Scientists at 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.
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my girl crazy, man!
my girl crazy, man!
My grandfather always said “believe half of what you see and none of what you hear.” He was a smart man and has been telling me this all my life. Weather or not he knew the specifics of illusions pertaining to the way the senses work with together within the brain I don’t know. I do know it is amazing that this interaction within the brain and the senses can now be further studied. I agree that this opens more opportunities for illusionists and that it is a good aid for the explanation of how the mirror treatment works for the ghost-limb.
There is an alternate explanation. Over at the Duke U Purves Lab they’ve also been doing these sorts of experiments and have several clever parlour tricks now rendered online so you can try them yourself — by their reasoning, there is a paradox in how our experience of ‘now’ can seem so rich when our actual apparatus for detecting the “reality” behind that experience is really very lossy.
For example, our eyes may collect light and our rods and cones may generate signals, but it does not follow that all those signals reach the brain; what actually hits the brain is really not very rich at all!
So how could this be? How could the qualia appear so complete when we’re missing so much of the picture? If I can presume to summarize work I barely understand
the Purves view appears to be that we spontaneously augment our present-tense perceptions with bits from our total sum timeline of past experience, assimilating from not just the near-present context, but from all the contexts in our history, all are brought to bear on ‘present’ experience to flush it out enough for us to model the actual reality.
And not just in the visual experience. Hearing too, as Diane Deutch’s research illustrates, is teeming with sensory ambiguities resolved on the fly by context expectations, which I believe could be a reason why Jazz/Gospel tritone-pivot substitutions work. There are similar real-time quandries in touch perception (curve illusions), in haptics (the famous Exploratorium exhibit that makes you think the brass hand is your own or the mirror treatment for ghost-limb) and probably others to be found in every other cognitive sense.
Needless to say, that leaves oodles of opportunities for illusionists, marketing and other shamanisms