http://www.sciencedaily.com/releases/2008/02/080215103210.htm
ScienceDaily (Feb. 19, 2008) — Scientists
at UCL (University College London) have found the link between what
we expect to see, and what our brain tells us we actually saw. The
study reveals that the context surrounding what we see is all
important -- sometimes overriding the evidence gathered by our eyes
and even causing us to imagine things which aren't really
there.
The paper reveals that a vague background context is more
influential and helps us to fill in more blanks than a bright,
well-defined context. This may explain why we are prone to 'see'
imaginary shapes in the shadows when the light is poor.
Eighteen observers were asked to concentrate on the centre of a
black computer screen. Every time a buzzer sounded they 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. It did
not appear every time, but when it did appear it was displayed for
just 80 milliseconds (80 one thousandths of a second).
"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 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.
"Illusionists have been alive to this phenomenon for years,"
continued Professor Zhaoping. "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.
"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."
Journal reference: Filling-in and suppression of visual
perception from context -- a Bayesian account of perceptual biases
by contextual influences, by Professor Li Zhaoping and Dr Li
Jingling appears in the 15 February issue of the journal PLoS
Computational Biology.
The research was funded by the Gatsby Charitable Foundation and
a Cognitive Science Foresight Grant.