Washington, February 16 : University College London scientists have unearthed reason why people often see things that do not actually present around the object on which their eyes are concentrated.
Published in PLoS Journal of Computational Biology, the study reveals that the context surrounding an object may sometimes override the evidence gathered by the eyes, and even cause a person to imagine things that are not really there.
The study even reveals that a vague background context is more influential and helps a person fill in more blanks than a bright, well-defined context.
According to the researchers, this may help explain why people are prone to "see" imaginary shapes in the shadows when the light is poor.
During the study, 18 observers were assigned to focus their attention on the centre of a black computer screen. Each time the buzzer sounded, the participants pressed one of two buttons to record whether or not they had just seen a small, dim, grey "target" rectangle in the middle of the screen.
The rectangle would not appear every time, and even when it did appear, it would be displayed for just 80 milliseconds.
"People saw the target much more often if it appeared in the middle of a vertical line of similar looking, grey rectangles, compared to when it appeared in the middle of a pattern of bright, white rectangles. They even registered 'seeing' the target when it wasn't actually there," said Professor Li Zhaoping, lead author of the paper.
"This is because people are mentally better prepared to see something vague when the surrounding context is also vague. It made sense for them to see it - so that's what happened. When the target didn't match the expectations set by the surrounding context, they saw it much less often," the researcher added.
Professor Zhaoping says that magicians often exploit background contexts in their tricks.
"Illusionists have been alive to this phenomenon for years. When you see them throw a ball into the air, followed by a second ball, and then a third ball which 'magically' disappears, you wonder how they did it. In truth, there's often no third ball - it's just our brain being deceived by the context, telling us that we really did see three balls launched into the air, one after the other," said Professor Zhaoping.
"Contrary to what one might expect, it is a vague rather than a bright and clearly visible context that most strongly permits our beliefs to override the evidence and fill in the blanks. In fact, a bright and clearly visible context actually overrides the evidence in the opposite direction - suppressing our 'seeing' of the vague target even when it is present.
"Mathematical modelling suggests that visual inference through context is processed in the brain beyond the primary visual cortex. By starting with a relatively simple experiment such as this, where visual input can be more easily and systematically manipulated, we are gaining a better understanding of how context influences what we see. Further studies along these lines can hopefully enable us to dissect the workings behind more complex and wondrous illusions," Professor Zhaoping added.