If the brain works like this in other ways, it suggests we should be less trusting of the evidence from our senses, says Christoph Teufel of Cardiff University, who wasn’t involved in the study. “Perception is not providing us with a [true] representation of the world,” he says. “It is contaminated by what we already know.”
The blind spot is caused by a patch at the back of each eye where there are no light-sensitive cells, just a gap where neurons exit the eye on their way to the brain.
We normally don’t notice blind spots because our two eyes can fill in for each other. When vision is obscured in one eye, the brain makes up what’s in the missing area by assuming that whatever is in the regions around the spot continues inwards.
Trick of the mind
But do we subconsciously know that this filled-in vision is less trustworthy than real visual information? Benedikt Ehinger of the University of Osnabrück in Germany and his colleagues set out to answer this question by asking 100 people to look at a picture of a circle of vertical stripes, which contained a small patch of horizontal stripes.
The circle was positioned so that with one eye obscured, the patch of horizontal stripes fell within the other eye’s blind spot. As a result, the circle appeared as though there was no patch and the vertical stripes were continuous.
Next to this was another circle of vertical stripes without a patch of horizontal stripes. People were asked to choose which circle seemed most likely to have continuous stripes.
Ehinger’s team were expecting that people would choose the circle without a patch more often. “It would be more logical to choose the one where they can really see all the information,” he says.
In fact, people chose the circle that had a filled-in patch 65 per cent of the time. “We never expected this,” says Ehinger. “The brain trusts its own generated information more than what it sees outside in the world.”
This fits in with what we know about cognitive biases, says Ehinger. When people hold strong beliefs, they are likely to ignore any evidence to the contrary.
There is no obvious benefit to our brains ignoring external information, says Ehinger. “I’ve talked to many people about it, and they all say it doesn’t make sense,” he says.
As well as blind spots in vision, there are other ways in which our perceptions are changed by the brain’s expectations, says Teufel. For instance, if a well-known song is converted to MIDI format, which strips out the vocals, people can usually “hear” words that aren’t really there.
Journal reference: eLife, DOI: 10.7554/eLife.21761