016 at Barangaroo.
“With mirror-like, shiny round balls, on the other hand, the sky and the ground appear the right way up, and this is something we automatically learn throughout life,” says Dr Kim.
To produce the new optical illusion, he created a computer image of an object with a refractive index equivalent to Crown glass as viewed in outdoor light.
The object was rated as having a similar transparency to ice or glass on average by eight observers.
But when the object was viewed upside down (see below), and the bright highlights were at the top where you would expect the sky to be, it was mistaken as opaque and reflective, with some saying it looked like liquid metal.
“Our brain is sophisticated, but when it is trying to infer the material composition of an object just from the distorted pattern of light reflected from the surface, it makes guesses based on previous experience,” says Dr Kim.
“This optical illusion appears to be driven by two main biases in the brain. The first is the assumption that a light source usually comes from above. This bias is the reason people look scary and very different when they put a torch under their chin. The brain is used to processing faces with the light coming from above,” says Dr Kim.
“And the second bias is that most objects tend to be convex, or round overall.”
Apart from revealing the hidden working of the brain, the research could have applications for computer animators wanting to produce realistic images of objects, or for people making visual judgements about the composition of objects on a processing line.
Source: Juno Kim – University of New South Wales
Image Source: NeuroscienceNews.com images are adapted from the University of New South Wales press release.
Original Research: Full open access research for “Turning the World Upside Down to Understand Perceived Transparency” by Juno Kim and Phillip J. Marlow in i-Perception. Published online September 26 2016 doi:10.1177/2041669516671566
Turning the World Upside Down to Understand Perceived Transparency
Specular surfaces and refractive media are difficult to distinguish from each other because they both generate distorted images of the surrounding lighting environment. Whereas convex refractive objects invert the orientation of the horizon so the sky appears beneath the ground plane, convex specular surfaces preserve the orientation of the horizon so the sky appears above the ground. Here, we show that a refractive transparent object can be made to appear specular and opaque simply by rotating the image by 180°. This result suggests that the visual system relies on information tied to the orientation of the horizon to distinguish between refractive and specular objects.
“Turning the World Upside Down to Understand Perceived Transparency” by Juno Kim and Phillip J. Marlow in i-Perception. Published online September 26 2016 doi:10.1177/2041669516671566