Neuroscientists developed a method to study how the brains distinguishes real from imagined images.Credit: Zephyr/Science Photo Library
Real and imagined images are processed using the same systems in the brain, yet most people can distinguish between the two. Neuroscientists have identified two brain regions that keep imagined images separate from reality, described in a study1 published in Neuron on 5 June.
“Being able to keep apart your inner world and actual reality is quite useful for normal functioning in daily life,” says co-author Nadine Dijkstra, a cognitive neuroscientist at University College London, UK. “This is something that does go wrong in psychosis and schizophrenia,” she adds.
Dijkstra and her colleagues developed a method to probe the limit of people’s ability to distinguish real and imagined images. This involved showing volunteers black and white stripes of varying transparency over a background resembling television static. In some trials, the researchers asked the participants to imagine the stripes as they looked at the image, and to report whether they really saw the stripes and, if so, how vivid they were.
When participants thought the stripes were more vivid, they were more likely to report them as real, regardless of whether the stripes were actually there.
The researchers are trying to measure and then model an important yet abstract concept — the interaction between imagination and vision — that until now has been a philosophical question, says Thomas Naselaris, a neuroscientist at the University of Minnesota Twin Cities.
Brain imaging
To correlate the participants’ observations with brain activity, the researchers took functional magnetic resonance imaging (fMRI) readings of the participants while they viewed the images with and without stripes. The fMRI readings, which rely on tracking changes in blood flow as a proxy for brain activity, showed that activity in a region called the fusiform gyrus correlated highly with the reported vividness of the stripes. Neuroscientists already knew that the fusiform gyrus processes high-level visual information, but its role in distinguishing imagined images from reality was unknown.
This suggests that the fusiform gyrus sums up the total imagined and real stimulus, says Dijkstra. “The fusiform gyrus is tracking this ‘reality signal’, this vividness signal that then predicts reality judgments [whether participants judged the image to be real],” she says.
